Added dimension stacking

- Dimension files define upright top-to-bottom terrain layers
- Spacer values set exact air gaps between layers
- Optional noise styles shape blended layer boundaries
- Runtime queries resolve stacked terrain and biome ownership
- Pack validation rejects invalid stack definitions
- Project copy and hotload paths include stack dependencies
- Superflat fixtures cover gaps, blending, clipping, and fluids
- Platform tests cover stacked world generation contracts

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/CustomBiomeSource.java

@@ -4,6 +4,9 @@ import art.arcane.iris.platform.bukkit.BukkitWorldBinding;
 import com.mojang.datafixers.util.Pair;
 import com.mojang.serialization.MapCodec;
 import art.arcane.iris.spi.IrisLogging;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.data.cache.AtomicCache;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.GenerationSessionException;
@@ -16,6 +19,7 @@ import art.arcane.iris.util.project.context.IrisContext;
 import art.arcane.volmlib.util.collection.KMap;
 import art.arcane.volmlib.util.math.RNG;
 import net.minecraft.core.BlockPos;
+import net.minecraft.core.Direction;
 import net.minecraft.core.Holder;
 import net.minecraft.core.QuartPos;
 import net.minecraft.core.Registry;
@@ -24,6 +28,8 @@ import net.minecraft.core.registries.Registries;
 import net.minecraft.resources.Identifier;
 import net.minecraft.resources.ResourceKey;
 import net.minecraft.util.RandomSource;
+import net.minecraft.util.Mth;
+import net.minecraft.world.level.LevelReader;
 import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.biome.BiomeSource;
 import net.minecraft.world.level.biome.Climate;
@@ -80,27 +86,31 @@ public class CustomBiomeSource extends BiomeSource {
         this.cacheRuntimeId = engine.getCacheID();
     }
 
-    private static List<Holder<Biome>> getAllBiomes(Registry<Biome> customRegistry, Registry<Biome> registry, Engine engine) {
+    private static List<Holder<Biome>> getAllBiomes(
+            Registry<Biome> customRegistry,
+            Registry<Biome> registry,
+            Engine engine,
+            boolean includeDimensionStack
+    ) {
         LinkedHashSet<Holder<Biome>> biomes = new LinkedHashSet<>();
 
-        for (IrisBiome i : engine.getAllBiomes()) {
+        for (OwnedBiome ownedBiome : getHostOwnedBiomes(engine)) {
+            IrisBiome i = ownedBiome.biome();
             Holder<Biome> vanillaHolder = resolveBiomeHolder(registry, i.getStructureDerivativeKey());
             if (vanillaHolder == null) {
                 throw new IllegalStateException("Iris structure biome derivative '"
                         + i.getStructureDerivativeKey() + "' is not registered for biome '" + i.getLoadKey() + "'");
             }
             biomes.add(vanillaHolder);
+        }
 
-            if (i.isCustom()) {
+        if (includeDimensionStack) {
-                for (IrisBiomeCustom j : i.getCustomDerivitives()) {
+            biomes.addAll(getVisibleBiomes(
-                    Holder<Biome> customHolder = resolveCustomBiomeHolder(customRegistry, engine, j.getId());
+                    customRegistry,
-                    if (customHolder == null) {
+                    registry,
-                        throw new IllegalStateException("Iris custom structure biome '"
+                    engine,
-                                + engine.getDimension().getLoadKey() + ":" + j.getId() + "' is not registered");
+                    resolveFallbackBiome(registry, customRegistry)
-                    }
+            ));
-                    biomes.add(customHolder);
-                }
-            }
         }
 
         if (biomes.isEmpty()) {
@@ -110,6 +120,57 @@ public class CustomBiomeSource extends BiomeSource {
         return new ArrayList<>(biomes);
     }
 
+    private static List<Holder<Biome>> getVisibleBiomes(
+            Registry<Biome> customRegistry,
+            Registry<Biome> registry,
+            Engine engine,
+            Holder<Biome> fallback
+    ) {
+        LinkedHashSet<Holder<Biome>> biomes = new LinkedHashSet<>();
+        boolean fallbackPossible = false;
+        for (OwnedBiome ownedBiome : getOwnedBiomes(engine)) {
+            IrisBiome irisBiome = ownedBiome.biome();
+            if (irisBiome.isCustom()) {
+                for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
+                    Holder<Biome> customHolder = resolveCustomBiomeHolder(
+                            customRegistry, ownedBiome.dimension(), customBiome.getId());
+                    if (customHolder == null) {
+                        throw new IllegalStateException("Iris custom visible biome '"
+                                + customBiomeKey(ownedBiome.dimension(), customBiome.getId())
+                                + "' is not registered");
+                    }
+                    biomes.add(customHolder);
+                }
+                continue;
+            }
+            fallbackPossible |= !addVisibleBiomeHolder(
+                    biomes, registry, irisBiome.getDerivativeKey());
+            for (String scatter : irisBiome.getBiomeScatter()) {
+                fallbackPossible |= !addVisibleBiomeHolder(biomes, registry, scatter);
+            }
+            for (String scatter : irisBiome.getBiomeSkyScatter()) {
+                fallbackPossible |= !addVisibleBiomeHolder(biomes, registry, scatter);
+            }
+        }
+        if (fallbackPossible && fallback != null) {
+            biomes.add(fallback);
+        }
+        return new ArrayList<>(biomes);
+    }
+
+    private static boolean addVisibleBiomeHolder(
+            Set<Holder<Biome>> biomes,
+            Registry<Biome> registry,
+            String biomeKey
+    ) {
+        Holder<Biome> holder = resolveBiomeHolder(registry, biomeKey);
+        if (holder == null) {
+            return false;
+        }
+        biomes.add(holder);
+        return true;
+    }
+
     private static Object getFor(Class<?> type, Object source) {
         Object o = fieldFor(type, source);
 
@@ -175,10 +236,19 @@ public class CustomBiomeSource extends BiomeSource {
 
     @Override
     protected Stream<Holder<Biome>> collectPossibleBiomes() {
-        return possibleStructureBiomes().stream();
+        return possibleBiomes(true).stream();
+    }
+
+    @Override
+    public Set<Holder<Biome>> possibleBiomes() {
+        return possibleBiomes(true);
     }
 
     Set<Holder<Biome>> possibleStructureBiomes() {
+        return possibleBiomes(false);
+    }
+
+    private Set<Holder<Biome>> possibleBiomes(boolean includeDimensionStack) {
         GenerationSessionLease lease = tryAcquireGenerationLease("bukkit_possible_biomes");
         if (lease == null) {
             throw new IllegalStateException("Iris possible biome lookup was rejected during an engine transition");
@@ -197,7 +267,8 @@ public class CustomBiomeSource extends BiomeSource {
                     .lookup(Registries.BIOME).orElse(null);
             Registry<Biome> worldRegistry = ((CraftWorld) world).getHandle().registryAccess()
                     .lookup(Registries.BIOME).orElse(null);
-            return Set.copyOf(getAllBiomes(customRegistry, worldRegistry, engine));
+            return Set.copyOf(getAllBiomes(
+                    customRegistry, worldRegistry, engine, includeDimensionStack));
         }
     }
 
@@ -207,18 +278,22 @@ public class CustomBiomeSource extends BiomeSource {
             return m;
         }
 
-        for (IrisBiome i : engine.getAllBiomes()) {
+        for (OwnedBiome ownedBiome : getOwnedBiomes(engine)) {
+            IrisBiome i = ownedBiome.biome();
             if (i.isCustom()) {
                 for (IrisBiomeCustom j : i.getCustomDerivitives()) {
-                    Holder<Biome> holder = resolveCustomBiomeHolder(customRegistry, engine, j.getId());
+                    String key = customBiomeKey(ownedBiome.dimension(), j.getId());
+                    Holder<Biome> holder = resolveCustomBiomeHolder(
+                            customRegistry, ownedBiome.dimension(), j.getId());
                     if (holder == null) {
                         if (fallback != null) {
-                            m.put(j.getId(), fallback);
+                            m.put(key, fallback);
                         }
-                        IrisLogging.error("Cannot find biome for IrisBiomeCustom " + j.getId() + " from engine " + engine.getName());
+                        IrisLogging.error("Cannot find biome for IrisBiomeCustom " + key
+                                + " from engine " + engine.getName());
                         continue;
                     }
-                    m.put(j.getId(), holder);
+                    m.put(key, holder);
                 }
             }
         }
@@ -232,7 +307,8 @@ public class CustomBiomeSource extends BiomeSource {
             return Collections.unmodifiableMap(spawnBiomes);
         }
 
-        for (IrisBiome irisBiome : engine.getAllBiomes()) {
+        for (OwnedBiome ownedBiome : getOwnedBiomes(engine)) {
+            IrisBiome irisBiome = ownedBiome.biome();
             if (!irisBiome.isCustom()) {
                 continue;
             }
@@ -241,7 +317,8 @@ public class CustomBiomeSource extends BiomeSource {
                 continue;
             }
             for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
-                Holder<Biome> customHolder = resolveCustomBiomeHolder(customRegistry, engine, customBiome.getId());
+                Holder<Biome> customHolder = resolveCustomBiomeHolder(
+                        customRegistry, ownedBiome.dimension(), customBiome.getId());
                 if (customHolder != null) {
                     spawnBiomes.putIfAbsent(customHolder.value(), vanillaHolder);
                 }
@@ -268,6 +345,34 @@ public class CustomBiomeSource extends BiomeSource {
         }
     }
 
+    Holder<Biome> getVisibleSurfaceBiome(int blockX, int blockZ) {
+        GenerationSessionLease lease = tryAcquireGenerationLease("bukkit_surface_spawn_biome");
+        if (lease == null) {
+            return null;
+        }
+        try (lease; IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
+            if (!isRuntimeAvailable()) {
+                return null;
+            }
+            ensureCachesCurrent();
+            int quartX = QuartPos.fromBlock(blockX);
+            int quartZ = QuartPos.fromBlock(blockZ);
+            int sampleX = QuartPos.toBlock(quartX);
+            int sampleZ = QuartPos.toBlock(quartZ);
+            boolean cacheable = isBiomeCacheable(engine, sampleX, sampleZ);
+            DimensionStackContext stackContext = engine.getDimensionStackContext();
+            DimensionStackLayout.Layer layer = stackContext == null
+                    ? null
+                    : stackContext.getLayout(sampleX, sampleZ).surfaceLayer();
+            int internalY = layer == null
+                    ? Engine.hostHeight(engine, sampleX, sampleZ, true)
+                    : layer.clippedSurfaceY();
+            int quartY = QuartPos.fromBlock(internalY + engine.getWorld().minHeight());
+            return getVisibleNoiseBiomeWithActiveGenerationLease(
+                    quartX, quartY, quartZ, null, null, cacheable);
+        }
+    }
+
     private RegistryAccess registry() {
         RegistryAccess access = registryAccess.aquire(() -> (RegistryAccess) getFor(RegistryAccess.Frozen.class, ((CraftServer) Bukkit.getServer()).getHandle().getServer()));
 
@@ -294,28 +399,38 @@ public class CustomBiomeSource extends BiomeSource {
                 throw new IllegalStateException("Iris structure biome lookup has no active engine runtime");
             }
             ensureCachesCurrent();
-            if (isGuaranteedSurfaceBiome(y)) {
+            return getStructureNoiseBiomeWithActiveGenerationLease(x, y, z);
-                return getSurfaceStructureBiomeHolder(x, z);
+        }
-            }
+    }
 
-            long cacheKey = packNoiseKey(x, y, z);
+    private Holder<Biome> getStructureNoiseBiomeWithActiveGenerationLease(int x, int y, int z) {
+        if (isGuaranteedSurfaceBiome(y)) {
+            return getSurfaceStructureBiomeHolder(x, z);
+        }
+
+        boolean cacheable = isBiomeCacheable(x, z);
+        long cacheKey = packNoiseKey(x, y, z);
+        if (cacheable) {
             Holder<Biome> cachedHolder = structureBiomeCache.get(cacheKey);
             if (cachedHolder != null) {
                 return cachedHolder;
             }
+        }
 
-            Holder<Biome> resolvedHolder = resolveStructureBiomeHolder(x, y, z);
+        Holder<Biome> resolvedHolder = resolveStructureBiomeHolder(x, y, z);
-            Holder<Biome> existingHolder = structureBiomeCache.putIfAbsent(cacheKey, resolvedHolder);
+        if (!cacheable) {
-            if (existingHolder != null) {
-                return existingHolder;
-            }
-
-            if (structureBiomeCache.size() > NOISE_BIOME_CACHE_MAX) {
-                structureBiomeCache.clear();
-            }
-
             return resolvedHolder;
         }
+        Holder<Biome> existingHolder = structureBiomeCache.putIfAbsent(cacheKey, resolvedHolder);
+        if (existingHolder != null) {
+            return existingHolder;
+        }
+
+        if (structureBiomeCache.size() > NOISE_BIOME_CACHE_MAX) {
+            structureBiomeCache.clear();
+        }
+
+        return resolvedHolder;
     }
 
     @Override
@@ -346,6 +461,62 @@ public class CustomBiomeSource extends BiomeSource {
         }
     }
 
+    @Override
+    public Pair<BlockPos, Holder<Biome>> findClosestBiome3d(
+            BlockPos origin,
+            int searchRadius,
+            int sampleResolutionHorizontal,
+            int sampleResolutionVertical,
+            Predicate<Holder<Biome>> allowed,
+            Climate.Sampler sampler,
+            LevelReader level
+    ) {
+        GenerationSessionLease lease = tryAcquireGenerationLease("bukkit_locate_visible_biome");
+        if (lease == null) {
+            throw new IllegalStateException("Iris visible biome search was rejected during an engine transition");
+        }
+        try (lease; IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
+            prepareVisibleBiomeBatch();
+            DimensionStackContext stackContext = engine.getDimensionStackContext();
+            if (stackContext == null) {
+                return super.findClosestBiome3d(
+                        origin, searchRadius, sampleResolutionHorizontal, sampleResolutionVertical,
+                        allowed, sampler, level);
+            }
+            Set<Holder<Biome>> candidates = new HashSet<>();
+            for (Holder<Biome> biome : getVisibleBiomes(
+                    biomeCustomRegistry, biomeRegistry, engine, fallbackBiome)) {
+                if (allowed.test(biome)) {
+                    candidates.add(biome);
+                }
+            }
+            if (candidates.isEmpty()) {
+                return null;
+            }
+            int sampleRadius = Math.floorDiv(searchRadius, sampleResolutionHorizontal);
+            int[] sampleYs = Mth.outFromOrigin(
+                    origin.getY(), level.getMinY() + 1, level.getMaxY() + 1, sampleResolutionVertical).toArray();
+            for (BlockPos.MutableBlockPos sampleColumn : BlockPos.spiralAround(
+                    BlockPos.ZERO, sampleRadius, Direction.EAST, Direction.SOUTH)) {
+                int blockX = origin.getX() + sampleColumn.getX() * sampleResolutionHorizontal;
+                int blockZ = origin.getZ() + sampleColumn.getZ() * sampleResolutionHorizontal;
+                int quartX = QuartPos.fromBlock(blockX);
+                int quartZ = QuartPos.fromBlock(blockZ);
+                DimensionStackLayout stackLayout = stackContext.sample(
+                        QuartPos.toBlock(quartX), QuartPos.toBlock(quartZ));
+                for (int blockY : sampleYs) {
+                    int quartY = QuartPos.fromBlock(blockY);
+                    Holder<Biome> visibleBiome = resolveVisibleBiomeHolder(
+                            quartX, quartY, quartZ, null, stackLayout);
+                    if (candidates.contains(visibleBiome)) {
+                        return Pair.of(new BlockPos(blockX, blockY, blockZ), visibleBiome);
+                    }
+                }
+            }
+            return null;
+        }
+    }
+
     static int horizontalBiomeSearchQuartStep(int blockY, int searchRadius) {
         return blockY == STRONGHOLD_RING_SEARCH_Y && searchRadius == STRONGHOLD_RING_SEARCH_RADIUS
                 ? STRONGHOLD_RING_SEARCH_QUART_STEP
@@ -468,7 +639,7 @@ public class CustomBiomeSource extends BiomeSource {
     private Holder<Biome> resolveSurfaceStructureBiomeHolder(int x, int z) {
         int blockX = x << 2;
         int blockZ = z << 2;
-        IrisBiome irisBiome = engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+        IrisBiome irisBiome = resolveSurfaceStructureBiome(engine, blockX, blockZ);
         if (irisBiome == null) {
             throw new IllegalStateException("Iris returned no surface structure biome at block "
                     + blockX + "," + blockZ);
@@ -532,14 +703,32 @@ public class CustomBiomeSource extends BiomeSource {
             int z,
             Climate.Sampler sampler,
             IrisDimensionCarvingResolver.State resolverState
+    ) {
+        boolean cacheable = isBiomeCacheable(x, z);
+        return getVisibleNoiseBiomeWithActiveGenerationLease(
+                x, y, z, sampler, resolverState, cacheable);
+    }
+
+    private Holder<Biome> getVisibleNoiseBiomeWithActiveGenerationLease(
+            int x,
+            int y,
+            int z,
+            Climate.Sampler sampler,
+            IrisDimensionCarvingResolver.State resolverState,
+            boolean cacheable
     ) {
         long cacheKey = packNoiseKey(x, y, z);
-        Holder<Biome> cachedHolder = noiseBiomeCache.get(cacheKey);
+        if (cacheable) {
-        if (cachedHolder != null) {
+            Holder<Biome> cachedHolder = noiseBiomeCache.get(cacheKey);
-            return cachedHolder;
+            if (cachedHolder != null) {
+                return cachedHolder;
+            }
         }
 
         Holder<Biome> resolvedHolder = resolveVisibleBiomeHolder(x, y, z, resolverState);
+        if (!cacheable) {
+            return resolvedHolder;
+        }
         Holder<Biome> existingHolder = noiseBiomeCache.putIfAbsent(cacheKey, resolvedHolder);
         if (existingHolder != null) {
             return existingHolder;
@@ -552,6 +741,18 @@ public class CustomBiomeSource extends BiomeSource {
         return resolvedHolder;
     }
 
+    private boolean isBiomeCacheable(int quartX, int quartZ) {
+        return isBiomeCacheable(
+                engine,
+                QuartPos.toBlock(quartX),
+                QuartPos.toBlock(quartZ)
+        );
+    }
+
+    static boolean isBiomeCacheable(Engine engine, int blockX, int blockZ) {
+        return !engine.answersFromNaturalTerrain(blockX, blockZ);
+    }
+
     private GenerationSessionLease tryAcquireGenerationLease(String operation) {
         if (engine.isClosed()) {
             return null;
@@ -597,7 +798,7 @@ public class CustomBiomeSource extends BiomeSource {
     }
 
     private Holder<Biome> resolveStructureBiomeHolder(int x, int y, int z) {
-        BiomeResolution resolution = resolveBiomeResolution(x, y, z);
+        BiomeResolution resolution = resolveStructureBiomeResolution(x, y, z);
         if (resolution == null) {
             throw new IllegalStateException("Iris returned no structure biome at quart "
                     + x + "," + y + "," + z);
@@ -619,11 +820,26 @@ public class CustomBiomeSource extends BiomeSource {
             int z,
             IrisDimensionCarvingResolver.State resolverState
     ) {
-        BiomeResolution resolution = resolveBiomeResolution(x, y, z, resolverState);
+        return resolveVisibleBiomeHolder(x, y, z, resolverState, null);
+    }
+
+    private Holder<Biome> resolveVisibleBiomeHolder(
+            int x,
+            int y,
+            int z,
+            IrisDimensionCarvingResolver.State resolverState,
+            DimensionStackLayout stackLayout
+    ) {
+        BiomeResolution resolution = resolveBiomeResolution(
+                x, y, z, resolverState, true, stackLayout);
         if (resolution == null) {
             return getFallbackBiome();
         }
 
+        return resolveVisibleBiomeHolder(resolution);
+    }
+
+    private Holder<Biome> resolveVisibleBiomeHolder(BiomeResolution resolution) {
         if (resolution.irisBiome.isCustom()) {
             return resolveCustomHolder(resolution);
         }
@@ -642,7 +858,8 @@ public class CustomBiomeSource extends BiomeSource {
     private Holder<Biome> resolveCustomHolder(BiomeResolution resolution) {
         IrisBiomeCustom customBiome = resolution.irisBiome.getCustomBiome(resolution.rng, engine, resolution.blockX, resolution.blockY, resolution.blockZ);
         if (customBiome != null) {
-            Holder<Biome> holder = customBiomes.get(customBiome.getId());
+            Holder<Biome> holder = customBiomes.get(customBiomeKey(
+                    resolution.dimension, customBiome.getId()));
             if (holder != null) {
                 return holder;
             }
@@ -651,8 +868,8 @@ public class CustomBiomeSource extends BiomeSource {
         return getFallbackBiome();
     }
 
-    private BiomeResolution resolveBiomeResolution(int x, int y, int z) {
+    private BiomeResolution resolveStructureBiomeResolution(int x, int y, int z) {
-        return resolveBiomeResolution(x, y, z, null);
+        return resolveBiomeResolution(x, y, z, null, false);
     }
 
     private BiomeResolution resolveBiomeResolution(
@@ -660,6 +877,28 @@ public class CustomBiomeSource extends BiomeSource {
             int y,
             int z,
             IrisDimensionCarvingResolver.State resolverState
+    ) {
+        return resolveBiomeResolution(x, y, z, resolverState, true);
+    }
+
+    private BiomeResolution resolveBiomeResolution(
+            int x,
+            int y,
+            int z,
+            IrisDimensionCarvingResolver.State resolverState,
+            boolean includeDimensionStack
+    ) {
+        return resolveBiomeResolution(
+                x, y, z, resolverState, includeDimensionStack, null);
+    }
+
+    private BiomeResolution resolveBiomeResolution(
+            int x,
+            int y,
+            int z,
+            IrisDimensionCarvingResolver.State resolverState,
+            boolean includeDimensionStack,
+            DimensionStackLayout stackLayout
     ) {
         if (engine == null || engine.isClosed()) {
             return null;
@@ -675,31 +914,63 @@ public class CustomBiomeSource extends BiomeSource {
         int worldMinHeight = engine.getWorld().minHeight();
         int internalY = blockY - worldMinHeight;
         int caveSwitchInternalY = Math.max(-8 - worldMinHeight, 40);
+        DimensionStackLayout.Layer stackLayer = includeDimensionStack
+                ? stackLayout == null
+                        ? resolveDimensionStackLayer(engine, blockX, internalY, blockZ)
+                        : stackLayout.layerAt(internalY)
+                : null;
+        IrisDimension owningDimension = stackLayer == null
+                ? engine.getDimension()
+                : stackLayer.terrainContext().getDimension();
+        if (stackLayer != null && !stackLayer.terrainContext().isSelfReferencing()) {
+            IrisBiome stackedBiome = stackLayer.biome();
+            return stackedBiome == null
+                    ? null
+                    : createBiomeResolution(
+                            stackedBiome, false, owningDimension, blockX, blockY, blockZ);
+        }
         boolean deepUnderground = internalY <= caveSwitchInternalY;
         boolean underground = false;
         IrisBiome irisBiome;
-        if (deepUnderground) {
+        if (stackLayer != null) {
-            int surfaceInternalY = engine.getComplex().getHeightStream().get(blockX, blockZ).intValue();
+            int surfaceInternalY = stackLayer.surfaceY();
+            underground = internalY <= surfaceInternalY - 8;
+            irisBiome = underground
+                    ? engine.getCaveBiome(
+                            blockX,
+                            internalY,
+                            blockZ,
+                            resolverState,
+                            stackLayer.biome(),
+                            surfaceInternalY
+                    )
+                    : stackLayer.biome();
+        } else if (deepUnderground) {
+            int surfaceInternalY = Engine.hostHeight(engine, blockX, blockZ, true);
             underground = internalY <= surfaceInternalY - 8;
             irisBiome = underground
                     ? engine.getCaveBiome(blockX, internalY, blockZ, resolverState)
-                    : engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+                    : engine.getHostSurfaceBiome(blockX, blockZ);
         } else {
-            irisBiome = engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+            irisBiome = engine.getHostSurfaceBiome(blockX, blockZ);
         }
         if (irisBiome == null && underground) {
-            irisBiome = engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+            irisBiome = stackLayer == null
+                    ? engine.getHostSurfaceBiome(blockX, blockZ)
+                    : stackLayer.biome();
         }
         if (irisBiome == null) {
             return null;
         }
 
-        return createBiomeResolution(irisBiome, underground, blockX, blockY, blockZ);
+        return createBiomeResolution(
+                irisBiome, underground, owningDimension, blockX, blockY, blockZ);
     }
 
     private BiomeResolution createBiomeResolution(
             IrisBiome irisBiome,
             boolean underground,
+            IrisDimension dimension,
             int blockX,
             int blockY,
             int blockZ
@@ -708,7 +979,8 @@ public class CustomBiomeSource extends BiomeSource {
                 ^ (((long) blockX) * 341873128712L)
                 ^ (((long) blockY) * 132897987541L)
                 ^ (((long) blockZ) * 42317861L));
-        return new BiomeResolution(irisBiome, underground, blockX, blockY, blockZ, noiseRng);
+        return new BiomeResolution(
+                irisBiome, underground, dimension, blockX, blockY, blockZ, noiseRng);
     }
 
     private Holder<Biome> getFallbackBiome() {
@@ -734,15 +1006,20 @@ public class CustomBiomeSource extends BiomeSource {
         return ((long) x << 32) ^ ((long) z & 4294967295L);
     }
 
-    private static Holder<Biome> resolveCustomBiomeHolder(Registry<Biome> customRegistry, Engine engine, String customBiomeId) {
+    private static Holder<Biome> resolveCustomBiomeHolder(
-        if (customRegistry == null || engine == null || customBiomeId == null || customBiomeId.isBlank()) {
+            Registry<Biome> customRegistry,
+            IrisDimension dimension,
+            String customBiomeId
+    ) {
+        if (customRegistry == null || dimension == null
+                || customBiomeId == null || customBiomeId.isBlank()) {
             return null;
         }
 
-        Identifier resourceLocation = Identifier.fromNamespaceAndPath(
+        Identifier resourceLocation = Identifier.tryParse(customBiomeKey(dimension, customBiomeId));
-                engine.getDimension().getLoadKey().toLowerCase(java.util.Locale.ROOT),
+        if (resourceLocation == null) {
-                customBiomeId.toLowerCase(java.util.Locale.ROOT)
+            return null;
-        );
+        }
         Biome biome = customRegistry.getValue(resourceLocation);
         if (biome == null) {
             return null;
@@ -761,6 +1038,63 @@ public class CustomBiomeSource extends BiomeSource {
         return optionalReferenceHolder.get();
     }
 
+    static String customBiomeKey(IrisDimension dimension, String customBiomeId) {
+        return dimension.getCustomBiomeKey(customBiomeId);
+    }
+
+    private static List<OwnedBiome> getOwnedBiomes(Engine engine) {
+        ArrayList<OwnedBiome> biomes = new ArrayList<>(getHostOwnedBiomes(engine));
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        if (stackContext == null) {
+            return biomes;
+        }
+        for (DimensionTerrainContext terrainContext : stackContext.getLayersBottomToTop()) {
+            if (terrainContext.isSelfReferencing()) {
+                continue;
+            }
+            IrisDimension dimension = terrainContext.getDimension();
+            addOwnedBiomes(
+                    biomes,
+                    dimension,
+                    dimension.getReachableBiomes(terrainContext)
+            );
+        }
+        return biomes;
+    }
+
+    private static List<OwnedBiome> getHostOwnedBiomes(Engine engine) {
+        IrisDimension hostDimension = engine.getDimension();
+        ArrayList<OwnedBiome> biomes = new ArrayList<>();
+        addOwnedBiomes(biomes, hostDimension, hostDimension.getReachableBiomes(engine));
+        return biomes;
+    }
+
+    private static void addOwnedBiomes(
+            List<OwnedBiome> target,
+            IrisDimension dimension,
+            Iterable<IrisBiome> biomes
+    ) {
+        for (IrisBiome biome : biomes) {
+            if (biome != null) {
+                target.add(new OwnedBiome(dimension, biome));
+            }
+        }
+    }
+
+    private static DimensionStackLayout.Layer resolveDimensionStackLayer(
+            Engine engine,
+            int blockX,
+            int internalY,
+            int blockZ
+    ) {
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        return stackContext == null ? null : stackContext.getLayerAt(blockX, internalY, blockZ);
+    }
+
+    private static IrisBiome resolveSurfaceStructureBiome(Engine engine, int blockX, int blockZ) {
+        return engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+    }
+
     private static Holder<Biome> resolveBiomeHolder(Registry<Biome> registry, String biomeKey) {
         if (registry == null || biomeKey == null || biomeKey.isBlank()) {
             return null;
@@ -809,18 +1143,31 @@ public class CustomBiomeSource extends BiomeSource {
     private static final class BiomeResolution {
         private final IrisBiome irisBiome;
         private final boolean underground;
+        private final IrisDimension dimension;
         private final int blockX;
         private final int blockY;
         private final int blockZ;
         private final RNG rng;
 
-        private BiomeResolution(IrisBiome irisBiome, boolean underground, int blockX, int blockY, int blockZ, RNG rng) {
+        private BiomeResolution(
+                IrisBiome irisBiome,
+                boolean underground,
+                IrisDimension dimension,
+                int blockX,
+                int blockY,
+                int blockZ,
+                RNG rng
+        ) {
             this.irisBiome = irisBiome;
             this.underground = underground;
+            this.dimension = dimension;
             this.blockX = blockX;
             this.blockY = blockY;
             this.blockZ = blockZ;
             this.rng = rng;
         }
     }
+
+    private record OwnedBiome(IrisDimension dimension, IrisBiome biome) {
+    }
 }

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/ImportedFeatureStage.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.core.nms.v26_2_R1;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.NativeFeatureGenerationPolicy;
 import art.arcane.iris.engine.object.IrisBiome;
@@ -40,6 +42,9 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.IntBinaryOperator;
+import java.util.function.Predicate;
 
 /**
  * Bukkit twin of the modded imported-feature stage. Same control, same semantics, same seeds: with
@@ -198,8 +203,7 @@ final class ImportedFeatureStage {
      */
     private Set<String> visibleBiomeKeys() {
         Set<String> keys = new LinkedHashSet<>();
-        String namespace = engine.getDimension().getLoadKey().toLowerCase(Locale.ROOT);
+        for (IrisBiome irisBiome : engine.getDimension().getReachableBiomes(engine)) {
-        for (IrisBiome irisBiome : engine.getAllBiomes()) {
             addKey(keys, irisBiome.getStructureDerivativeKey());
             addKey(keys, irisBiome.getDerivativeKey());
             addKey(keys, irisBiome.getVanillaDerivativeKey());
@@ -213,7 +217,7 @@ final class ImportedFeatureStage {
                 continue;
             }
             for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
-                addKey(keys, namespace + ":" + customBiome.getId());
+                addKey(keys, engine.getDimension().getCustomBiomeKey(customBiome.getId()));
             }
         }
         return keys;
@@ -226,8 +230,7 @@ final class ImportedFeatureStage {
     private Map<String, Holder<Biome>> customBiomeDerivatives(Registry<Biome> registry,
                                                               Map<String, Holder<Biome>> byKey) {
         Map<String, Holder<Biome>> derivatives = new HashMap<>();
-        String namespace = engine.getDimension().getLoadKey().toLowerCase(Locale.ROOT);
+        for (IrisBiome irisBiome : engine.getDimension().getReachableBiomes(engine)) {
-        for (IrisBiome irisBiome : engine.getAllBiomes()) {
             if (!irisBiome.isCustom()) {
                 continue;
             }
@@ -246,7 +249,7 @@ final class ImportedFeatureStage {
                 continue;
             }
             for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
-                derivatives.put(namespace + ":" + customBiome.getId().toLowerCase(Locale.ROOT), derivative);
+                derivatives.put(engine.getDimension().getCustomBiomeKey(customBiome.getId()), derivative);
             }
         }
         return derivatives;
@@ -288,14 +291,24 @@ final class ImportedFeatureStage {
         List<FeatureSorter.StepFeatureData> steps = table.steps();
         WorldgenRandom random = new WorldgenRandom(new XoroshiroRandomSource(RandomSupport.generateUniqueSeed()));
         long decorationSeed = random.setDecorationSeed(level.getSeed(), origin.getX(), origin.getZ());
-        Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table);
         IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
         int staticMinY = engine.getMinHeight();
-        WorldGenLevel placementLevel = staticObjects.isEmpty() ? level : NativeStructureWorldgenAccess.create(
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table, stackContext);
+        IntBinaryOperator surfaceFirstFreeY = stackContext == null
+                ? (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z)
+                : (x, z) -> Engine.hostHeight(engine, x, z, false) + staticMinY + 1;
+        IntBinaryOperator floorFirstFreeY = stackContext == null
+                ? (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z)
+                : (x, z) -> Engine.hostHeight(engine, x, z, true) + staticMinY + 1;
+        WorldGenLevel placementLevel = staticObjects.isEmpty() && stackContext == null
+                ? level
+                : NativeStructureWorldgenAccess.create(
                 level, centerPos,
-                (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z),
+                surfaceFirstFreeY,
-                (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z),
+                floorFirstFreeY,
-                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
+                stackContext != null,
+                importedFeatureProtection(staticObjects, stackContext, staticMinY));
 
         try {
             for (int stepIndex = 0; stepIndex < steps.size(); stepIndex++) {
@@ -313,6 +326,30 @@ final class ImportedFeatureStage {
         }
     }
 
+    private static Predicate<BlockPos> importedFeatureProtection(
+            IrisStaticObjectLayer staticObjects,
+            DimensionStackContext stackContext,
+            int minimumY
+    ) {
+        Map<Long, DimensionStackLayout> layouts = new ConcurrentHashMap<>();
+        return position -> {
+            if (!staticObjects.isEmpty() && staticObjects.contains(
+                    position.getX(), position.getY() - minimumY, position.getZ())) {
+                return true;
+            }
+            if (stackContext == null) {
+                return false;
+            }
+            long columnKey = ((long) position.getX() << 32)
+                    ^ (position.getZ() & 0xFFFFFFFFL);
+            DimensionStackLayout layout = layouts.computeIfAbsent(
+                    columnKey,
+                    ignored -> stackContext.sample(position.getX(), position.getZ())
+            );
+            return layout.isHostFeatureProtectedY(position.getY() - minimumY);
+        };
+    }
+
     private void placeStep(WorldGenLevel level, FeatureTable table, FeatureSorter.StepFeatureData stepData,
                            Registry<PlacedFeature> featureRegistry, Set<Holder<Biome>> chunkBiomes,
                            ChunkGenerator owner, WorldgenRandom random, long decorationSeed,
@@ -353,12 +390,37 @@ final class ImportedFeatureStage {
         return id == null ? feature.toString() : id.toString();
     }
 
-    private Set<Holder<Biome>> chunkBiomes(WorldGenLevel level, SectionPos sectionPos, FeatureTable table) {
+    private Set<Holder<Biome>> chunkBiomes(
+            WorldGenLevel level,
+            SectionPos sectionPos,
+            FeatureTable table,
+            DimensionStackContext stackContext
+    ) {
         List<Holder<Biome>> collected = new ArrayList<>();
         ChunkPos.rangeClosed(sectionPos.chunk(), 1).forEach((ChunkPos chunkPos) -> {
             ChunkAccess neighbour = level.getChunk(chunkPos.x(), chunkPos.z());
-            for (LevelChunkSection section : neighbour.getSections()) {
+            LevelChunkSection[] sections = neighbour.getSections();
-                section.getBiomes().getAll(collected::add);
+            for (int sectionIndex = 0; sectionIndex < sections.length; sectionIndex++) {
+                LevelChunkSection section = sections[sectionIndex];
+                if (stackContext == null) {
+                    section.getBiomes().getAll(collected::add);
+                    continue;
+                }
+                int sectionMinimumY = neighbour.getSectionYFromSectionIndex(sectionIndex) << 4;
+                for (int quartY = 0; quartY < 4; quartY++) {
+                    int internalY = sectionMinimumY + (quartY << 2) - level.getMinY();
+                    for (int quartX = 0; quartX < 4; quartX++) {
+                        int blockX = chunkPos.getMinBlockX() + (quartX << 2);
+                        for (int quartZ = 0; quartZ < 4; quartZ++) {
+                            int blockZ = chunkPos.getMinBlockZ() + (quartZ << 2);
+                            DimensionStackLayout.Layer layer = stackContext.getLayerAt(
+                                    blockX, internalY, blockZ);
+                            if (layer.terrainContext().isSelfReferencing()) {
+                                collected.add(section.getBiomes().get(quartX, quartY, quartZ));
+                            }
+                        }
+                    }
+                }
             }
         });
         Set<Holder<Biome>> present = new LinkedHashSet<>();

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/IrisChunkGenerator.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.core.nms.v26_2_R1;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.GenerationSessionException;
 import art.arcane.iris.engine.framework.GenerationSessionLease;
@@ -115,6 +117,7 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CompletionException;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.IntBinaryOperator;
+import java.util.function.Predicate;
 
 public class IrisChunkGenerator extends CustomChunkGenerator {
     private static final WrappedField<ChunkGenerator, BiomeSource> BIOME_SOURCE;
@@ -429,7 +432,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
                         undergroundStep,
                         decision.preserveSourceY(),
                         decision.yBand(),
-                        (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight());
+                        (x, z) -> Engine.hostHeight(engine, x, z, true) + engine.getMinHeight());
                 StructureStart wrapped = NativeStructureReferenceEnvelope.wrapForPublication(
                         start, structure, start.getReferences(),
                         NativeStructureTerrainIntegrator.resolveNativeTerrain(start, decision.terrain()),
@@ -726,6 +729,14 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         return (x, z) -> engine.getHeight(x, z, true) + runtimeMinY + 1;
     }
 
+    private IntBinaryOperator hostWorldgenSurfaceHeight() {
+        return (x, z) -> Engine.hostHeight(engine, x, z, false) + runtimeMinY + 1;
+    }
+
+    private IntBinaryOperator hostWorldgenFloorHeight() {
+        return (x, z) -> Engine.hostHeight(engine, x, z, true) + runtimeMinY + 1;
+    }
+
     @Override
     public WeightedList<MobSpawnSettings.SpawnerData> getMobsAt(Holder<Biome> holder, StructureManager structuremanager, MobCategory enumcreaturetype, BlockPos blockposition) {
         Holder<Biome> vanillaSpawnBiome = customBiomeSource.getVanillaSpawnBiome(holder);
@@ -868,17 +879,22 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         }
         try {
             WorldgenTerrainHeightmaps.primeStructurePlacement(
-                    world, chunkPos, heightmapStarts, worldgenSurfaceHeight(), worldgenFloorHeight());
+                    world, chunkPos, heightmapStarts,
+                    hostWorldgenSurfaceHeight(), hostWorldgenFloorHeight());
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "heightmap priming", nativeStructureBatchContext(placementGroups),
                     chunkPos.x(), chunkPos.z(), error);
         }
         IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
-        int staticMinY = engine.getMinHeight();
+        Predicate<BlockPos> protectedPosition = nativeStructureProtection(staticObjects);
-        WorldGenLevel boundedWorld = staticObjects.isEmpty() ? world : NativeStructureWorldgenAccess.create(
+        WorldGenLevel boundedWorld = staticObjects.isEmpty()
-                world, chunkPos, worldgenSurfaceHeight(), worldgenFloorHeight(),
+                && engine.getDimensionStackContext() == null
-                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
+                ? world
+                : NativeStructureWorldgenAccess.create(
+                world, chunkPos, hostWorldgenSurfaceHeight(), hostWorldgenFloorHeight(),
+                engine.getDimensionStackContext() != null,
+                protectedPosition);
         try {
             NativeStructureVegetationClearer.clearIntersectingVegetation(
                     boundedWorld, chunk, area, vegetationTargets);
@@ -891,7 +907,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
         try {
             vacuumFoundationPlan = NativeStructureSurfaceFitter.prepareSurfaceStructures(
                     boundedWorld, area, terrainTargets,
-                    (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight());
+                    (x, z) -> Engine.hostHeight(engine, x, z, true) + engine.getMinHeight());
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "terrain integration", nativeStructureBatchContext(placementGroups),
@@ -945,17 +961,45 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
                                        BoundingBox area, ChunkPos chunkPos, String structureId, StructureStart start,
                                        IrisNativeStructureDecision decision) {
         WorldGenLevel boundedWorld = world instanceof NativeStructureWorldgenAccess ? world : NativeStructureWorldgenAccess.create(
-                world, chunkPos, worldgenSurfaceHeight(), worldgenFloorHeight(), position -> false);
+                world,
+                chunkPos,
+                hostWorldgenSurfaceHeight(),
+                hostWorldgenFloorHeight(),
+                engine.getDimensionStackContext() != null,
+                nativeStructureProtection(
+                        engine.getDimension().getStaticObjectLayer(engine.getData())));
         world.setCurrentlyGenerating(() -> "Iris native structure " + structureId);
         try {
             NativeStructurePostProcessor.place(boundedWorld, structureManager, this, random, area, chunkPos,
                     structureId, start, decision, this::resolvePaletteBlock,
-                    (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight());
+                    (x, z) -> Engine.hostHeight(engine, x, z, true) + engine.getMinHeight());
         } finally {
             world.setCurrentlyGenerating(null);
         }
     }
 
+    private Predicate<BlockPos> nativeStructureProtection(IrisStaticObjectLayer staticObjects) {
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        int minimumY = engine.getMinHeight();
+        Map<Long, DimensionStackLayout> layouts = new ConcurrentHashMap<>();
+        return position -> {
+            if (!staticObjects.isEmpty() && staticObjects.contains(
+                    position.getX(), position.getY() - minimumY, position.getZ())) {
+                return true;
+            }
+            if (stackContext == null) {
+                return false;
+            }
+            long columnKey = ((long) position.getX() << 32)
+                    ^ (position.getZ() & 0xFFFFFFFFL);
+            DimensionStackLayout layout = layouts.computeIfAbsent(
+                    columnKey,
+                    ignored -> stackContext.sample(position.getX(), position.getZ())
+            );
+            return layout.isHostFeatureProtectedY(position.getY() - minimumY);
+        };
+    }
+
     private List<List<Structure>> structuresByStep(Registry<Structure> registry) {
         StructureStepCache cached = structureStepCache;
         if (cached != null && cached.registry() == registry) {
@@ -1041,7 +1085,17 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
     @Override
     public void spawnOriginalMobs(WorldGenRegion region) {
         ChunkPos center = region.getCenter();
-        Holder<Biome> visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+        Holder<Biome> visibleBiome;
+        if (engine.getDimensionStackContext() == null) {
+            visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+        } else {
+            visibleBiome = customBiomeSource.getVisibleSurfaceBiome(
+                    center.getMinBlockX() + 8,
+                    center.getMinBlockZ() + 8);
+            if (visibleBiome == null) {
+                visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+            }
+        }
         Holder<Biome> vanillaBiome = customBiomeSource.getVanillaSpawnBiome(visibleBiome);
         WorldgenRandom random = new WorldgenRandom(new LegacyRandomSource(RandomSupport.generateUniqueSeed()));
         random.setDecorationSeed(region.getSeed(), center.getMinBlockX(), center.getMinBlockZ());
@@ -1081,9 +1135,12 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
     public int getBaseHeight(int i, int j, Heightmap.Types heightmap_type, LevelHeightAccessor levelheightaccessor, RandomState randomstate) {
         try (GenerationSessionLease lease = engine.acquireGenerationLease("bukkit_nms_base_height");
              IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
+            boolean ignoreFluid = !heightmap_type.isOpaque().test(Blocks.WATER.defaultBlockState());
+            int height = engine.getDimensionStackContext() == null
+                    ? engine.getHeight(i, j, ignoreFluid)
+                    : Engine.hostHeight(engine, i, j, ignoreFluid);
             return levelheightaccessor.getMinY()
-                    + engine.getHeight(i, j, !heightmap_type.isOpaque().test(Blocks.WATER.defaultBlockState()))
+                    + height + 1;
-                    + 1;
         } catch (GenerationSessionException e) {
             throw new IllegalStateException("Iris base height query could not acquire its engine runtime.", e);
         }
@@ -1093,8 +1150,13 @@ public class IrisChunkGenerator extends CustomChunkGenerator {
     public NoiseColumn getBaseColumn(int i, int j, LevelHeightAccessor levelheightaccessor, RandomState randomstate) {
         try (GenerationSessionLease lease = engine.acquireGenerationLease("bukkit_nms_base_column");
              IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
-            int block = engine.getHeight(i, j, true);
+            boolean dimensionStack = engine.getDimensionStackContext() != null;
-            int water = engine.getHeight(i, j, false);
+            int block = dimensionStack
+                    ? Engine.hostHeight(engine, i, j, true)
+                    : engine.getHeight(i, j, true);
+            int water = dimensionStack
+                    ? Engine.hostHeight(engine, i, j, false)
+                    : engine.getHeight(i, j, false);
             BlockState[] column = new BlockState[levelheightaccessor.getHeight()];
             for (int k = 0; k < column.length; k++) {
                 if (k <= block) {

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/NMSBinding.java

@@ -824,6 +824,11 @@ public class NMSBinding implements INMSBinding {
         return NativeStructureVolumeIndex.volumes(engine, minX, minZ, maxX, maxZ);
     }
 
+    @Override
+    public void invalidateNativeStructureVolumeIndex(Engine engine) {
+        NativeStructureVolumeIndex.invalidate(engine);
+    }
+
     @Override
     public int getBiomeId(Biome biome) {
         for (World i : Bukkit.getWorlds()) {

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/NativeStructureWorldgenAccess.java

@@ -64,6 +64,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
     private final ChunkPos generationCenter;
     private final IntBinaryOperator surfaceFirstFreeY;
     private final IntBinaryOperator floorFirstFreeY;
+    private final boolean virtualizeHeights;
     private final Predicate<BlockPos> protectedPosition;
     private final Holder<Biome> fallbackBiome;
     private final BiomeManager biomeManager;
@@ -81,6 +82,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
                 boundary.surfaceFirstFreeY(), "Native structure world access requires a surface resolver");
         this.floorFirstFreeY = Objects.requireNonNull(
                 boundary.floorFirstFreeY(), "Native structure world access requires an ocean-floor resolver");
+        this.virtualizeHeights = boundary.virtualizeHeights();
         this.protectedPosition = Objects.requireNonNull(
                 boundary.protectedPosition(), "World access requires a protected-position predicate");
         BlockPos biomeSample = generationCenter.getMiddleBlockPosition(delegate.getSeaLevel());
@@ -103,9 +105,10 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
     static NativeStructureWorldgenAccess create(WorldGenLevel delegate, ChunkPos generationCenter,
                                                 IntBinaryOperator surfaceFirstFreeY,
                                                 IntBinaryOperator floorFirstFreeY,
+                                                boolean virtualizeHeights,
                                                 Predicate<BlockPos> protectedPosition) {
         return new NativeStructureWorldgenAccess(delegate, new Boundary(
-                generationCenter, surfaceFirstFreeY, floorFirstFreeY, protectedPosition));
+                generationCenter, surfaceFirstFreeY, floorFirstFreeY, virtualizeHeights, protectedPosition));
     }
 
     @Override
@@ -250,7 +253,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
 
     @Override
     public int getHeight(Heightmap.Types type, int x, int z) {
-        if (isInsideGenerationRegion(x >> 4, z >> 4)) {
+        if (!virtualizeHeights && isInsideGenerationRegion(x >> 4, z >> 4)) {
             return delegate.getHeight(type, x, z);
         }
         return height(type, x, z);
@@ -549,6 +552,7 @@ final class NativeStructureWorldgenAccess implements WorldGenLevel {
     private record Boundary(ChunkPos generationCenter,
                             IntBinaryOperator surfaceFirstFreeY,
                             IntBinaryOperator floorFirstFreeY,
+                            boolean virtualizeHeights,
                             Predicate<BlockPos> protectedPosition) {
     }
 

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/core/nms/v26_2_R1/CustomBiomeSourceStructureContractTest.java

@@ -1,15 +1,29 @@
 package art.arcane.iris.core.nms.v26_2_R1;
 
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.object.IrisDimension;
+import net.minecraft.SharedConstants;
+import net.minecraft.server.Bootstrap;
+import org.junit.BeforeClass;
 import org.junit.Test;
 
 import java.io.IOException;
+import java.lang.reflect.Proxy;
 import java.nio.file.Files;
 import java.nio.file.Path;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertTrue;
 
 public class CustomBiomeSourceStructureContractTest {
+    @BeforeClass
+    public static void bootstrapMinecraftRegistries() {
+        SharedConstants.tryDetectVersion();
+        Bootstrap.bootStrap();
+    }
+
     @Test
     public void nativeStructuresUseTerrainSafeDerivativeAtEveryBiomeBoundary() throws IOException {
         String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
@@ -69,14 +83,53 @@ public class CustomBiomeSourceStructureContractTest {
         String realResolution = source.substring(realResolutionStart, naturalResolutionStart);
         String naturalResolution = source.substring(naturalResolutionStart, resolutionEnd);
 
-        assertTrue(realResolution.contains("engine.getComplex().getTrueBiomeStream().get(blockX, blockZ)"));
+        assertTrue(realResolution.contains("resolveSurfaceStructureBiome(engine, blockX, blockZ)"));
         assertFalse(realResolution.contains("getNaturalTrueBiomeStream()"));
         assertTrue(naturalResolution.contains(
                 "engine.getComplex().getNaturalTrueBiomeStream().get(blockX, blockZ)"));
         assertFalse(naturalResolution.contains("getTrueBiomeStream()"));
+        int helperStart = source.indexOf("private static IrisBiome resolveSurfaceStructureBiome(");
+        int helperEnd = source.indexOf("private static Holder<Biome> resolveBiomeHolder(", helperStart);
+        String helper = source.substring(helperStart, helperEnd);
+        assertTrue(helper.contains("engine.getComplex().getTrueBiomeStream().get(blockX, blockZ)"));
+        assertFalse(helper.contains("getDimensionStackContext()"));
+        assertFalse(helper.contains("getSurfaceBiome("));
         assertFalse(source.contains("studioBootstrapSurfaceStructureBiomeCache"));
     }
 
+    @Test
+    public void stackedCustomBiomesUseTheirOwningDimensionNamespace() {
+        IrisDimension host = new IrisDimension();
+        host.setLoadKey("Host");
+        IrisDimension upper = new IrisDimension();
+        upper.setLoadKey("Layers/Upper");
+
+        assertEquals("layers:upper/aurora", CustomBiomeSource.customBiomeKey(upper, "Aurora"));
+        assertNotEquals(
+                CustomBiomeSource.customBiomeKey(host, "Aurora"),
+                CustomBiomeSource.customBiomeKey(upper, "Aurora")
+        );
+    }
+
+    @Test
+    public void visibleBiomeResolutionSelectsStackOwnershipBeforeHostCaves() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
+        int resolutionStart = source.indexOf("private BiomeResolution resolveBiomeResolution(");
+        int resolutionEnd = source.indexOf("private BiomeResolution createBiomeResolution(", resolutionStart);
+
+        assertTrue(resolutionStart >= 0);
+        assertTrue(resolutionEnd > resolutionStart);
+        String resolution = source.substring(resolutionStart, resolutionEnd);
+
+        assertTrue(resolution.contains("resolveDimensionStackLayer("));
+        assertTrue(resolution.contains("!stackLayer.terrainContext().isSelfReferencing()"));
+        assertTrue(resolution.indexOf("resolveDimensionStackLayer(")
+                < resolution.indexOf("boolean deepUnderground"));
+        assertTrue(resolution.contains("includeDimensionStack"));
+        assertTrue(source.contains("resolveBiomeResolution(x, y, z, null, false)"));
+        assertTrue(source.contains("resolution.dimension, customBiome.getId()"));
+    }
+
     @Test
     public void visibleBiomeBatchReusesTheCallersGenerationLease() throws IOException {
         String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
@@ -94,4 +147,126 @@ public class CustomBiomeSourceStructureContractTest {
                 < lookup.indexOf("getVisibleNoiseBiomeWithActiveGenerationLease(x, y, z, sampler)"));
         assertFalse(source.contains("studioBootstrapNoiseBiomeCache"));
     }
+
+    @Test
+    public void stackBiomesAreVisibleButExcludedFromStructureReachability() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
+
+        assertTrue(source.contains("return possibleBiomes(true).stream()"));
+        assertTrue(source.contains("public Set<Holder<Biome>> possibleBiomes()"));
+        assertTrue(source.contains("irisBiome.getDerivativeKey()"));
+        assertTrue(source.contains("irisBiome.getBiomeScatter()"));
+        assertTrue(source.contains("irisBiome.getBiomeSkyScatter()"));
+        assertTrue(source.contains("return possibleBiomes(false)"));
+        assertTrue(source.contains("for (OwnedBiome ownedBiome : getHostOwnedBiomes(engine))"));
+        assertTrue(source.contains("for (OwnedBiome ownedBiome : getOwnedBiomes(engine))"));
+    }
+
+    @Test
+    public void biomeLocatorSearchesTheVisibleStack() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
+        int start = source.indexOf("public Pair<BlockPos, Holder<Biome>> findClosestBiome3d(");
+        int end = source.indexOf("static int horizontalBiomeSearchQuartStep", start);
+        String method = source.substring(start, end);
+
+        assertTrue(method.contains("bukkit_locate_visible_biome"));
+        assertTrue(method.contains("return super.findClosestBiome3d("));
+        assertTrue(method.contains("getVisibleBiomes("));
+        assertTrue(method.contains("QuartPos.toBlock(quartX), QuartPos.toBlock(quartZ)"));
+        assertTrue(method.contains("resolveVisibleBiomeHolder("));
+        assertFalse(method.contains("getStructureNoiseBiomeWithActiveGenerationLease("));
+        assertTrue(method.indexOf("tryAcquireGenerationLease(\"bukkit_locate_visible_biome\")")
+                < method.indexOf("engine.getDimensionStackContext()"));
+    }
+
+    @Test
+    public void surfaceSpawnBiomeUsesTheVisibleQuartCell() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
+        int start = source.indexOf("Holder<Biome> getVisibleSurfaceBiome(");
+        int end = source.indexOf("private RegistryAccess registry()", start);
+        String method = source.substring(start, end);
+
+        assertTrue(method.contains("QuartPos.fromBlock(blockX)"));
+        assertTrue(method.contains("QuartPos.toBlock(quartX)"));
+        assertTrue(method.contains("QuartPos.fromBlock(internalY + engine.getWorld().minHeight())"));
+        assertTrue(method.contains("stackContext.getLayout(sampleX, sampleZ).surfaceLayer()"));
+        assertTrue(method.contains("getVisibleNoiseBiomeWithActiveGenerationLease("));
+        assertTrue(method.contains("quartX, quartY, quartZ, null, null, cacheable"));
+        assertTrue(method.indexOf("boolean cacheable = isBiomeCacheable(engine, sampleX, sampleZ)")
+                < method.indexOf("Engine.hostHeight(engine, sampleX, sampleZ, true)"));
+    }
+
+    @Test
+    public void temporaryNaturalBiomeAnswersAreNotMemoized() throws IOException {
+        Engine temporary = engineAnsweringNaturalFallback(true, 12, -8);
+        Engine stable = engineAnsweringNaturalFallback(false, 12, -8);
+        assertFalse(CustomBiomeSource.isBiomeCacheable(temporary, 12, -8));
+        assertTrue(CustomBiomeSource.isBiomeCacheable(stable, 12, -8));
+
+        String source = Files.readString(Path.of(System.getProperty("iris.customBiomeSource")));
+        int structureStart = source.indexOf(
+                "private Holder<Biome> getStructureNoiseBiomeWithActiveGenerationLease(");
+        int structureEnd = source.indexOf("@Override\n    public Pair<BlockPos, Holder<Biome>> findBiomeHorizontal(",
+                structureStart);
+        assertCacheGuarded(source.substring(structureStart, structureEnd), "structureBiomeCache");
+
+        int firstVisible = source.indexOf(
+                "Holder<Biome> getVisibleNoiseBiomeWithActiveGenerationLease(");
+        int visibleStart = source.indexOf(
+                "Holder<Biome> getVisibleNoiseBiomeWithActiveGenerationLease(", firstVisible + 1);
+        int visibleEnd = source.indexOf("private boolean isBiomeCacheable(", visibleStart);
+        assertCacheGuarded(source.substring(visibleStart, visibleEnd), "noiseBiomeCache");
+
+        String predicate = source.substring(
+                visibleEnd,
+                source.indexOf("private GenerationSessionLease tryAcquireGenerationLease(", visibleEnd)
+        );
+        assertTrue(predicate.contains("QuartPos.toBlock(quartX)"));
+        assertTrue(predicate.contains("QuartPos.toBlock(quartZ)"));
+        assertTrue(predicate.contains("return !engine.answersFromNaturalTerrain(blockX, blockZ)"));
+        assertFalse(predicate.contains("getDimensionStackContext()"));
+    }
+
+    @Test
+    public void importedFeaturesCollectOnlyHostOwnedStackCells() throws IOException {
+        Path biomeSource = Path.of(System.getProperty("iris.customBiomeSource"));
+        String source = Files.readString(biomeSource.getParent().resolve("ImportedFeatureStage.java"));
+
+        assertTrue(source.contains("stackContext.getLayerAt("));
+        assertTrue(source.contains("layer.terrainContext().isSelfReferencing()"));
+        assertTrue(source.contains("section.getBiomes().get(quartX, quartY, quartZ)"));
+    }
+
+    private static void assertCacheGuarded(String method, String cacheName) {
+        int predicate = method.indexOf("boolean cacheable = isBiomeCacheable(");
+        int read = method.indexOf(cacheName + ".get(", predicate);
+        int readGuard = method.lastIndexOf("if (cacheable)", read);
+        int bypass = method.indexOf("if (!cacheable)", read);
+        int write = method.indexOf(cacheName + ".putIfAbsent(", bypass);
+
+        assertTrue(predicate >= 0);
+        assertTrue(read > predicate);
+        assertTrue(readGuard >= predicate);
+        assertTrue(bypass > read);
+        assertTrue(write > bypass);
+    }
+
+    private static Engine engineAnsweringNaturalFallback(
+            boolean naturalFallback,
+            int expectedX,
+            int expectedZ
+    ) {
+        return (Engine) Proxy.newProxyInstance(
+                Engine.class.getClassLoader(),
+                new Class<?>[]{Engine.class},
+                (proxy, method, arguments) -> {
+                    if (!method.getName().equals("answersFromNaturalTerrain")) {
+                        throw new UnsupportedOperationException(method.getName());
+                    }
+                    assertEquals(Integer.valueOf(expectedX), arguments[0]);
+                    assertEquals(Integer.valueOf(expectedZ), arguments[1]);
+                    return naturalFallback;
+                }
+        );
+    }
 }

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/core/nms/v26_2_R1/IrisChunkGeneratorFailureContractTest.java

@@ -78,6 +78,8 @@ public class IrisChunkGeneratorFailureContractTest {
                 < placement.indexOf("for (NativePlacementGroup group"));
         assertTrue(placement.indexOf("repairVacuumFoundations")
                 > placement.indexOf("for (NativePlacementGroup group"));
+        assertTrue(placement.contains("nativeStructureProtection(staticObjects)"));
+        assertTrue(placement.contains("Engine.hostHeight(engine, x, z, true)"));
         assertFalse(placement.contains("IrisLogging.reportError"));
     }
 
@@ -542,8 +544,8 @@ public class IrisChunkGeneratorFailureContractTest {
         String spawn = source.substring(spawnStart, spawnEnd);
 
         assertTrue(spawn.contains("customBiomeSource.getVanillaSpawnBiome(visibleBiome)"));
+        assertTrue(spawn.contains("customBiomeSource.getVisibleSurfaceBiome("));
         assertTrue(spawn.contains("NaturalSpawner.spawnMobsForChunkGeneration("));
-        assertTrue(spawn.contains("region.getBiome(center.getWorldPosition().atY(region.getMaxY()))"));
         assertTrue(spawn.contains("new LegacyRandomSource(RandomSupport.generateUniqueSeed())"));
         assertTrue(spawn.contains("random.setDecorationSeed(region.getSeed()"));
         assertFalse(spawn.contains("delegate.spawnOriginalMobs"));

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/core/nms/v26_2_R1/NativeStructureWorldgenAccessTest.java

@@ -135,6 +135,24 @@ public class NativeStructureWorldgenAccessTest {
         assertEquals(1, recording.blockTicks.count());
     }
 
+    @Test
+    public void virtualizedHeightsHideInRegionTerrainFromTheDelegate() {
+        RecordingDelegate recording = new RecordingDelegate();
+        ChunkPos generationCenter = generationCenter();
+        NativeStructureWorldgenAccess access = NativeStructureWorldgenAccess.create(
+                recording.world(), generationCenter,
+                (x, z) -> SURFACE_FIRST_FREE_Y,
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                true,
+                position -> false);
+        int x = generationCenter.getMiddleBlockX();
+        int z = generationCenter.getMiddleBlockZ();
+
+        assertEquals(SURFACE_FIRST_FREE_Y, access.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z));
+        assertEquals(FLOOR_FIRST_FREE_Y, access.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z));
+        assertEquals(0, recording.heightReads);
+    }
+
     @Test
     public void protectedCellsRemainReadableButRejectWritesTilesAndTicks() {
         RecordingDelegate recording = new RecordingDelegate();
@@ -142,7 +160,9 @@ public class NativeStructureWorldgenAccessTest {
         NativeStructureWorldgenAccess access = NativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y, position::equals);
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                false,
+                position::equals);
 
         assertSame(Blocks.DIRT, access.getBlockState(position).getBlock());
         assertFalse(access.ensureCanWrite(position));
@@ -211,7 +231,9 @@ public class NativeStructureWorldgenAccessTest {
         return NativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y, position -> false);
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                false,
+                position -> false);
     }
 
     private static ChunkPos generationCenter() {

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/nativegen/NativeStructureFactoryTest.java

@@ -332,6 +332,18 @@ public class NativeStructureFactoryTest {
         assertTrue(failureStart < source.indexOf("NativeStructureOwnershipStore.record("));
     }
 
+    @Test
+    public void manualNativeStructureGenerationUsesTheRootTerrainHeight() throws Exception {
+        String source = Files.readString(
+                Path.of(System.getProperty("iris.nativeStructureStartInjectorSource")),
+                StandardCharsets.UTF_8);
+
+        assertTrue(source.contains(
+                "Engine.hostHeight(context.engine(), x, z, true)"));
+        assertFalse(source.contains(
+                "context.engine().getHeight(x, z, true)"));
+    }
+
     @Test
     public void failedManualStartPublicationClearsRecordedOwnershipAndPreservesTheFailure() throws Exception {
         String source = Files.readString(

adapters/bukkit/plugin/src/main/java/art/arcane/iris/core/runtime/BukkitEnginePlatformHooks.java

@@ -84,6 +84,11 @@ public final class BukkitEnginePlatformHooks implements EnginePlatformHooks {
                 "iris");
     }
 
+    @Override
+    public void prepareRuntimeHotload(Engine engine) {
+        INMS.get().invalidateNativeStructureVolumeIndex(engine);
+    }
+
     @Override
     public void applyWorldBoundary(Engine engine) {
         IrisDimension expectedDimension = engine.getDimension();

adapters/minecraft-common/src/main/java/art/arcane/iris/nativegen/NativeStructureOwnershipRecovery.java

@@ -163,7 +163,7 @@ public final class NativeStructureOwnershipRecovery {
                         level,
                         biome -> true,
                         generator.getSeaLevel(),
-                        (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight()
+                        (x, z) -> Engine.hostHeight(engine, x, z, true) + engine.getMinHeight()
                 );
         return NativeStructureFactory.generate(generationContext, holder, plan, references);
     }

adapters/minecraft-common/src/main/java/art/arcane/iris/nativegen/NativeStructureStartInjector.java

@@ -90,7 +90,7 @@ public final class NativeStructureStartInjector {
                             chunk,
                             biome -> true,
                             context.generator().getSeaLevel(),
-                            (x, z) -> context.engine().getHeight(x, z, true)
+                            (x, z) -> Engine.hostHeight(context.engine(), x, z, true)
                                     + context.engine().getMinHeight()
                     );
             StructureStart generated = NativeStructureFactory.generate(

adapters/minecraft-common/src/main/java/art/arcane/iris/nativegen/NativeStructureVolumeIndex.java

@@ -89,6 +89,18 @@ public final class NativeStructureVolumeIndex {
         }
     }
 
+    public static void invalidate(Engine engine) {
+        if (engine == null) {
+            return;
+        }
+        synchronized (INDEXES) {
+            NativeStructureVolumeIndex current = INDEXES.get(engine);
+            if (current != null) {
+                INDEXES.put(engine, current.fresh());
+            }
+        }
+    }
+
     public static KList<NativeStructureVolume> volumes(Engine engine, int minX, int minZ, int maxX, int maxZ) {
         NativeStructureVolumeIndex index = engine == null ? null : INDEXES.get(engine);
         return index == null ? NativeStructureVolume.NONE : index.resolve(engine, minX, minZ, maxX, maxZ);
@@ -102,6 +114,12 @@ public final class NativeStructureVolumeIndex {
         return ORIGIN_REACH_CHUNKS;
     }
 
+    private NativeStructureVolumeIndex fresh() {
+        return context == null
+                ? new NativeStructureVolumeIndex(origins)
+                : new NativeStructureVolumeIndex(context);
+    }
+
     KList<NativeStructureVolume> resolve(Engine engine, int minX, int minZ, int maxX, int maxZ) {
         int fromChunkX = Math.min(minX, maxX) >> 4;
         int toChunkX = Math.max(minX, maxX) >> 4;
@@ -323,7 +341,7 @@ public final class NativeStructureVolumeIndex {
     }
 
     private static IntBinaryOperator surfaceHeight(Engine engine) {
-        return (x, z) -> engine.getHeight(x, z, true) + engine.getMinHeight();
+        return (x, z) -> Engine.hostHeight(engine, x, z, true) + engine.getMinHeight();
     }
 
     static KList<NativeStructureVolume> appendPieces(KList<NativeStructureVolume> volumes,

adapters/modded-common/src/main/java/art/arcane/iris/modded/IrisModdedBiomeSource.java

@@ -18,16 +18,22 @@
 
 package art.arcane.iris.modded;
 
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.GenerationSessionException;
 import art.arcane.iris.engine.framework.GenerationSessionLease;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeCustom;
+import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.util.project.context.IrisContext;
 import art.arcane.volmlib.util.math.RNG;
 import com.mojang.datafixers.util.Pair;
 import com.mojang.serialization.MapCodec;
 import net.minecraft.core.BlockPos;
+import net.minecraft.core.Direction;
 import net.minecraft.core.Holder;
 import net.minecraft.core.HolderLookup;
 import net.minecraft.core.QuartPos;
@@ -37,12 +43,15 @@ import net.minecraft.resources.Identifier;
 import net.minecraft.resources.ResourceKey;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.util.RandomSource;
+import net.minecraft.util.Mth;
+import net.minecraft.world.level.LevelReader;
 import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.biome.BiomeSource;
 import net.minecraft.world.level.biome.Climate;
 import net.minecraft.world.level.levelgen.structure.Structure;
 import net.minecraft.world.level.levelgen.structure.StructureSet;
 
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.LinkedHashSet;
@@ -196,11 +205,16 @@ final class IrisModdedBiomeSource extends BiomeSource {
 
     private List<Holder<Biome>> collectPossibleBiomeHolders() {
         BiomeKeySets keys = biomeKeySets();
-        Set<String> generatedBiomeKeys = requireConfiguredStructureBiomeKeys(keys.required());
+        LinkedHashSet<String> generatedBiomeKeys = new LinkedHashSet<>(
-        Set<String> visibleBiomeKeys = keys.visibleOnly();
+                requireConfiguredStructureBiomeKeys(keys.structureRequired()));
+        generatedBiomeKeys.addAll(keys.visibleRequired());
+        LinkedHashSet<String> visibleBiomeKeys = new LinkedHashSet<>(keys.visibleOnly());
         Registry<Biome> registry = biomeRegistry();
         LinkedHashSet<Holder<Biome>> possible = new LinkedHashSet<>();
         if (registry == null) {
+            if (keys.visibleFallbackPossible()) {
+                visibleBiomeKeys.add("minecraft:plains");
+            }
             for (Holder<Biome> biome : serializedSource.possibleBiomes()) {
                 String key = holderKey(biome);
                 if (isGeneratedBiomeKey(key, generatedBiomeKeys)
@@ -215,6 +229,9 @@ final class IrisModdedBiomeSource extends BiomeSource {
                 throw new IllegalStateException("Iris structure biomes are not registered: "
                         + missingBiomeKeys);
             }
+            if (needsVisibleFallback(keys, registry)) {
+                visibleBiomeKeys.add("minecraft:plains");
+            }
             // Registry-ordered, never a hash-ordered walk: see possibleBiomes().
             registry.listElements().forEach((Holder.Reference<Biome> reference) -> {
                 String key = holderKey(reference);
@@ -223,7 +240,7 @@ final class IrisModdedBiomeSource extends BiomeSource {
                     possible.add(reference);
                 }
             });
-            warnUnregisteredVisibleBiomes(registry, visibleBiomeKeys);
+            warnUnregisteredVisibleBiomes(registry, keys.visibleOnly());
         }
         if (possible.isEmpty()) {
             String phase = registry == null ? "serialized biome bootstrap" : "biome registry";
@@ -233,6 +250,36 @@ final class IrisModdedBiomeSource extends BiomeSource {
         return List.copyOf(possible);
     }
 
+    private List<Holder<Biome>> collectVisibleBiomeHolders() {
+        BiomeKeySets keys = biomeKeySets();
+        Set<String> visibleBiomeKeys = keys.visibleEmittable();
+        Registry<Biome> registry = biomeRegistry();
+        boolean includeFallback = registry == null
+                ? keys.visibleFallbackPossible()
+                : needsVisibleFallback(keys, registry);
+        ArrayList<Holder<Biome>> visible = new ArrayList<>();
+        for (Holder<Biome> biome : resolvePossibleBiomes().ordered()) {
+            String key = holderKey(biome);
+            if (isGeneratedBiomeKey(key, visibleBiomeKeys)
+                    || includeFallback && "minecraft:plains".equals(key)) {
+                visible.add(biome);
+            }
+        }
+        return List.copyOf(visible);
+    }
+
+    private static boolean needsVisibleFallback(BiomeKeySets keys, Registry<Biome> registry) {
+        if (keys.visibleFallbackPossible()) {
+            return true;
+        }
+        for (String key : keys.visibleEmittable()) {
+            if (resolveHolder(registry, key) == null) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Scatter and derivative keys are advisory: a typo there must not stop a world from loading the way a
      * missing structure biome does, so they are reported once and skipped.
@@ -273,19 +320,91 @@ final class IrisModdedBiomeSource extends BiomeSource {
         }
     }
 
+    @Override
+    public Pair<BlockPos, Holder<Biome>> findClosestBiome3d(
+            BlockPos origin,
+            int searchRadius,
+            int sampleResolutionHorizontal,
+            int sampleResolutionVertical,
+            Predicate<Holder<Biome>> allowed,
+            Climate.Sampler sampler,
+            LevelReader level
+    ) {
+        Engine engine = engineOrNull();
+        if (engine == null) {
+            return serializedSource.findClosestBiome3d(
+                    origin, searchRadius, sampleResolutionHorizontal, sampleResolutionVertical,
+                    allowed, sampler, level);
+        }
+        GenerationSessionLease lease = tryAcquireGenerationLease(engine, "modded_locate_visible_biome");
+        if (lease == null) {
+            throw new IllegalStateException("Iris visible biome search was rejected during an engine transition");
+        }
+        try (lease; IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
+            if (!isReady(engine)) {
+                throw new IllegalStateException("Iris visible biome search has no active engine runtime");
+            }
+            DimensionStackContext stackContext = engine.getDimensionStackContext();
+            if (stackContext == null) {
+                return super.findClosestBiome3d(
+                        origin, searchRadius, sampleResolutionHorizontal, sampleResolutionVertical,
+                        allowed, sampler, level);
+            }
+            Set<Holder<Biome>> candidates = new HashSet<>();
+            for (Holder<Biome> biome : collectVisibleBiomeHolders()) {
+                if (allowed.test(biome)) {
+                    candidates.add(biome);
+                }
+            }
+            if (candidates.isEmpty()) {
+                return null;
+            }
+            int sampleRadius = Math.floorDiv(searchRadius, sampleResolutionHorizontal);
+            int[] sampleYs = Mth.outFromOrigin(
+                    origin.getY(), level.getMinY() + 1, level.getMaxY() + 1, sampleResolutionVertical).toArray();
+            for (BlockPos.MutableBlockPos sampleColumn : BlockPos.spiralAround(
+                    BlockPos.ZERO, sampleRadius, Direction.EAST, Direction.SOUTH)) {
+                int blockX = origin.getX() + sampleColumn.getX() * sampleResolutionHorizontal;
+                int blockZ = origin.getZ() + sampleColumn.getZ() * sampleResolutionHorizontal;
+                int quartX = QuartPos.fromBlock(blockX);
+                int quartZ = QuartPos.fromBlock(blockZ);
+                DimensionStackLayout stackLayout = stackContext.sample(
+                        QuartPos.toBlock(quartX), QuartPos.toBlock(quartZ));
+                for (int blockY : sampleYs) {
+                    int quartY = QuartPos.fromBlock(blockY);
+                    Holder<Biome> visibleBiome = resolveVisibleBiome(
+                            engine, quartX, quartY, quartZ, sampler, stackLayout);
+                    if (candidates.contains(visibleBiome)) {
+                        return Pair.of(new BlockPos(blockX, blockY, blockZ), visibleBiome);
+                    }
+                }
+            }
+            return null;
+        }
+    }
+
     private Holder<Biome> getNoiseBiome(Engine engine, int quartX, int quartY, int quartZ,
                                         Climate.Sampler sampler) {
         if (isGuaranteedSurfaceBiome(quartY, engine.getMinHeight())) {
             return getSurfaceStructureBiome(engine, quartX, quartZ, sampler);
         }
+        boolean cacheable = isBiomeCacheable(
+                engine,
+                QuartPos.toBlock(quartX),
+                QuartPos.toBlock(quartZ)
+        );
         long key = packNoiseKey(quartX, quartY, quartZ);
         BiomeHolderTable cache = structureBiomeCache;
-        Holder<Biome> cached = cache.get(key);
+        if (cacheable) {
-        if (cached != null) {
+            Holder<Biome> cached = cache.get(key);
-            return cached;
+            if (cached != null) {
+                return cached;
+            }
         }
         Holder<Biome> resolved = resolveStructureBiome(engine, quartX, quartY, quartZ, sampler);
-        cache.put(key, resolved);
+        if (cacheable) {
+            cache.put(key, resolved);
+        }
         return resolved;
     }
 
@@ -302,14 +421,81 @@ final class IrisModdedBiomeSource extends BiomeSource {
             if (!isReady(engine)) {
                 throw new IllegalStateException("Iris visible biome lookup has no active engine runtime");
             }
-            long key = packNoiseKey(quartX, quartY, quartZ);
+            return getVisibleNoiseBiomeWithActiveGenerationLease(
-            BiomeHolderTable cache = visibleBiomeCache;
+                    engine, quartX, quartY, quartZ, sampler);
+        }
+    }
+
+    private Holder<Biome> getVisibleNoiseBiomeWithActiveGenerationLease(
+            Engine engine,
+            int quartX,
+            int quartY,
+            int quartZ,
+            Climate.Sampler sampler
+    ) {
+        boolean cacheable = isBiomeCacheable(
+                engine,
+                QuartPos.toBlock(quartX),
+                QuartPos.toBlock(quartZ)
+        );
+        long key = packNoiseKey(quartX, quartY, quartZ);
+        BiomeHolderTable cache = visibleBiomeCache;
+        if (cacheable) {
             Holder<Biome> cached = cache.get(key);
             if (cached != null) {
                 return cached;
             }
-            Holder<Biome> resolved = resolveVisibleBiome(engine, quartX, quartY, quartZ, sampler);
+        }
+        Holder<Biome> resolved = resolveVisibleBiome(engine, quartX, quartY, quartZ, sampler);
+        if (cacheable) {
             cache.put(key, resolved);
+        }
+        return resolved;
+    }
+
+    Holder<Biome> getVisibleSurfaceBiome(int blockX, int blockZ) {
+        Engine engine = engineOrNull();
+        if (engine == null) {
+            return null;
+        }
+        GenerationSessionLease lease = tryAcquireGenerationLease(engine, "modded_surface_spawn_biome");
+        if (lease == null) {
+            return null;
+        }
+        try (lease; IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
+            if (!isReady(engine)) {
+                return null;
+            }
+            Registry<Biome> registry = biomeRegistry();
+            if (registry == null) {
+                return null;
+            }
+            int quartX = QuartPos.fromBlock(blockX);
+            int quartZ = QuartPos.fromBlock(blockZ);
+            int sampleX = QuartPos.toBlock(quartX);
+            int sampleZ = QuartPos.toBlock(quartZ);
+            boolean cacheable = isBiomeCacheable(engine, sampleX, sampleZ);
+            DimensionStackContext stackContext = engine.getDimensionStackContext();
+            DimensionStackLayout.Layer layer = stackContext == null
+                    ? null
+                    : stackContext.getLayout(sampleX, sampleZ).surfaceLayer();
+            int internalY = layer == null
+                    ? Engine.hostHeight(engine, sampleX, sampleZ, true)
+                    : layer.clippedSurfaceY();
+            int quartY = QuartPos.fromBlock(internalY + engine.getMinHeight());
+            long key = packNoiseKey(quartX, quartY, quartZ);
+            BiomeHolderTable cache = visibleBiomeCache;
+            if (cacheable) {
+                Holder<Biome> cached = cache.get(key);
+                if (cached != null) {
+                    return cached;
+                }
+            }
+            Holder<Biome> resolved = resolveVisibleBiome(
+                    engine, quartX, quartY, quartZ, null);
+            if (cacheable) {
+                cache.put(key, resolved);
+            }
             return resolved;
         }
     }
@@ -395,7 +581,9 @@ final class IrisModdedBiomeSource extends BiomeSource {
         if (registry == null) {
             throw new IllegalStateException("Iris structure biome lookup has no biome registry");
         }
-        IrisBiome irisBiome = engine.getComplex().getTrueBiomeStream().get(quartX << 2, quartZ << 2);
+        int blockX = quartX << 2;
+        int blockZ = quartZ << 2;
+        IrisBiome irisBiome = resolveSurfaceStructureBiome(engine, blockX, blockZ);
         if (irisBiome == null) {
             throw new IllegalStateException("Iris returned no surface structure biome at quart "
                     + quartX + "," + quartZ);
@@ -412,7 +600,8 @@ final class IrisModdedBiomeSource extends BiomeSource {
     private Holder<Biome> resolveStructureBiome(Engine engine, int quartX, int quartY, int quartZ,
                                                 Climate.Sampler sampler) {
         Registry<Biome> registry = biomeRegistry();
-        BiomeResolution resolution = resolveBiomeResolution(engine, quartX, quartY, quartZ);
+        BiomeResolution resolution = resolveStructureBiomeResolution(
+                engine, quartX, quartY, quartZ);
         if (resolution == null) {
             throw new IllegalStateException("Iris returned no structure biome at quart "
                     + quartX + "," + quartY + "," + quartZ);
@@ -431,11 +620,36 @@ final class IrisModdedBiomeSource extends BiomeSource {
 
     private Holder<Biome> resolveVisibleBiome(Engine engine, int quartX, int quartY, int quartZ,
                                               Climate.Sampler sampler) {
+        return resolveVisibleBiome(engine, quartX, quartY, quartZ, sampler, null);
+    }
+
+    private Holder<Biome> resolveVisibleBiome(
+            Engine engine,
+            int quartX,
+            int quartY,
+            int quartZ,
+            Climate.Sampler sampler,
+            DimensionStackLayout stackLayout
+    ) {
         Registry<Biome> registry = biomeRegistry();
-        BiomeResolution resolution = resolveBiomeResolution(engine, quartX, quartY, quartZ);
+        BiomeResolution resolution = resolveBiomeResolution(
+                engine, quartX, quartY, quartZ, true, stackLayout);
         if (resolution == null || registry == null) {
             return serializedSource.getNoiseBiome(quartX, quartY, quartZ, sampler);
         }
+        return resolveVisibleBiome(
+                engine, registry, resolution, quartX, quartY, quartZ, sampler);
+    }
+
+    private Holder<Biome> resolveVisibleBiome(
+            Engine engine,
+            Registry<Biome> registry,
+            BiomeResolution resolution,
+            int quartX,
+            int quartY,
+            int quartZ,
+            Climate.Sampler sampler
+    ) {
         String biomeKey;
         if (resolution.irisBiome().isCustom()) {
             IrisBiomeCustom customBiome = resolution.irisBiome().getCustomBiome(
@@ -444,7 +658,8 @@ final class IrisModdedBiomeSource extends BiomeSource {
                 return fallbackBiome(registry, "custom derivative of '"
                         + resolution.irisBiome().getLoadKey() + "'", quartX, quartY, quartZ, sampler);
             }
-            biomeKey = ModdedWorldgenIds.biomeRef(engine, customBiome.getId());
+            biomeKey = customBiomeRef(
+                    resolution.packName(), resolution.dimension(), customBiome.getId());
         } else if (resolution.underground()) {
             biomeKey = resolution.irisBiome().getGroundBiomeKey(
                     resolution.rng(), engine, resolution.blockX(), resolution.blockY(), resolution.blockZ());
@@ -459,6 +674,37 @@ final class IrisModdedBiomeSource extends BiomeSource {
     }
 
     private BiomeResolution resolveBiomeResolution(Engine engine, int quartX, int quartY, int quartZ) {
+        return resolveBiomeResolution(engine, quartX, quartY, quartZ, true);
+    }
+
+    private BiomeResolution resolveStructureBiomeResolution(
+            Engine engine,
+            int quartX,
+            int quartY,
+            int quartZ
+    ) {
+        return resolveBiomeResolution(engine, quartX, quartY, quartZ, false);
+    }
+
+    private BiomeResolution resolveBiomeResolution(
+            Engine engine,
+            int quartX,
+            int quartY,
+            int quartZ,
+            boolean includeDimensionStack
+    ) {
+        return resolveBiomeResolution(
+                engine, quartX, quartY, quartZ, includeDimensionStack, null);
+    }
+
+    private BiomeResolution resolveBiomeResolution(
+            Engine engine,
+            int quartX,
+            int quartY,
+            int quartZ,
+            boolean includeDimensionStack,
+            DimensionStackLayout stackLayout
+    ) {
         if (!isReady(engine)) {
             return null;
         }
@@ -467,27 +713,75 @@ final class IrisModdedBiomeSource extends BiomeSource {
         int blockZ = quartZ << 2;
         int internalY = blockY - engine.getMinHeight();
         int caveSwitchY = Math.max(-8 - engine.getMinHeight(), 40);
+        DimensionStackLayout.Layer stackLayer = includeDimensionStack
+                ? stackLayout == null
+                        ? resolveDimensionStackLayer(engine, blockX, internalY, blockZ)
+                        : stackLayout.layerAt(internalY)
+                : null;
+        IrisDimension owningDimension = stackLayer == null
+                ? engine.getDimension()
+                : stackLayer.terrainContext().getDimension();
+        IrisData owningData = stackLayer == null
+                ? engine.getData()
+                : stackLayer.terrainContext().getData();
+        if (stackLayer != null && !stackLayer.terrainContext().isSelfReferencing()) {
+            IrisBiome stackedBiome = stackLayer.biome();
+            if (stackedBiome == null) {
+                return null;
+            }
+            return createBiomeResolution(
+                    engine,
+                    stackedBiome,
+                    false,
+                    owningDimension,
+                    owningData,
+                    blockX,
+                    blockY,
+                    blockZ
+            );
+        }
         boolean underground = false;
         IrisBiome irisBiome;
-        if (internalY <= caveSwitchY) {
+        if (stackLayer != null) {
-            int surfaceY = engine.getComplex().getHeightStream().get(blockX, blockZ).intValue();
+            int surfaceY = stackLayer.surfaceY();
+            underground = isUnderground(internalY, surfaceY);
+            irisBiome = underground
+                    ? engine.getCaveBiome(
+                            blockX,
+                            internalY,
+                            blockZ,
+                            null,
+                            stackLayer.biome(),
+                            surfaceY
+                    )
+                    : stackLayer.biome();
+        } else if (internalY <= caveSwitchY) {
+            int surfaceY = Engine.hostHeight(engine, blockX, blockZ, true);
             underground = isUnderground(internalY, surfaceY);
             irisBiome = underground
                     ? engine.getCaveBiome(blockX, internalY, blockZ)
-                    : engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+                    : engine.getHostSurfaceBiome(blockX, blockZ);
         } else {
-            irisBiome = engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+            irisBiome = engine.getHostSurfaceBiome(blockX, blockZ);
         }
         if (irisBiome == null && underground) {
-            irisBiome = engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
+            irisBiome = stackLayer == null
+                    ? engine.getHostSurfaceBiome(blockX, blockZ)
+                    : stackLayer.biome();
         }
         if (irisBiome == null) {
             return null;
         }
-        IrisModdedChunkGenerator current = generator;
+        return createBiomeResolution(
-        long worldSeed = current == null ? engine.getWorld().getRawWorldSeed() : current.visibleBiomeSeed();
+                engine,
-        long seed = biomeResolutionSeed(worldSeed, blockX, blockY, blockZ);
+                irisBiome,
-        return new BiomeResolution(irisBiome, underground, blockX, blockY, blockZ, new RNG(seed));
+                underground,
+                owningDimension,
+                owningData,
+                blockX,
+                blockY,
+                blockZ
+        );
     }
 
     private Holder<Biome> resolveRequiredStructureBiome(int quartX, int quartY, int quartZ) {
@@ -510,9 +804,10 @@ final class IrisModdedBiomeSource extends BiomeSource {
             }
             IrisBiome irisBiome;
             if (isGuaranteedSurfaceBiome(quartY, engine.getMinHeight())) {
-                irisBiome = engine.getComplex().getTrueBiomeStream().get(quartX << 2, quartZ << 2);
+                irisBiome = resolveSurfaceStructureBiome(engine, quartX << 2, quartZ << 2);
             } else {
-                BiomeResolution resolution = resolveBiomeResolution(engine, quartX, quartY, quartZ);
+                BiomeResolution resolution = resolveStructureBiomeResolution(
+                        engine, quartX, quartY, quartZ);
                 irisBiome = resolution == null ? null : resolution.irisBiome();
             }
             if (irisBiome == null) {
@@ -557,6 +852,10 @@ final class IrisModdedBiomeSource extends BiomeSource {
         return internalY <= surfaceY - 8;
     }
 
+    static boolean isBiomeCacheable(Engine engine, int blockX, int blockZ) {
+        return !engine.answersFromNaturalTerrain(blockX, blockZ);
+    }
+
     static boolean isMonumentSurfaceBiomeQuery(int blockY, int radius, int minHeight, int fluidHeight) {
         return radius == 29 && blockY == minHeight + fluidHeight;
     }
@@ -612,7 +911,7 @@ final class IrisModdedBiomeSource extends BiomeSource {
     }
 
     private Set<String> exactStructureBiomeKeys() {
-        return biomeKeySets().required();
+        return biomeKeySets().structureRequired();
     }
 
     /**
@@ -636,11 +935,12 @@ final class IrisModdedBiomeSource extends BiomeSource {
     private BiomeKeySets collectBiomeKeySets(long generation) {
         IrisModdedChunkGenerator current = generator;
         if (current == null) {
-            return new BiomeKeySets(generation, Set.of(), Set.of());
+            return new BiomeKeySets(generation, Set.of(), Set.of(), Set.of(), Set.of(), false);
         }
         Engine engine = current.structureEngineOrNull();
         if (engine == null) {
-            return new BiomeKeySets(generation, current.configuredStructureBiomeKeys(), Set.of());
+            return new BiomeKeySets(
+                    generation, current.configuredStructureBiomeKeys(), Set.of(), Set.of(), Set.of(), false);
         }
         GenerationSessionLease lease = tryAcquireGenerationLease(engine, "modded_structure_biome_keys");
         if (lease == null) {
@@ -650,36 +950,154 @@ final class IrisModdedBiomeSource extends BiomeSource {
             if (!isReady(engine)) {
                 throw new IllegalStateException("Iris structure biome key lookup has no active engine runtime");
             }
-            LinkedHashSet<String> required = new LinkedHashSet<>();
+            Set<String> structureRequired = collectStructureBiomeKeys(
-            LinkedHashSet<String> visible = new LinkedHashSet<>();
+                    engine.getDimension(), engine.getData());
-            for (IrisBiome irisBiome : engine.getAllBiomes()) {
+            LinkedHashSet<String> visibleRequired = new LinkedHashSet<>();
-                String derivative = normalizeKey(irisBiome.getStructureDerivativeKey());
+            LinkedHashSet<String> visibleEmittable = new LinkedHashSet<>();
-                if (derivative != null) {
+            boolean visibleFallbackPossible = false;
-                    required.add(derivative);
+            for (OwnedBiome ownedBiome : getOwnedBiomes(engine)) {
-                }
+                IrisBiome irisBiome = ownedBiome.biome();
                 if (irisBiome.isCustom()) {
                     for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
-                        required.add(ModdedWorldgenIds.biomeRef(engine, customBiome.getId()));
+                        String customKey = customBiomeRef(
+                                ownedBiome.data().getDataFolder().getName(),
+                                ownedBiome.dimension(),
+                                customBiome.getId()
+                        );
+                        visibleRequired.add(customKey);
+                        visibleEmittable.add(customKey);
                     }
+                    continue;
                 }
-                addVisibleBiomeKey(visible, irisBiome.getDerivativeKey());
+                visibleFallbackPossible |= !addVisibleBiomeKey(
+                        visibleEmittable, irisBiome.getDerivativeKey());
                 for (String scatter : irisBiome.getBiomeScatter()) {
-                    addVisibleBiomeKey(visible, scatter);
+                    visibleFallbackPossible |= !addVisibleBiomeKey(visibleEmittable, scatter);
                 }
                 for (String scatter : irisBiome.getBiomeSkyScatter()) {
-                    addVisibleBiomeKey(visible, scatter);
+                    visibleFallbackPossible |= !addVisibleBiomeKey(visibleEmittable, scatter);
                 }
             }
-            visible.removeAll(required);
+            LinkedHashSet<String> visible = new LinkedHashSet<>(visibleEmittable);
-            return new BiomeKeySets(generation, Set.copyOf(required), Set.copyOf(visible));
+            visible.removeAll(structureRequired);
+            visible.removeAll(visibleRequired);
+            return new BiomeKeySets(
+                    generation,
+                    Set.copyOf(structureRequired),
+                    Set.copyOf(visibleRequired),
+                    Set.copyOf(visible),
+                    Set.copyOf(visibleEmittable),
+                    visibleFallbackPossible
+            );
         }
     }
 
-    private static void addVisibleBiomeKey(Set<String> target, String key) {
+    static Set<String> collectStructureBiomeKeys(IrisDimension dimension, IrisData data) {
-        String normalized = normalizeKey(key);
+        LinkedHashSet<String> keys = new LinkedHashSet<>();
-        if (normalized != null) {
+        for (IrisBiome irisBiome : dimension.getReachableBiomes(() -> data)) {
-            target.add(normalized);
+            String derivative = normalizeKey(irisBiome.getStructureDerivativeKey());
+            if (derivative != null) {
+                keys.add(derivative);
+            }
         }
+        return Set.copyOf(keys);
+    }
+
+    private static boolean addVisibleBiomeKey(Set<String> target, String key) {
+        String normalized = normalizeKey(key);
+        if (normalized == null) {
+            return false;
+        }
+        target.add(normalized);
+        return true;
+    }
+
+    static String customBiomeRef(String packName, IrisDimension dimension, String customBiomeId) {
+        return ModdedWorldgenIds.biomeRef(packName, dimension.getLoadKey(), customBiomeId);
+    }
+
+    private BiomeResolution createBiomeResolution(
+            Engine engine,
+            IrisBiome irisBiome,
+            boolean underground,
+            IrisDimension dimension,
+            IrisData data,
+            int blockX,
+            int blockY,
+            int blockZ
+    ) {
+        IrisModdedChunkGenerator current = generator;
+        long worldSeed = current == null
+                ? engine.getWorld().getRawWorldSeed()
+                : current.visibleBiomeSeed();
+        long seed = biomeResolutionSeed(worldSeed, blockX, blockY, blockZ);
+        return new BiomeResolution(
+                irisBiome,
+                underground,
+                dimension,
+                data.getDataFolder().getName(),
+                blockX,
+                blockY,
+                blockZ,
+                new RNG(seed)
+        );
+    }
+
+    private static List<OwnedBiome> getOwnedBiomes(Engine engine) {
+        ArrayList<OwnedBiome> biomes = new ArrayList<>();
+        IrisDimension hostDimension = engine.getDimension();
+        IrisData hostData = engine.getData();
+        addOwnedBiomes(
+                biomes,
+                hostDimension,
+                hostData,
+                hostDimension.getReachableBiomes(engine)
+        );
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        if (stackContext == null) {
+            return biomes;
+        }
+        for (DimensionTerrainContext terrainContext : stackContext.getLayersBottomToTop()) {
+            if (terrainContext.isSelfReferencing()) {
+                continue;
+            }
+            IrisDimension dimension = terrainContext.getDimension();
+            IrisData data = terrainContext.getData();
+            addOwnedBiomes(
+                    biomes,
+                    dimension,
+                    data,
+                    dimension.getReachableBiomes(terrainContext)
+            );
+        }
+        return biomes;
+    }
+
+    private static void addOwnedBiomes(
+            List<OwnedBiome> target,
+            IrisDimension dimension,
+            IrisData data,
+            Iterable<IrisBiome> biomes
+    ) {
+        for (IrisBiome biome : biomes) {
+            if (biome != null) {
+                target.add(new OwnedBiome(dimension, data, biome));
+            }
+        }
+    }
+
+    private static DimensionStackLayout.Layer resolveDimensionStackLayer(
+            Engine engine,
+            int blockX,
+            int internalY,
+            int blockZ
+    ) {
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        return stackContext == null ? null : stackContext.getLayerAt(blockX, internalY, blockZ);
+    }
+
+    private static IrisBiome resolveSurfaceStructureBiome(Engine engine, int blockX, int blockZ) {
+        return engine.getComplex().getTrueBiomeStream().get(blockX, blockZ);
     }
 
     private static Set<String> registeredBiomeKeys(Registry<Biome> registry) {
@@ -726,7 +1144,13 @@ final class IrisModdedBiomeSource extends BiomeSource {
         Holder<Biome> plains = resolveHolder(registry, "minecraft:plains");
         warnUnresolvedBiome(unresolvedKey, plains == null ? "the serialized biome source" : "minecraft:plains",
                 quartX, quartY, quartZ);
-        return plains == null ? serializedSource.getNoiseBiome(quartX, quartY, quartZ, sampler) : plains;
+        if (plains != null) {
+            return plains;
+        }
+        if (sampler == null) {
+            throw new IllegalStateException("Iris cannot resolve a registered fallback biome");
+        }
+        return serializedSource.getNoiseBiome(quartX, quartY, quartZ, sampler);
     }
 
     private void warnUnresolvedBiome(String unresolvedKey, String fallback,
@@ -753,11 +1177,29 @@ final class IrisModdedBiomeSource extends BiomeSource {
         return ((long) x << 32) ^ ((long) z & 4294967295L);
     }
 
-    private record BiomeResolution(IrisBiome irisBiome, boolean underground, int blockX, int blockY,
+    private record BiomeResolution(
-                                   int blockZ, RNG rng) {
+            IrisBiome irisBiome,
+            boolean underground,
+            IrisDimension dimension,
+            String packName,
+            int blockX,
+            int blockY,
+            int blockZ,
+            RNG rng
+    ) {
     }
 
-    private record BiomeKeySets(long generation, Set<String> required, Set<String> visibleOnly) {
+    private record OwnedBiome(IrisDimension dimension, IrisData data, IrisBiome biome) {
+    }
+
+    private record BiomeKeySets(
+            long generation,
+            Set<String> structureRequired,
+            Set<String> visibleRequired,
+            Set<String> visibleOnly,
+            Set<String> visibleEmittable,
+            boolean visibleFallbackPossible
+    ) {
     }
 
     private record PossibleBiomes(long generation, List<Holder<Biome>> ordered, Set<Holder<Biome>> set) {

adapters/modded-common/src/main/java/art/arcane/iris/modded/IrisModdedChunkGenerator.java

@@ -30,6 +30,7 @@ import art.arcane.iris.engine.object.IrisImportedStructureControl;
 import art.arcane.iris.nativegen.NativeStructureStartInjector;
 import art.arcane.iris.nativegen.NativeStructureReferenceRepair;
 import art.arcane.iris.nativegen.NativeStructureVanillaLocator;
+import art.arcane.iris.nativegen.NativeStructureVolumeIndex;
 import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.spi.PlatformBiome;
 import art.arcane.iris.spi.PlatformBlockState;
@@ -89,7 +90,9 @@ import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
+import java.util.TreeSet;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -181,11 +184,18 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         }
         requireGlobalStructureGeneration(
                 level.getServer().getWorldGenSettings().options().generateStructures(), dimensionKey);
+        Engine activeEngine = engineIfBound();
         Engine replacement = ModdedWorldEngines.prepareReplacement(level, pack, packDimensionKey, seed);
         try {
+            if (activeEngine != null) {
+                requireStructureBiomeUniverseCompatible(
+                        activeEngine.getDimension(), replacement.getDimension());
+            }
+            nativeStructures.installVolumeIndex(level, replacement);
             ModdedWorldEngines.installReplacement(level, replacement);
             replacement.getPlatformHooks().applyWorldBoundary(replacement);
         } catch (Throwable error) {
+            NativeStructureVolumeIndex.uninstall(replacement);
             try {
                 ModdedWorldEngines.closeUnregistered(replacement);
             } catch (Throwable cleanupError) {
@@ -203,17 +213,14 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         this.activePack = pack;
         this.activeDimensionKey = packDimensionKey;
         this.seedOverride = seed;
-        this.engine = replacement;
         this.configuredStructureBiomeKeys = null;
         this.configuredPack = null;
         this.heightMetadata = engineHeights(replacement);
         this.engineBinding.reset();
-        this.engineBinding.complete(replacement);
         this.announced.set(false);
-        this.structureBiomeSource.clearCaches();
+        resetRuntimeCaches();
-        this.importedFeatures.invalidate();
+        this.engine = replacement;
-        this.nativeStructures.clearWorldCheckStructureShifts();
+        this.engineBinding.complete(replacement);
-        this.spawnTables.resetVanillaSpawnBiomes();
         // Bind time: a feature-order cycle in the new pack is reported here, once, and degrades to features-off.
         // Never waits on a build owned by another thread: this method owns the generator monitor and the build
         // path can need it.
@@ -751,7 +758,38 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         return lastChunkGenAt;
     }
 
-    public void onHotload() {
+    public void prepareRuntimeHotload(ServerLevel level, Engine current) {
+        if (this.engine != current || level.getChunkSource().getGenerator() != this) {
+            throw new IllegalStateException("Iris generator '" + dimensionKey
+                    + "' cannot prepare caches for an unrelated engine hotload");
+        }
+        resetRuntimeCaches();
+        nativeStructures.installVolumeIndex(level, current);
+    }
+
+    public static void requireStructureBiomeUniverseCompatible(
+            IrisDimension active,
+            IrisDimension replacement
+    ) {
+        IrisData activeData = Objects.requireNonNull(active.getLoader(),
+                "Active Iris dimension has no pack loader");
+        IrisData replacementData = Objects.requireNonNull(replacement.getLoader(),
+                "Replacement Iris dimension has no pack loader");
+        requireStructureBiomeUniverseCompatible(
+                IrisModdedBiomeSource.collectStructureBiomeKeys(active, activeData),
+                IrisModdedBiomeSource.collectStructureBiomeKeys(replacement, replacementData));
+    }
+
+    static void requireStructureBiomeUniverseCompatible(Set<String> active, Set<String> replacement) {
+        if (active.equals(replacement)) {
+            return;
+        }
+        throw new IllegalArgumentException("Iris cannot hotload a changed host structure-biome derivative set "
+                + "from " + new TreeSet<>(active) + " to " + new TreeSet<>(replacement)
+                + ". Restart the server so Minecraft can rebuild its immutable structure state.");
+    }
+
+    private void resetRuntimeCaches() {
         configuredStructureBiomeKeys = null;
         structureBiomeSource.clearCaches();
         importedFeatures.invalidate();
@@ -986,7 +1024,17 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         Registry<Biome> registry = region.registryAccess().lookupOrThrow(Registries.BIOME);
         spawnTables.initializeVanillaSpawnBiomes(registry);
         ChunkPos center = region.getCenter();
-        Holder<Biome> visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+        Holder<Biome> visibleBiome;
+        if (engine == null || engine.getDimensionStackContext() == null) {
+            visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+        } else {
+            visibleBiome = structureBiomeSource.getVisibleSurfaceBiome(
+                    center.getMinBlockX() + 8,
+                    center.getMinBlockZ() + 8);
+            if (visibleBiome == null) {
+                visibleBiome = region.getBiome(center.getWorldPosition().atY(region.getMaxY()));
+            }
+        }
         Holder<Biome> vanillaBiome = spawnTables.vanillaSpawnBiome(visibleBiome.value());
         WorldgenRandom random = new WorldgenRandom(new LegacyRandomSource(RandomSupport.generateUniqueSeed()));
         random.setDecorationSeed(region.getSeed(), center.getMinBlockX(), center.getMinBlockZ());
@@ -1029,7 +1077,10 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         boolean ignoreFluid = !type.isOpaque().test(Blocks.WATER.defaultBlockState());
         try (GenerationSessionLease lease = current.acquireGenerationLease("modded_base_height");
              IrisContext.Scope ignored = IrisContext.open(current, lease.sessionId(), null)) {
-            return heightAccessor.getMinY() + current.getHeight(x, z, ignoreFluid) + 1;
+            int height = current.getDimensionStackContext() == null
+                    ? current.getHeight(x, z, ignoreFluid)
+                    : Engine.hostHeight(current, x, z, ignoreFluid);
+            return heightAccessor.getMinY() + height + 1;
         } catch (GenerationSessionException e) {
             throw new IllegalStateException("Iris base height query could not acquire its engine runtime.", e);
         }
@@ -1043,8 +1094,13 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
         try (GenerationSessionLease lease = current.acquireGenerationLease("modded_base_column");
              IrisContext.Scope ignored = IrisContext.open(current, lease.sessionId(), null)) {
             BlockState airState = Blocks.AIR.defaultBlockState();
-            int surface = current.getHeight(x, z, true);
+            boolean dimensionStack = current.getDimensionStackContext() != null;
-            int fluid = current.getHeight(x, z, false);
+            int surface = dimensionStack
+                    ? Engine.hostHeight(current, x, z, true)
+                    : current.getHeight(x, z, true);
+            int fluid = dimensionStack
+                    ? Engine.hostHeight(current, x, z, false)
+                    : current.getHeight(x, z, false);
             BlockState stone = Blocks.STONE.defaultBlockState();
             BlockState water = Blocks.WATER.defaultBlockState();
             int solidEnd = Math.max(0, Math.min(states.length, surface + 1));

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedBiomeWriter.java

@@ -21,6 +21,7 @@ package art.arcane.iris.modded;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeCustom;
+import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.spi.PlatformBiome;
 import art.arcane.iris.spi.PlatformBiomeWriter;
 import art.arcane.iris.util.project.context.IrisContext;
@@ -97,7 +98,7 @@ public final class ModdedBiomeWriter implements PlatformBiomeWriter {
             return key;
         }
         Engine engine = context.getEngine();
-        String prefix = engine.getDimension().getLoadKey() + ":";
+        String prefix = engine.getDimension().getCustomBiomeKeyPrefix();
         if (!key.regionMatches(true, 0, prefix, 0, prefix.length())) {
             return key;
         }
@@ -138,13 +139,7 @@ public final class ModdedBiomeWriter implements PlatformBiomeWriter {
     }
 
     private int idForDerivative(Registry<Biome> registry, String key) {
-        int colon = key.indexOf(':');
+        IrisBiome owner = findCustomBiomeOwner(key);
-        if (colon <= 0 || colon >= key.length() - 1) {
-            return -1;
-        }
-        String dimensionLoadKey = key.substring(0, colon);
-        String customBiomeId = key.substring(colon + 1);
-        IrisBiome owner = findCustomBiomeOwner(dimensionLoadKey, customBiomeId);
         if (owner == null) {
             return -1;
         }
@@ -155,20 +150,18 @@ public final class ModdedBiomeWriter implements PlatformBiomeWriter {
         return idForKey(registry, derivativeKey);
     }
 
-    private IrisBiome findCustomBiomeOwner(String dimensionLoadKey, String customBiomeId) {
+    private IrisBiome findCustomBiomeOwner(String key) {
         for (Engine engine : ModdedWorldEngines.activeEngines()) {
             if (engine == null || engine.isClosed()) {
                 continue;
             }
-            if (!dimensionLoadKey.equalsIgnoreCase(engine.getDimension().getLoadKey())) {
+            IrisDimension dimension = engine.getDimension();
-                continue;
+            for (IrisBiome biome : dimension.getAllBiomes(engine)) {
-            }
-            for (IrisBiome biome : engine.getDimension().getAllBiomes(engine)) {
                 if (!biome.isCustom()) {
                     continue;
                 }
                 for (IrisBiomeCustom custom : biome.getCustomDerivitives()) {
-                    if (customBiomeId.equals(custom.getId())) {
+                    if (key.equalsIgnoreCase(dimension.getCustomBiomeKey(custom.getId()))) {
                         return biome;
                     }
                 }

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedBlockBreakHandler.java

@@ -258,7 +258,7 @@ public final class ModdedBlockBreakHandler {
         if (skipsParents(providers)) {
             return providers;
         }
-        IrisRegion region = engine.getRegion(position.getX(), position.getZ());
+        IrisRegion region = engine.getRegion(position.getX(), relativeY, position.getZ());
         if (region != null) {
             addMatching(providers, region.getBlockDrops(), brokenState, data);
         }

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedDimensionMetadata.java

@@ -62,7 +62,6 @@ final class ModdedDimensionMetadata {
 
     static Set<String> collectConfiguredBiomeKeys(Iterable<IrisBiome> biomes, String dimensionLoadKey) {
         LinkedHashSet<String> keys = new LinkedHashSet<>();
-        String namespace = dimensionLoadKey.toLowerCase(Locale.ROOT);
         for (IrisBiome irisBiome : biomes) {
             if (irisBiome == null) {
                 continue;
@@ -75,7 +74,7 @@ final class ModdedDimensionMetadata {
                 continue;
             }
             for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
-                keys.add(namespace + ":" + customBiome.getId().toLowerCase(Locale.ROOT));
+                keys.add(IrisDimension.customBiomeKey(dimensionLoadKey, customBiome.getId()));
             }
         }
         return Set.copyOf(keys);

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedEngineEffects.java

@@ -226,7 +226,7 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
         int blockY = floor(player.getY()) - getEngine().getWorld().minHeight();
         int blockZ = floor(player.getZ());
         state.biome(getEngine().getBiome(blockX, blockY, blockZ));
-        state.region(getEngine().getRegion(blockX, blockZ));
+        state.region(getEngine().getRegion(blockX, blockY, blockZ));
     }
 
     private void applyEffects(ServerLevel level, ServerPlayer player, Iterable<IrisEffect> effects) {

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedForcedDatapack.java

@@ -52,7 +52,6 @@ import java.nio.file.attribute.BasicFileAttributes;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Comparator;
 import java.util.HexFormat;
 import java.util.LinkedHashMap;
@@ -514,18 +513,13 @@ public final class ModdedForcedDatapack {
         if (!dimensionsDirectory.isDirectory()) {
             return false;
         }
-        File[] dimensionFiles = dimensionsDirectory.listFiles(
+        IrisData data = IrisData.get(packFolder);
-                (File file) -> file.isFile() && file.getName().endsWith(".json"));
+        String[] dimensionKeys = data.getDimensionLoader().getPossibleKeys();
-        if (dimensionFiles == null) {
+        if (dimensionKeys == null || dimensionKeys.length == 0) {
-            throw new IOException("Iris could not read dimensions for pack '" + packName + "'");
-        }
-        if (dimensionFiles.length == 0) {
             return false;
         }
-        Arrays.sort(dimensionFiles, Comparator.comparing(File::getName));
+        List<String> sortedDimensionKeys = Stream.of(dimensionKeys).sorted().toList();
-        IrisData data = IrisData.get(packFolder);
+        for (String dimensionKey : sortedDimensionKeys) {
-        for (File dimensionFile : dimensionFiles) {
-            String dimensionKey = dimensionFile.getName().substring(0, dimensionFile.getName().length() - ".json".length());
             IrisDimension dimension = data.getDimensionLoader().load(dimensionKey);
             if (dimension == null) {
                 throw new IllegalStateException("Iris pack '" + packName + "' dimension '"

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedImportedFeatureStage.java

@@ -18,6 +18,8 @@
 
 package art.arcane.iris.modded;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.GenerationSessionLease;
 import art.arcane.iris.engine.framework.NativeFeatureGenerationPolicy;
@@ -57,7 +59,10 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.locks.ReentrantLock;
+import java.util.function.IntBinaryOperator;
+import java.util.function.Predicate;
 
 /**
  * Native placed-feature passthrough for one Iris dimension, gated on {@code importedFeatures.enabled}.
@@ -219,7 +224,14 @@ final class ModdedImportedFeatureStage {
     private FeatureTable buildTable(Engine engine, IrisImportedFeatureControl control, long generation) {
         // Registry-ordered biome list. FeatureSorter's cycle detection walks it, so an unordered list makes
         // detection depend on JVM hash order and turns a real cycle into an intermittent one.
-        List<Holder<Biome>> biomes = biomeSource.orderedPossibleBiomes();
+        Set<String> hostBiomeKeys = hostBiomeKeys(engine);
+        List<Holder<Biome>> biomes = new ArrayList<>();
+        for (Holder<Biome> biome : biomeSource.orderedPossibleBiomes()) {
+            String key = holderKey(biome);
+            if (key != null && hostBiomeKeys.contains(key)) {
+                biomes.add(biome);
+            }
+        }
         if (biomes.isEmpty()) {
             ModdedIrisLog.error("Iris importedFeatures is on but {} exposes no biomes; features off",
                     dimensionKey(engine));
@@ -262,7 +274,7 @@ final class ModdedImportedFeatureStage {
      */
     private Map<String, Holder<Biome>> customBiomeDerivatives(Engine engine, Map<String, Holder<Biome>> byKey) {
         Map<String, Holder<Biome>> derivatives = new HashMap<>();
-        for (IrisBiome irisBiome : engine.getAllBiomes()) {
+        for (IrisBiome irisBiome : engine.getDimension().getReachableBiomes(engine)) {
             if (!irisBiome.isCustom()) {
                 continue;
             }
@@ -322,14 +334,24 @@ final class ModdedImportedFeatureStage {
         List<FeatureSorter.StepFeatureData> steps = table.steps();
         WorldgenRandom random = new WorldgenRandom(new XoroshiroRandomSource(RandomSupport.generateUniqueSeed()));
         long decorationSeed = random.setDecorationSeed(level.getSeed(), origin.getX(), origin.getZ());
-        Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table);
         IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
         int staticMinY = engine.getMinHeight();
-        WorldGenLevel placementLevel = staticObjects.isEmpty() ? level : ModdedNativeStructureWorldgenAccess.create(
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        Set<Holder<Biome>> chunkBiomes = chunkBiomes(level, sectionPos, table, stackContext);
+        IntBinaryOperator surfaceFirstFreeY = stackContext == null
+                ? (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z)
+                : (x, z) -> Engine.hostHeight(engine, x, z, false) + staticMinY + 1;
+        IntBinaryOperator floorFirstFreeY = stackContext == null
+                ? (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z)
+                : (x, z) -> Engine.hostHeight(engine, x, z, true) + staticMinY + 1;
+        WorldGenLevel placementLevel = staticObjects.isEmpty() && stackContext == null
+                ? level
+                : ModdedNativeStructureWorldgenAccess.create(
                 level, centerPos,
-                (x, z) -> level.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z),
+                surfaceFirstFreeY,
-                (x, z) -> level.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z),
+                floorFirstFreeY,
-                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
+                stackContext != null,
+                importedFeatureProtection(staticObjects, stackContext, staticMinY));
 
         try {
             for (int stepIndex = 0; stepIndex < steps.size(); stepIndex++) {
@@ -349,6 +371,58 @@ final class ModdedImportedFeatureStage {
         WorldCheckFeaturePlacement.recordPlacementPass();
     }
 
+    private static Set<String> hostBiomeKeys(Engine engine) {
+        Set<String> keys = new LinkedHashSet<>();
+        for (IrisBiome irisBiome : engine.getDimension().getReachableBiomes(engine)) {
+            addKey(keys, irisBiome.getStructureDerivativeKey());
+            addKey(keys, irisBiome.getDerivativeKey());
+            for (String scatter : irisBiome.getBiomeScatter()) {
+                addKey(keys, scatter);
+            }
+            for (String scatter : irisBiome.getBiomeSkyScatter()) {
+                addKey(keys, scatter);
+            }
+            if (!irisBiome.isCustom()) {
+                continue;
+            }
+            for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
+                addKey(keys, ModdedWorldgenIds.biomeRef(engine, customBiome.getId()));
+            }
+        }
+        return keys;
+    }
+
+    private static void addKey(Set<String> keys, String key) {
+        String normalized = normalizeKey(key);
+        if (normalized != null) {
+            keys.add(normalized);
+        }
+    }
+
+    private static Predicate<BlockPos> importedFeatureProtection(
+            IrisStaticObjectLayer staticObjects,
+            DimensionStackContext stackContext,
+            int minimumY
+    ) {
+        Map<Long, DimensionStackLayout> layouts = new ConcurrentHashMap<>();
+        return position -> {
+            if (!staticObjects.isEmpty() && staticObjects.contains(
+                    position.getX(), position.getY() - minimumY, position.getZ())) {
+                return true;
+            }
+            if (stackContext == null) {
+                return false;
+            }
+            long columnKey = ((long) position.getX() << 32)
+                    ^ (position.getZ() & 0xFFFFFFFFL);
+            DimensionStackLayout layout = layouts.computeIfAbsent(
+                    columnKey,
+                    ignored -> stackContext.sample(position.getX(), position.getZ())
+            );
+            return layout.isHostFeatureProtectedY(position.getY() - minimumY);
+        };
+    }
+
     private void placeStep(WorldGenLevel level, FeatureTable table, FeatureSorter.StepFeatureData stepData,
                            Registry<PlacedFeature> featureRegistry, Set<Holder<Biome>> chunkBiomes,
                            IrisModdedChunkGenerator owner, WorldgenRandom random, long decorationSeed,
@@ -394,12 +468,37 @@ final class ModdedImportedFeatureStage {
      * shape as vanilla, which drops any section biome its biome source does not claim. Holders are canonicalised
      * back onto the table's own holders so the index mapping cannot be handed a holder it never saw.
      */
-    private Set<Holder<Biome>> chunkBiomes(WorldGenLevel level, SectionPos sectionPos, FeatureTable table) {
+    private Set<Holder<Biome>> chunkBiomes(
+            WorldGenLevel level,
+            SectionPos sectionPos,
+            FeatureTable table,
+            DimensionStackContext stackContext
+    ) {
         List<Holder<Biome>> collected = new ArrayList<>();
         ChunkPos.rangeClosed(sectionPos.chunk(), 1).forEach((ChunkPos chunkPos) -> {
             ChunkAccess neighbour = level.getChunk(chunkPos.x(), chunkPos.z());
-            for (LevelChunkSection section : neighbour.getSections()) {
+            LevelChunkSection[] sections = neighbour.getSections();
-                section.getBiomes().getAll(collected::add);
+            for (int sectionIndex = 0; sectionIndex < sections.length; sectionIndex++) {
+                LevelChunkSection section = sections[sectionIndex];
+                if (stackContext == null) {
+                    section.getBiomes().getAll(collected::add);
+                    continue;
+                }
+                int sectionMinimumY = neighbour.getSectionYFromSectionIndex(sectionIndex) << 4;
+                for (int quartY = 0; quartY < 4; quartY++) {
+                    int internalY = sectionMinimumY + (quartY << 2) - level.getMinY();
+                    for (int quartX = 0; quartX < 4; quartX++) {
+                        int blockX = chunkPos.getMinBlockX() + (quartX << 2);
+                        for (int quartZ = 0; quartZ < 4; quartZ++) {
+                            int blockZ = chunkPos.getMinBlockZ() + (quartZ << 2);
+                            DimensionStackLayout.Layer layer = stackContext.getLayerAt(
+                                    blockX, internalY, blockZ);
+                            if (layer.terrainContext().isSelfReferencing()) {
+                                collected.add(section.getBiomes().get(quartX, quartY, quartZ));
+                            }
+                        }
+                    }
+                }
             }
         });
         Set<Holder<Biome>> present = new LinkedHashSet<>();

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedNativeStructureStage.java

@@ -18,6 +18,8 @@
 
 package art.arcane.iris.modded;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.IrisStructureLocator;
 import art.arcane.iris.engine.framework.NativeStructureGenerationPolicy;
@@ -78,6 +80,7 @@ import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.BiConsumer;
 import java.util.function.IntBinaryOperator;
+import java.util.function.Predicate;
 
 /**
  * Native (vanilla registry) structure stage for {@link IrisModdedChunkGenerator}. The generator keeps the
@@ -266,7 +269,7 @@ final class ModdedNativeStructureStage {
                         undergroundStep,
                         decision.preserveSourceY(),
                         decision.yBand(),
-                        (x, z) -> current.getHeight(x, z, true) + current.getMinHeight());
+                        (x, z) -> Engine.hostHeight(current, x, z, true) + current.getMinHeight());
                 StructureStart wrapped = NativeStructureReferenceEnvelope.wrapForPublication(
                         start, structure, start.getReferences(),
                         NativeStructureTerrainIntegrator.resolveNativeTerrain(start, decision.terrain()),
@@ -367,10 +370,14 @@ final class ModdedNativeStructureStage {
                     chunkPos.x(), chunkPos.z(), error);
         }
         IrisStaticObjectLayer staticObjects = current.getDimension().getStaticObjectLayer(current.getData());
-        int staticMinY = current.getMinHeight();
+        Predicate<BlockPos> protectedPosition = nativeStructureProtection(current, staticObjects);
-        WorldGenLevel boundedWorld = staticObjects.isEmpty() ? world : ModdedNativeStructureWorldgenAccess.create(
+        WorldGenLevel boundedWorld = staticObjects.isEmpty()
+                && current.getDimensionStackContext() == null
+                ? world
+                : ModdedNativeStructureWorldgenAccess.create(
                 world, chunkPos, worldgenSurfaceHeight(current, world.getMinY()), worldgenFloorHeight(current, world.getMinY()),
-                position -> staticObjects.contains(position.getX(), position.getY() - staticMinY, position.getZ()));
+                current.getDimensionStackContext() != null,
+                protectedPosition);
         try {
             NativeStructureVegetationClearer.clearIntersectingVegetation(
                     boundedWorld, chunk, area, vegetationTargets);
@@ -383,7 +390,7 @@ final class ModdedNativeStructureStage {
         try {
             vacuumFoundationPlan = NativeStructureSurfaceFitter.prepareSurfaceStructures(
                     boundedWorld, area, terrainTargets,
-                    (x, z) -> current.getHeight(x, z, true) + current.getMinHeight());
+                    (x, z) -> Engine.hostHeight(current, x, z, true) + current.getMinHeight());
         } catch (Throwable error) {
             throw NativeStructureGenerationException.failure(
                     "terrain integration", nativeStructureBatchContext(placementGroups),
@@ -443,18 +450,50 @@ final class ModdedNativeStructureStage {
                                        IrisNativeStructureDecision decision) {
         Engine current = generator.engine();
         WorldGenLevel boundedWorld = world instanceof ModdedNativeStructureWorldgenAccess ? world : ModdedNativeStructureWorldgenAccess.create(
-                world, chunkPos, worldgenSurfaceHeight(current, world.getMinY()), worldgenFloorHeight(current, world.getMinY()), position -> false);
+                world,
+                chunkPos,
+                worldgenSurfaceHeight(current, world.getMinY()),
+                worldgenFloorHeight(current, world.getMinY()),
+                current.getDimensionStackContext() != null,
+                nativeStructureProtection(
+                        current,
+                        current.getDimension().getStaticObjectLayer(current.getData())));
         world.setCurrentlyGenerating(() -> "Iris native structure " + structureId);
         try {
             NativeStructurePostProcessor.place(
                     boundedWorld, structureManager, generator, random, area, chunkPos,
                     structureId, start, decision, this::resolvePaletteBlock,
-                    (x, z) -> current.getHeight(x, z, true) + current.getMinHeight());
+                    (x, z) -> Engine.hostHeight(current, x, z, true) + current.getMinHeight());
         } finally {
             world.setCurrentlyGenerating(null);
         }
     }
 
+    private static Predicate<BlockPos> nativeStructureProtection(
+            Engine engine,
+            IrisStaticObjectLayer staticObjects
+    ) {
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        int minimumY = engine.getMinHeight();
+        Map<Long, DimensionStackLayout> layouts = new ConcurrentHashMap<>();
+        return position -> {
+            if (!staticObjects.isEmpty() && staticObjects.contains(
+                    position.getX(), position.getY() - minimumY, position.getZ())) {
+                return true;
+            }
+            if (stackContext == null) {
+                return false;
+            }
+            long columnKey = ((long) position.getX() << 32)
+                    ^ (position.getZ() & 0xFFFFFFFFL);
+            DimensionStackLayout layout = layouts.computeIfAbsent(
+                    columnKey,
+                    ignored -> stackContext.sample(position.getX(), position.getZ())
+            );
+            return layout.isHostFeatureProtectedY(position.getY() - minimumY);
+        };
+    }
+
     private List<List<Structure>> structuresByStep(Registry<Structure> registry) {
         StructureStepCache cached = structureStepCache;
         if (cached != null && cached.registry() == registry) {
@@ -523,11 +562,11 @@ final class ModdedNativeStructureStage {
     }
 
     private IntBinaryOperator worldgenSurfaceHeight(Engine generationEngine, int runtimeMinY) {
-        return (x, z) -> generationEngine.getHeight(x, z, false) + runtimeMinY + 1;
+        return (x, z) -> Engine.hostHeight(generationEngine, x, z, false) + runtimeMinY + 1;
     }
 
     private IntBinaryOperator worldgenFloorHeight(Engine generationEngine, int runtimeMinY) {
-        return (x, z) -> generationEngine.getHeight(x, z, true) + runtimeMinY + 1;
+        return (x, z) -> Engine.hostHeight(generationEngine, x, z, true) + runtimeMinY + 1;
     }
 
     private record NativeStructureStartKey(String structureId, long chunkPosition) {

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedNativeStructureWorldgenAccess.java

@@ -63,6 +63,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
     private final ChunkPos generationCenter;
     private final IntBinaryOperator surfaceFirstFreeY;
     private final IntBinaryOperator floorFirstFreeY;
+    private final boolean virtualizeHeights;
     private final Predicate<BlockPos> protectedPosition;
     private final Holder<Biome> fallbackBiome;
     private final BiomeManager biomeManager;
@@ -80,6 +81,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
                 boundary.surfaceFirstFreeY(), "Native structure world access requires a surface resolver");
         this.floorFirstFreeY = Objects.requireNonNull(
                 boundary.floorFirstFreeY(), "Native structure world access requires an ocean-floor resolver");
+        this.virtualizeHeights = boundary.virtualizeHeights();
         this.protectedPosition = Objects.requireNonNull(
                 boundary.protectedPosition(), "World access requires a protected-position predicate");
         BlockPos biomeSample = generationCenter.getMiddleBlockPosition(delegate.getSeaLevel());
@@ -102,9 +104,10 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
     static ModdedNativeStructureWorldgenAccess create(WorldGenLevel delegate, ChunkPos generationCenter,
                                                 IntBinaryOperator surfaceFirstFreeY,
                                                 IntBinaryOperator floorFirstFreeY,
+                                                boolean virtualizeHeights,
                                                 Predicate<BlockPos> protectedPosition) {
         return new ModdedNativeStructureWorldgenAccess(delegate, new Boundary(
-                generationCenter, surfaceFirstFreeY, floorFirstFreeY, protectedPosition));
+                generationCenter, surfaceFirstFreeY, floorFirstFreeY, virtualizeHeights, protectedPosition));
     }
 
     @Override
@@ -241,7 +244,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
 
     @Override
     public int getHeight(Heightmap.Types type, int x, int z) {
-        if (isInsideGenerationRegion(x >> 4, z >> 4)) {
+        if (!virtualizeHeights && isInsideGenerationRegion(x >> 4, z >> 4)) {
             return delegate.getHeight(type, x, z);
         }
         return height(type, x, z);
@@ -526,6 +529,7 @@ final class ModdedNativeStructureWorldgenAccess implements WorldGenLevel {
     private record Boundary(ChunkPos generationCenter,
                             IntBinaryOperator surfaceFirstFreeY,
                             IntBinaryOperator floorFirstFreeY,
+                            boolean virtualizeHeights,
                             Predicate<BlockPos> protectedPosition) {
     }
 

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedScheduler.java

@@ -108,12 +108,17 @@ public final class ModdedScheduler implements PlatformScheduler {
         scheduler.drain();
     }
 
+    public static boolean isMainThread() {
+        Thread running = mainThread;
+        return running != null && Thread.currentThread() == running;
+    }
+
     @Override
     public void global(Runnable task) {
         if (task == null) {
             return;
         }
-        if (onMainThread()) {
+        if (isMainThread()) {
             runGuarded(task);
             return;
         }
@@ -250,11 +255,6 @@ public final class ModdedScheduler implements PlatformScheduler {
         }
     }
 
-    private boolean onMainThread() {
-        Thread main = mainThread;
-        return main != null && Thread.currentThread() == main;
-    }
-
     private void runGuarded(Runnable task) {
         try {
             task.run();

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedSpawnTableMerger.java

@@ -18,10 +18,14 @@
 
 package art.arcane.iris.modded;
 
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.GenerationSessionLease;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeCustom;
+import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.util.project.context.IrisContext;
 import net.minecraft.core.Holder;
 import net.minecraft.core.Registry;
@@ -31,32 +35,24 @@ import net.minecraft.world.entity.MobCategory;
 import net.minecraft.world.level.biome.Biome;
 import net.minecraft.world.level.biome.MobSpawnSettings;
 
-import java.util.Locale;
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
 
-/**
- * Vanilla-derivative spawn table state for {@link IrisModdedChunkGenerator}. Every mutator locks the
- * generator monitor because the repoint/bind/reset paths already hold it while resetting this state.
- */
 final class ModdedSpawnTableMerger {
     private final IrisModdedChunkGenerator generator;
-    private final ConcurrentHashMap<Biome, Holder<Biome>> vanillaSpawnBiomes = new ConcurrentHashMap<>();
+    private volatile SpawnTableState state = new SpawnTableState();
-    private final ConcurrentHashMap<SpawnTableKey, WeightedList<MobSpawnSettings.SpawnerData>> mergedSpawnTables = new ConcurrentHashMap<>();
-    private volatile boolean vanillaSpawnBiomesInitialized;
 
     ModdedSpawnTableMerger(IrisModdedChunkGenerator generator) {
         this.generator = generator;
     }
 
     void initializeVanillaSpawnBiomes(Registry<Biome> registry) {
-        // Volatile fast path BEFORE the monitor: this runs per mob category per spawn attempt
+        SpawnTableState active = state;
-        // on the server thread, and the generator monitor is contended by binds/hotloads.
+        if (active.initialized) {
-        if (vanillaSpawnBiomesInitialized) {
             return;
         }
-        synchronized (generator) {
+        synchronized (active) {
-            if (vanillaSpawnBiomesInitialized) {
+            if (active != state || active.initialized) {
                 return;
             }
             Engine current = generator.engineOrNull();
@@ -66,29 +62,70 @@ final class ModdedSpawnTableMerger {
 
             try (GenerationSessionLease lease = generator.requireGenerationLease(current, "modded_spawn_biomes");
                  IrisContext.Scope ignored = IrisContext.open(current, lease.sessionId(), null)) {
-                String namespace = current.getDimension().getLoadKey().toLowerCase(Locale.ROOT);
+                IrisDimension hostDimension = current.getDimension();
-                for (IrisBiome irisBiome : current.getDimension().getReachableBiomes(current)) {
+                addVanillaSpawnBiomes(
-                    if (irisBiome == null || !irisBiome.isCustom()) {
+                        registry,
-                        continue;
+                        hostDimension,
-                    }
+                        current.getData(),
-                    Holder<Biome> vanillaHolder = resolveBiomeHolder(registry, irisBiome.getVanillaDerivativeKey());
+                        hostDimension.getReachableBiomes(current),
-                    if (vanillaHolder == null) {
+                        active
-                        continue;
+                );
-                    }
+                DimensionStackContext stackContext = current.getDimensionStackContext();
-                    for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
+                if (stackContext != null) {
-                        Holder<Biome> customHolder = resolveBiomeHolder(registry, namespace + ":" + customBiome.getId());
+                    for (DimensionTerrainContext terrainContext : stackContext.getLayersBottomToTop()) {
-                        if (customHolder != null) {
+                        if (terrainContext.isSelfReferencing()) {
-                            vanillaSpawnBiomes.putIfAbsent(customHolder.value(), vanillaHolder);
+                            continue;
                         }
+                        IrisDimension dimension = terrainContext.getDimension();
+                        addVanillaSpawnBiomes(
+                                registry,
+                                dimension,
+                                terrainContext.getData(),
+                                dimension.getReachableBiomes(terrainContext),
+                                active
+                        );
                     }
                 }
-                vanillaSpawnBiomesInitialized = true;
+                active.initialized = true;
+            }
+        }
+    }
+
+    private void addVanillaSpawnBiomes(
+            Registry<Biome> registry,
+            IrisDimension dimension,
+            IrisData data,
+            Iterable<IrisBiome> biomes,
+            SpawnTableState state
+    ) {
+        for (IrisBiome irisBiome : biomes) {
+            if (irisBiome == null || !irisBiome.isCustom()) {
+                continue;
+            }
+            Holder<Biome> vanillaHolder = resolveBiomeHolder(
+                    registry, irisBiome.getVanillaDerivativeKey());
+            if (vanillaHolder == null) {
+                continue;
+            }
+            for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
+                Holder<Biome> customHolder = resolveBiomeHolder(
+                        registry,
+                        ModdedWorldgenIds.biomeRef(
+                                data.getDataFolder().getName(),
+                                dimension.getLoadKey(),
+                                customBiome.getId()
+                        )
+                );
+                if (customHolder == null) {
+                    continue;
+                }
+                state.vanillaSpawnBiomes.putIfAbsent(customHolder.value(), vanillaHolder);
             }
         }
     }
 
     Holder<Biome> vanillaSpawnBiome(Biome biome) {
-        return vanillaSpawnBiomes.get(biome);
+        return state.vanillaSpawnBiomes.get(biome);
     }
 
     WeightedList<MobSpawnSettings.SpawnerData> mergedSpawnTable(
@@ -96,7 +133,8 @@ final class ModdedSpawnTableMerger {
             WeightedList<MobSpawnSettings.SpawnerData> vanillaSpawns,
             WeightedList<MobSpawnSettings.SpawnerData> explicitSpawns) {
         SpawnTableKey key = new SpawnTableKey(biome, category);
-        return mergedSpawnTables.computeIfAbsent(key, ignored -> NativeSpawnTableMerger.merge(vanillaSpawns, explicitSpawns));
+        return state.mergedSpawnTables.computeIfAbsent(
+                key, ignored -> NativeSpawnTableMerger.merge(vanillaSpawns, explicitSpawns));
     }
 
     private Holder<Biome> resolveBiomeHolder(Registry<Biome> registry, String key) {
@@ -112,13 +150,16 @@ final class ModdedSpawnTableMerger {
     }
 
     void resetVanillaSpawnBiomes() {
-        synchronized (generator) {
+        state = new SpawnTableState();
-            vanillaSpawnBiomes.clear();
-            mergedSpawnTables.clear();
-            vanillaSpawnBiomesInitialized = false;
-        }
     }
 
     private record SpawnTableKey(Biome biome, MobCategory category) {
     }
+
+    private static final class SpawnTableState {
+        private final ConcurrentHashMap<Biome, Holder<Biome>> vanillaSpawnBiomes = new ConcurrentHashMap<>();
+        private final ConcurrentHashMap<SpawnTableKey, WeightedList<MobSpawnSettings.SpawnerData>> mergedSpawnTables =
+                new ConcurrentHashMap<>();
+        private volatile boolean initialized;
+    }
 }

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/ModdedEditCommands.java

@@ -84,7 +84,8 @@ final class ModdedEditCommands {
             }
             BlockPos pos = player.blockPosition();
             try {
-                region = engine.getRegion(pos.getX(), pos.getZ());
+                region = engine.getRegion(
+                        pos.getX(), pos.getY() - engine.getMinHeight(), pos.getZ());
             } catch (Throwable e) {
                 IrisModdedCommands.fail(source, IrisLanguage.plain(ModdedCommandMessages.IRIS_MODDED_COMMANDS_REGION_LOOKUP_FAILED, MessageArgument.untrusted("value", e.getClass().getSimpleName())));
                 return 0;

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/ModdedStudioCommands.java

@@ -73,6 +73,8 @@ import org.zeroturnaround.zip.ZipUtil;
 import java.awt.Desktop;
 import java.io.File;
 import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -85,6 +87,7 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Predicate;
 import java.util.regex.Pattern;
+import java.util.stream.Stream;
 
 import art.arcane.iris.core.localization.IrisLanguage;
 import art.arcane.iris.core.localization.ModdedCommandMessages;
@@ -891,21 +894,30 @@ public final class ModdedStudioCommands {
         LinkedHashSet<String> objectKeys = new LinkedHashSet<>();
         LinkedHashSet<String> markerKeys = new LinkedHashSet<>();
         LinkedHashSet<String> structureKeys = new LinkedHashSet<>();
+        List<IrisDimension> dimensions = new ArrayList<>();
 
-        dimension.getAllRegions(() -> dm).forEach((IrisRegion region) -> {
+        for (String dimensionKey : PackExportClosure.collectDimensionKeys(dimension).stream().sorted().toList()) {
-            if (region != null && region.getLoadKey() != null) {
+            IrisDimension exportedDimension = dm.getDimensionLoader().load(dimensionKey);
-                regionKeys.add(region.getLoadKey());
+            if (exportedDimension != null) {
+                dimensions.add(exportedDimension);
             }
-        });
+        }
-        dimension.getReachableBiomes(() -> dm).forEach((IrisBiome biome) -> {
+        for (IrisDimension exportedDimension : dimensions) {
-            if (biome != null && biome.getLoadKey() != null) {
+            exportedDimension.getAllRegions(() -> dm).forEach((IrisRegion region) -> {
-                biomeKeys.add(biome.getLoadKey());
+                if (region != null && region.getLoadKey() != null) {
-            }
+                    regionKeys.add(region.getLoadKey());
-        });
+                }
-        lootKeys.addAll(dimension.getLoot().getTables());
+            });
-        spawnerKeys.addAll(dimension.getEntitySpawners());
+            exportedDimension.getReachableBiomes(() -> dm).forEach((IrisBiome biome) -> {
-        collectStructureKeys(structureKeys, dimension.getStructures());
+                if (biome != null && biome.getLoadKey() != null) {
-        objectKeys.addAll(PackExportClosure.collectStaticObjectKeys(dimension.getStaticObjects()));
+                    biomeKeys.add(biome.getLoadKey());
+                }
+            });
+            lootKeys.addAll(exportedDimension.getLoot().getTables());
+            spawnerKeys.addAll(exportedDimension.getEntitySpawners());
+            collectStructureKeys(structureKeys, exportedDimension.getStructures());
+            objectKeys.addAll(PackExportClosure.collectStaticObjectKeys(exportedDimension.getStaticObjects()));
+        }
 
         for (String regionKey : regionKeys) {
             IrisRegion region = dm.getRegionLoader().load(regionKey);
@@ -975,10 +987,17 @@ public final class ModdedStudioCommands {
             }
         }
 
-        hashes.append(copyJson(folder, "dimensions", dimension.getLoadKey(), dm.getDimensionLoader().findFile(dimension.getLoadKey())));
+        for (IrisDimension exportedDimension : dimensions) {
+            String key = exportedDimension.getLoadKey();
+            hashes.append(copyJson(folder, "dimensions", key, dm.getDimensionLoader().findFile(key)));
+        }
         for (String key : generatorKeys) {
             hashes.append(copyJson(folder, "generators", key, dm.getGeneratorLoader().findFile(key)));
         }
+        for (String key : Stream.of(dm.getExpressionLoader().getPossibleKeys()).sorted().toList()) {
+            hashes.append(copyJson(folder, "expressions", key, dm.getExpressionLoader().findFile(key)));
+        }
+        hashes.append(copyStyleSnippets(packFolder, folder));
         for (String key : regionKeys) {
             hashes.append(copyJson(folder, "regions", key, dm.getRegionLoader().findFile(key)));
         }
@@ -1041,6 +1060,25 @@ public final class ModdedStudioCommands {
         }
     }
 
+    private static String copyStyleSnippets(File packFolder, File folder) throws IOException {
+        Path sourceRoot = new File(packFolder, "snippet/style").toPath();
+        if (!Files.isDirectory(sourceRoot)) {
+            return "";
+        }
+        StringBuilder hashes = new StringBuilder();
+        try (Stream<Path> files = Files.walk(sourceRoot)) {
+            for (Path source : files.filter(Files::isRegularFile)
+                    .filter(path -> path.getFileName().toString().endsWith(".json"))
+                    .sorted()
+                    .toList()) {
+                String relative = sourceRoot.relativize(source).toString().replace(File.separatorChar, '/');
+                String key = relative.substring(0, relative.length() - ".json".length());
+                hashes.append(copyJson(folder, "snippet/style", key, source.toFile()));
+            }
+        }
+        return hashes.toString();
+    }
+
     private static int regions(CommandSourceStack source, int radius) {
         ServerPlayer player = source.getPlayer();
         if (player == null) {

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/ModdedWhatCommands.java

@@ -216,7 +216,8 @@ public final class ModdedWhatCommands {
         int centerX = (pos.getX() & ~15) + 8;
         int centerZ = (pos.getZ() & ~15) + 8;
         try {
-            IrisRegion region = engine.getRegion(centerX, centerZ);
+            IrisRegion region = engine.getRegion(
+                    centerX, pos.getY() - engine.getMinHeight(), centerZ);
             IrisModdedCommands.ok(source, IrisLanguage.plain(
                     RuntimeUiMessages.WHAT_IRIS_REGION,
                     MessageArgument.untrusted("region", region.getLoadKey()),

adapters/modded-common/src/main/java/art/arcane/iris/modded/service/ModdedStudioHotloadService.java

@@ -32,6 +32,7 @@ import art.arcane.iris.engine.object.IrisWorldBoundary;
 import art.arcane.iris.modded.IrisModdedChunkGenerator;
 import art.arcane.iris.modded.ModdedDimensionManager;
 import art.arcane.iris.modded.ModdedForcedDatapack;
+import art.arcane.iris.modded.ModdedScheduler;
 import art.arcane.iris.modded.ModdedWorkspaceGenerator;
 import art.arcane.iris.nativegen.NativeStructureVolumeIndex;
 import art.arcane.iris.modded.command.ModdedPregenJob;
@@ -100,6 +101,11 @@ public final class ModdedStudioHotloadService implements ModdedTickableService,
         return NativeStructureVolumeIndex.volumes(engine, minX, minZ, maxX, maxZ);
     }
 
+    @Override
+    public boolean isMainThread() {
+        return ModdedScheduler.isMainThread();
+    }
+
     @Override
     public void refreshDatapackWorkspace(Engine engine) {
         IrisData data = engine.getData();
@@ -121,6 +127,20 @@ public final class ModdedStudioHotloadService implements ModdedTickableService,
                 engine.getDimension(),
                 replacement,
                 "irisworldgen");
+        IrisModdedChunkGenerator.requireStructureBiomeUniverseCompatible(
+                engine.getDimension(), replacement);
+    }
+
+    @Override
+    public void prepareRuntimeHotload(Engine engine) {
+        IrisWorld world = engine.getWorld();
+        if (world == null || !world.hasPlatformWorld()
+                || !(world.platformWorld().nativeHandle() instanceof ServerLevel level)
+                || !(level.getChunkSource().getGenerator() instanceof IrisModdedChunkGenerator generator)) {
+            NativeStructureVolumeIndex.invalidate(engine);
+            return;
+        }
+        generator.prepareRuntimeHotload(level, engine);
     }
 
     @Override
@@ -285,7 +305,6 @@ public final class ModdedStudioHotloadService implements ModdedTickableService,
         long start = System.currentTimeMillis();
         try {
             engine.hotloadSilently();
-            generator.onHotload();
             ModdedIrisLog.info("Iris studio hotload {} pack={} {}ms", dimensionId, engine.getDimension().getLoadKey(), System.currentTimeMillis() - start);
         } catch (Throwable e) {
             ModdedIrisLog.error("Iris studio hotload failed for {}", dimensionId, e);

adapters/modded-common/src/test/java/art/arcane/iris/modded/IrisModdedChunkGeneratorSpawnTest.java

@@ -59,6 +59,7 @@ public class IrisModdedChunkGeneratorSpawnTest {
         String spawn = source.substring(spawnStart, spawnEnd);
 
         assertTrue(spawn.contains("spawnTables.initializeVanillaSpawnBiomes(registry)"));
+        assertTrue(spawn.contains("structureBiomeSource.getVisibleSurfaceBiome("));
         assertTrue(spawn.contains("spawnTables.vanillaSpawnBiome(visibleBiome.value())"));
         assertTrue(spawn.contains("NaturalSpawner.spawnMobsForChunkGeneration("));
         assertTrue(spawn.contains("new LegacyRandomSource(RandomSupport.generateUniqueSeed())"));

adapters/modded-common/src/test/java/art/arcane/iris/modded/IrisModdedStructureParityTest.java

@@ -5,6 +5,7 @@ import art.arcane.iris.engine.object.IrisBiomeCustom;
 import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisRange;
 import art.arcane.iris.engine.object.InferredType;
+import art.arcane.iris.engine.framework.Engine;
 import art.arcane.volmlib.util.collection.KList;
 import art.arcane.volmlib.util.mantle.flag.MantleFlag;
 import net.minecraft.SharedConstants;
@@ -29,6 +30,8 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 public class IrisModdedStructureParityTest {
     @BeforeClass
@@ -110,6 +113,144 @@ public class IrisModdedStructureParityTest {
         assertEquals(expected, IrisModdedBiomeSource.biomeResolutionSeed(worldSeed, blockX, blockY, blockZ));
     }
 
+    @Test
+    public void stackedCustomBiomesUseTheirOwningDimensionNamespace() {
+        IrisDimension host = new IrisDimension();
+        host.setLoadKey("Host");
+        IrisDimension upper = new IrisDimension();
+        upper.setLoadKey("Upper");
+
+        assertEquals(
+                ModdedWorldgenIds.biomeRef("pack", "Upper", "Aurora"),
+                IrisModdedBiomeSource.customBiomeRef("pack", upper, "Aurora")
+        );
+        assertFalse(IrisModdedBiomeSource.customBiomeRef("pack", host, "Aurora")
+                .equals(IrisModdedBiomeSource.customBiomeRef("pack", upper, "Aurora")));
+    }
+
+    @Test
+    public void visibleBiomeResolutionSelectsStackOwnershipBeforeHostCaves() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/IrisModdedBiomeSource.java"));
+        int resolutionStart = source.indexOf("private BiomeResolution resolveBiomeResolution(");
+        int resolutionEnd = source.indexOf("private Holder<Biome> resolveRequiredStructureBiome(", resolutionStart);
+
+        assertTrue(resolutionStart >= 0);
+        assertTrue(resolutionEnd > resolutionStart);
+        String resolution = source.substring(resolutionStart, resolutionEnd);
+
+        assertTrue(resolution.contains("resolveDimensionStackLayer("));
+        assertTrue(resolution.contains("!stackLayer.terrainContext().isSelfReferencing()"));
+        assertTrue(resolution.indexOf("resolveDimensionStackLayer(")
+                < resolution.indexOf("boolean underground"));
+        assertTrue(resolution.contains("includeDimensionStack"));
+        assertTrue(resolution.contains(
+                "return resolveBiomeResolution(engine, quartX, quartY, quartZ, false)"));
+        assertTrue(source.contains("resolution.packName(), resolution.dimension(), customBiome.getId()"));
+        int helperStart = source.indexOf("private static IrisBiome resolveSurfaceStructureBiome(");
+        int helperEnd = source.indexOf("private static Set<String> registeredBiomeKeys(", helperStart);
+        String helper = source.substring(helperStart, helperEnd);
+        assertTrue(helper.contains("engine.getComplex().getTrueBiomeStream().get(blockX, blockZ)"));
+        assertFalse(helper.contains("getDimensionStackContext()"));
+        assertFalse(helper.contains("getSurfaceBiome("));
+    }
+
+    @Test
+    public void stackBiomesAreVisibleButExcludedFromStructureReachability() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/IrisModdedBiomeSource.java"));
+
+        assertTrue(source.contains("return biomeKeySets().structureRequired()"));
+        assertTrue(source.contains("generatedBiomeKeys.addAll(keys.visibleRequired())"));
+        assertTrue(source.contains("collectStructureBiomeKeys("));
+        assertTrue(source.contains("dimension.getReachableBiomes(() -> data)"));
+    }
+
+    @Test
+    public void biomeLocatorSearchesTheVisibleStack() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/IrisModdedBiomeSource.java"));
+        int start = source.indexOf("public Pair<BlockPos, Holder<Biome>> findClosestBiome3d(");
+        int end = source.indexOf("private Holder<Biome> getNoiseBiome(", start);
+        String method = source.substring(start, end);
+
+        assertTrue(method.contains("modded_locate_visible_biome"));
+        assertTrue(method.contains("return super.findClosestBiome3d("));
+        assertTrue(method.contains("collectVisibleBiomeHolders()"));
+        assertTrue(method.contains("QuartPos.toBlock(quartX), QuartPos.toBlock(quartZ)"));
+        assertTrue(method.contains("resolveVisibleBiome("));
+        assertFalse(method.contains("Holder<Biome> structureBiome = getNoiseBiome("));
+        assertTrue(method.indexOf("tryAcquireGenerationLease(engine, \"modded_locate_visible_biome\")")
+                < method.indexOf("engine.getDimensionStackContext()"));
+    }
+
+    @Test
+    public void surfaceSpawnBiomeUsesTheVisibleQuartCell() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/IrisModdedBiomeSource.java"));
+        int start = source.indexOf("Holder<Biome> getVisibleSurfaceBiome(");
+        int end = source.indexOf("boolean isStructureReachable(", start);
+        String method = source.substring(start, end);
+
+        assertTrue(method.contains("QuartPos.fromBlock(blockX)"));
+        assertTrue(method.contains("QuartPos.toBlock(quartX)"));
+        assertTrue(method.contains("QuartPos.fromBlock(internalY + engine.getMinHeight())"));
+        assertTrue(method.contains("stackContext.getLayout(sampleX, sampleZ).surfaceLayer()"));
+        assertTrue(method.contains("visibleBiomeCache"));
+        assertTrue(method.contains("isBiomeCacheable(engine, sampleX, sampleZ)"));
+        assertTrue(method.indexOf("boolean cacheable = isBiomeCacheable(engine, sampleX, sampleZ)")
+                < method.indexOf("Engine.hostHeight(engine, sampleX, sampleZ, true)"));
+        assertCacheGuarded(method, "visibleBiomeCache");
+    }
+
+    @Test
+    public void temporaryNaturalBiomeAnswersAreNotMemoized() throws IOException {
+        Engine engine = mock(Engine.class);
+        when(engine.answersFromNaturalTerrain(12, -8)).thenReturn(true);
+        assertFalse(IrisModdedBiomeSource.isBiomeCacheable(engine, 12, -8));
+        when(engine.answersFromNaturalTerrain(12, -8)).thenReturn(false);
+        assertTrue(IrisModdedBiomeSource.isBiomeCacheable(engine, 12, -8));
+
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/IrisModdedBiomeSource.java"));
+        int structureStart = source.indexOf("private Holder<Biome> getNoiseBiome(Engine engine");
+        int structureEnd = source.indexOf("Holder<Biome> getVisibleNoiseBiome(", structureStart);
+        String structure = source.substring(structureStart, structureEnd);
+        assertTrue(structure.contains("QuartPos.toBlock(quartX)"));
+        assertTrue(structure.contains("QuartPos.toBlock(quartZ)"));
+        assertCacheGuarded(structure, "structureBiomeCache");
+
+        int visibleStart = source.indexOf(
+                "private Holder<Biome> getVisibleNoiseBiomeWithActiveGenerationLease(");
+        int visibleEnd = source.indexOf("Holder<Biome> getVisibleSurfaceBiome(", visibleStart);
+        String visible = source.substring(visibleStart, visibleEnd);
+        assertTrue(visible.contains("QuartPos.toBlock(quartX)"));
+        assertTrue(visible.contains("QuartPos.toBlock(quartZ)"));
+        assertCacheGuarded(visible, "visibleBiomeCache");
+
+        int predicateStart = source.indexOf("static boolean isBiomeCacheable(");
+        int predicateEnd = source.indexOf("static boolean isMonumentSurfaceBiomeQuery(", predicateStart);
+        String predicate = source.substring(predicateStart, predicateEnd);
+        assertTrue(predicate.contains("return !engine.answersFromNaturalTerrain(blockX, blockZ)"));
+        assertFalse(predicate.contains("getDimensionStackContext()"));
+    }
+
+    @Test
+    public void importedFeaturesCollectOnlyHostOwnedStackCells() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/ModdedImportedFeatureStage.java"));
+
+        assertTrue(source.contains("stackContext.getLayerAt("));
+        assertTrue(source.contains("layer.terrainContext().isSelfReferencing()"));
+        assertTrue(source.contains("section.getBiomes().get(quartX, quartY, quartZ)"));
+    }
+
     @Test
     public void configuredBiomeKeysContainOnlyPackDerivativesAndCustomBiomes() {
         IrisBiome ocean = new IrisBiome()
@@ -132,6 +273,21 @@ public class IrisModdedStructureParityTest {
                 "minecraft:the_void", "overworld:aurora"), keys);
         assertFalse(keys.contains("minecraft:desert"));
         assertFalse(keys.contains("minecraft:plains"));
+
+        Set<String> recursiveKeys = ModdedDimensionMetadata.collectConfiguredBiomeKeys(
+                List.of(custom), "Layers/Sky");
+        assertTrue(recursiveKeys.contains("layers:sky/aurora"));
+    }
+
+    @Test
+    public void biomeWriterDerivativeFallbackMatchesCanonicalRecursiveKeys() throws IOException {
+        String source = Files.readString(Path.of(
+                System.getProperty("iris.moddedCommonSources"),
+                "art/arcane/iris/modded/ModdedBiomeWriter.java"));
+
+        assertTrue(source.contains(
+                "key.equalsIgnoreCase(dimension.getCustomBiomeKey(custom.getId()))"));
+        assertFalse(source.contains("String dimensionLoadKey = key.substring(0, colon)"));
     }
 
     @Test
@@ -159,6 +315,19 @@ public class IrisModdedStructureParityTest {
         assertSame(configured, IrisModdedBiomeSource.requireConfiguredStructureBiomeKeys(configured));
     }
 
+    @Test
+    public void liveReplacementRequiresTheSameStructureBiomeUniverse() {
+        IrisModdedChunkGenerator.requireStructureBiomeUniverseCompatible(
+                Set.of("minecraft:plains", "minecraft:beach"),
+                Set.of("minecraft:beach", "minecraft:plains"));
+
+        IllegalArgumentException error = assertThrows(IllegalArgumentException.class,
+                () -> IrisModdedChunkGenerator.requireStructureBiomeUniverseCompatible(
+                        Set.of("minecraft:plains"), Set.of("minecraft:desert")));
+
+        assertTrue(error.getMessage().contains("Restart the server"));
+    }
+
     @Test
     public void configuredDimensionMetadataIsExactBeforeEngineBinding() {
         IrisDimension dimension = new IrisDimension()
@@ -240,4 +409,20 @@ public class IrisModdedStructureParityTest {
         assertSame(MantleFlag.INITIAL_SPAWNED_MARKER, ModdedWorldManager.INITIAL_SPAWN_COMPLETION_FLAG);
     }
 
+    private static void assertCacheGuarded(String method, String cacheName) {
+        int predicate = method.indexOf("boolean cacheable = isBiomeCacheable(");
+        int cache = method.indexOf("BiomeHolderTable cache = " + cacheName, predicate);
+        int read = method.indexOf("cache.get(", predicate);
+        int readGuard = method.lastIndexOf("if (cacheable)", read);
+        int write = method.indexOf("cache.put(", read);
+        int writeGuard = method.lastIndexOf("if (cacheable)", write);
+
+        assertTrue(predicate >= 0);
+        assertTrue(cache >= 0);
+        assertTrue(read > predicate);
+        assertTrue(readGuard >= predicate);
+        assertTrue(write > read);
+        assertTrue(writeGuard > read);
+    }
+
 }

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedNativeStructureWorldgenAccessTest.java

@@ -135,6 +135,24 @@ public class ModdedNativeStructureWorldgenAccessTest {
         assertEquals(1, recording.blockTicks.count());
     }
 
+    @Test
+    public void virtualizedHeightsHideInRegionTerrainFromTheDelegate() {
+        RecordingDelegate recording = new RecordingDelegate();
+        ChunkPos generationCenter = generationCenter();
+        ModdedNativeStructureWorldgenAccess access = ModdedNativeStructureWorldgenAccess.create(
+                recording.world(), generationCenter,
+                (x, z) -> SURFACE_FIRST_FREE_Y,
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                true,
+                position -> false);
+        int x = generationCenter.getMiddleBlockX();
+        int z = generationCenter.getMiddleBlockZ();
+
+        assertEquals(SURFACE_FIRST_FREE_Y, access.getHeight(Heightmap.Types.WORLD_SURFACE_WG, x, z));
+        assertEquals(FLOOR_FIRST_FREE_Y, access.getHeight(Heightmap.Types.OCEAN_FLOOR_WG, x, z));
+        assertEquals(0, recording.heightReads);
+    }
+
     @Test
     public void protectedCellsRemainReadableButRejectWritesTilesAndTicks() {
         RecordingDelegate recording = new RecordingDelegate();
@@ -142,7 +160,9 @@ public class ModdedNativeStructureWorldgenAccessTest {
         ModdedNativeStructureWorldgenAccess access = ModdedNativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y, position::equals);
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                false,
+                position::equals);
 
         assertSame(Blocks.DIRT, access.getBlockState(position).getBlock());
         assertFalse(access.ensureCanWrite(position));
@@ -211,7 +231,9 @@ public class ModdedNativeStructureWorldgenAccessTest {
         return ModdedNativeStructureWorldgenAccess.create(
                 recording.world(), generationCenter(),
                 (x, z) -> SURFACE_FIRST_FREE_Y,
-                (x, z) -> FLOOR_FIRST_FREE_Y, position -> false);
+                (x, z) -> FLOOR_FIRST_FREE_Y,
+                false,
+                position -> false);
     }
 
     private static ChunkPos generationCenter() {

adapters/modded-common/src/test/java/art/arcane/iris/modded/NativeStructureFailureContractTest.java

@@ -95,8 +95,9 @@ public class NativeStructureFailureContractTest {
         assertTrue(placement.contains("heightmapStarts.add(start);"));
         assertTrue(placement.indexOf("WorldgenTerrainHeightmaps.primeStructurePlacement(")
                 < placement.indexOf("prepareSurfaceStructures"));
-        assertTrue(source.contains("generationEngine.getHeight(x, z, false) + runtimeMinY + 1"));
+        assertTrue(source.contains("Engine.hostHeight(generationEngine, x, z, false) + runtimeMinY + 1"));
-        assertTrue(source.contains("generationEngine.getHeight(x, z, true) + runtimeMinY + 1"));
+        assertTrue(source.contains("Engine.hostHeight(generationEngine, x, z, true) + runtimeMinY + 1"));
+        assertTrue(placement.contains("current.getDimensionStackContext() != null"));
         assertTrue(source.contains("int minY = chunk.getMinY() + 1;"));
     }
 
@@ -108,6 +109,8 @@ public class NativeStructureFailureContractTest {
         String placement = source.substring(placementStart, placementEnd);
 
         assertTrue(placement.contains("ModdedNativeStructureWorldgenAccess.create("));
+        assertTrue(placement.contains("nativeStructureProtection(current, staticObjects)"));
+        assertTrue(placement.contains("Engine.hostHeight(current, x, z, true)"));
         assertTrue(placement.contains("placeVanillaStructure(boundedWorld, structureManager"));
         assertFalse(placement.contains("NativeStructurePostProcessor.place(world,"));
         assertTrue(placement.contains("world.setCurrentlyGenerating(null);"));

adapters/modded-common/src/test/java/art/arcane/iris/modded/service/ModdedStudioHotloadServiceTest.java

@@ -1,10 +1,12 @@
 package art.arcane.iris.modded.service;
 
 import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.modded.ModdedScheduler;
 import art.arcane.volmlib.util.collection.KList;
 import org.junit.Test;
 
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -26,4 +28,21 @@ public class ModdedStudioHotloadServiceTest {
         assertFalse(ModdedStudioHotloadService.hasDatapackImports(null));
         assertFalse(ModdedStudioHotloadService.hasDatapackImports(List.of(empty)));
     }
+
+    @Test
+    public void reportsTheCapturedServerThread() throws InterruptedException {
+        ModdedScheduler scheduler = new ModdedScheduler();
+        scheduler.reset();
+        try {
+            ModdedStudioHotloadService service = new ModdedStudioHotloadService();
+            assertTrue(service.isMainThread());
+            AtomicBoolean offThread = new AtomicBoolean(true);
+            Thread worker = new Thread(() -> offThread.set(service.isMainThread()));
+            worker.start();
+            worker.join();
+            assertFalse(offThread.get());
+        } finally {
+            scheduler.shutdown();
+        }
+    }
 }

core/src/main/java/art/arcane/iris/core/IrisDatapackCompiler.java

@@ -222,9 +222,45 @@ public final class IrisDatapackCompiler {
                 new LinkedHashSet<>(),
                 requiredDimension,
                 requiredFixer);
+        collectDimensionStackRegistryRequirements(
+                requirements,
+                requiredDimension,
+                requiredFixer);
         return Map.copyOf(requirements);
     }
 
+    private static void collectDimensionStackRegistryRequirements(
+            Map<String, String> requirements,
+            IrisDimension hostDimension,
+            IDataFixer fixer
+    ) throws IOException {
+        if (!hostDimension.hasDimensionStack()) {
+            return;
+        }
+        IrisData data = hostDimension.getLoader();
+        if (data == null) {
+            throw new IOException("Dimension stack host '" + hostDimension.getLoadKey()
+                    + "' has no pack loader");
+        }
+        LinkedHashSet<String> visited = new LinkedHashSet<>();
+        visited.add(hostDimension.getLoadKey());
+        for (String dimensionKey : hostDimension.getDimensionStack().getDimensions()) {
+            if (dimensionKey == null || dimensionKey.isBlank() || !visited.add(dimensionKey)) {
+                continue;
+            }
+            IrisDimension sourceDimension = data.getDimensionLoader().load(dimensionKey);
+            if (sourceDimension == null) {
+                throw new IOException("Dimension stack host '" + hostDimension.getLoadKey()
+                        + "' references missing dimension '" + dimensionKey + "'");
+            }
+            collectRegistryRequirements(
+                    requirements,
+                    new LinkedHashSet<>(),
+                    sourceDimension,
+                    fixer);
+        }
+    }
+
     private static void collectRegistryRequirements(
             Map<String, String> requirements,
             Set<String> installedBiomeKeys,
@@ -235,7 +271,6 @@ public final class IrisDatapackCompiler {
                 "dimension_type/iris:" + dimension.getDimensionTypeKey(),
                 fingerprintContent(dimension.getDimensionType().toJson(fixer)));
 
-        String namespace = dimension.getLoadKey().toLowerCase(Locale.ROOT);
         ContentGate contentGate = dimension.getLoader() == null ? null : dimension.getLoader().getContentGate();
         for (IrisBiome biome : dimension.getAllBiomes(dimension::getLoader)) {
             if (biome == null || !biome.isCustom()) {
@@ -246,7 +281,7 @@ public final class IrisDatapackCompiler {
                 if (customBiome == null) {
                     continue;
                 }
-                String biomeKey = namespace + ":" + customBiome.getId();
+                String biomeKey = dimension.getCustomBiomeKey(customBiome.getId());
                 if (!installedBiomeKeys.add(biomeKey)) {
                     continue;
                 }
@@ -533,7 +568,7 @@ public final class IrisDatapackCompiler {
                 || !Files.isDirectory(dimensions, LinkOption.NOFOLLOW_LINKS)) {
             return false;
         }
-        try (Stream<Path> stream = Files.list(dimensions)) {
+        try (Stream<Path> stream = Files.walk(dimensions)) {
             return stream.anyMatch(path -> !Files.isSymbolicLink(path)
                     && Files.isRegularFile(path, LinkOption.NOFOLLOW_LINKS)
                     && path.getFileName().toString().endsWith(".json"));

core/src/main/java/art/arcane/iris/core/ServerConfigurator.java

@@ -1121,7 +1121,7 @@ public class ServerConfigurator {
         for (IrisBiome i : dimension.getAllBiomes(dimension::getLoader)) {
             if (i.isCustom()) {
                 for (IrisBiomeCustom j : i.getCustomDerivitives()) {
-                    keys.add(dimension.getLoadKey() + ":" + j.getId());
+                    keys.add(dimension.getCustomBiomeKey(j.getId()));
                 }
             }
         }

core/src/main/java/art/arcane/iris/core/link/data/MythicMobsDataProvider.java

@@ -134,7 +134,8 @@ public class MythicMobsDataProvider extends ExternalDataProvider implements List
             if (access == null) return false;
             Engine engine = access.getEngine();
             if (engine == null) return false;
-            IrisRegion region = engine.getRegion(target.getBlockX(), target.getBlockZ());
+            IrisRegion region = engine.getRegion(
+                    target.getBlockX(), target.getBlockY() - engine.getMinHeight(), target.getBlockZ());
             return regions.contains(region.getLoadKey());
         }
     }

core/src/main/java/art/arcane/iris/core/nms/INMSBinding.java

@@ -156,6 +156,9 @@ public interface INMSBinding {
         return NativeStructureVolume.NONE;
     }
 
+    default void invalidateNativeStructureVolumeIndex(Engine engine) {
+    }
+
     int getBiomeId(Biome biome);
 
     MCABiomeContainer newBiomeContainer(int min, int max, int[] data);

core/src/main/java/art/arcane/iris/core/pack/PackDimensionValidator.java

@@ -26,7 +26,9 @@ import art.arcane.volmlib.util.json.JSONObject;
 import java.io.File;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
+import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 
 final class PackDimensionValidator {
     private PackDimensionValidator() {
@@ -35,9 +37,14 @@ final class PackDimensionValidator {
     static void validateDimensions(File packFolder, File[] dimensionFiles, List<String> blockingErrors, List<String> warnings) {
         File regionsFolder = new File(packFolder, "regions");
         File biomesFolder = new File(packFolder, "biomes");
+        File dimensionsFolder = new File(packFolder, PackValidator.DIMENSIONS_FOLDER);
+        Set<String> dimensionKeys = new LinkedHashSet<>();
+        for (File dimensionFile : dimensionFiles) {
+            dimensionKeys.add(PackValidationIo.deriveKey(dimensionsFolder, dimensionFile));
+        }
 
         for (File dimFile : dimensionFiles) {
-            String dimensionKey = PackValidationIo.stripExtension(dimFile.getName());
+            String dimensionKey = PackValidationIo.deriveKey(dimensionsFolder, dimFile);
             JSONObject dimJson;
             try {
                 dimJson = new JSONObject(Files.readString(dimFile.toPath(), StandardCharsets.UTF_8));
@@ -50,6 +57,7 @@ final class PackDimensionValidator {
             validateDimensionHeights(packFolder, dimensionKey, dimJson, blockingErrors);
             validateWorldBoundary(dimensionKey, dimJson, blockingErrors);
             validateStaticObjects(packFolder, dimensionKey, dimJson, blockingErrors);
+            validateDimensionStack(packFolder, dimensionKey, dimJson, dimensionKeys, blockingErrors);
 
             JSONArray regionsArray = dimJson.optJSONArray("regions");
             if (regionsArray == null || regionsArray.length() == 0) {
@@ -94,6 +102,108 @@ final class PackDimensionValidator {
         }
     }
 
+    static void validateDimensionStack(File packFolder, String dimensionKey, JSONObject dimension, Set<String> dimensionKeys,
+                                       List<String> blockingErrors) {
+        if (!dimension.has("dimensionStack")) {
+            return;
+        }
+        String context = "Dimension '" + dimensionKey + "' dimensionStack";
+        JSONObject stack = dimension.optJSONObject("dimensionStack");
+        if (stack == null) {
+            blockingErrors.add(context + " must be an object.");
+            return;
+        }
+
+        String upperDimension = dimension.optString("upperDimension", "none");
+        if (upperDimension != null && !upperDimension.isEmpty()
+                && !upperDimension.equalsIgnoreCase("none")) {
+            blockingErrors.add(context + " cannot be used with upperDimension.");
+        }
+
+        JSONArray dimensions = stack.optJSONArray("dimensions");
+        if (dimensions == null) {
+            blockingErrors.add(context + ".dimensions must be an array.");
+        } else {
+            int hostOccurrences = 0;
+            if (dimensions.length() < 2) {
+                blockingErrors.add(context + ".dimensions must contain at least two dimension keys.");
+            }
+            for (int index = 0; index < dimensions.length(); index++) {
+                Object rawKey = dimensions.opt(index);
+                if (!(rawKey instanceof String key) || key.isBlank()) {
+                    blockingErrors.add(context + ".dimensions[" + index + "] must be a nonblank dimension key.");
+                    continue;
+                }
+                if (!dimensionKeys.contains(key)) {
+                    blockingErrors.add(context + ".dimensions[" + index + "] references missing dimension '"
+                            + key + "'.");
+                }
+                if (dimensionKey.equals(key)) {
+                    hostOccurrences++;
+                }
+            }
+            if (hostOccurrences != 1) {
+                blockingErrors.add(context + ".dimensions must contain its host dimension key '"
+                        + dimensionKey + "' exactly once.");
+            }
+            if (dimensions.length() > 0) {
+                Object rawLast = dimensions.opt(dimensions.length() - 1);
+                if (rawLast instanceof String lastKey && !dimensionKey.equals(lastKey)) {
+                    blockingErrors.add(context + ".dimensions must end with its host dimension key '"
+                            + dimensionKey + "'.");
+                }
+            }
+        }
+
+        validateInteger(stack, "spacer", 0, 256, context, blockingErrors);
+        validateDimensionStackBlend(packFolder, context, stack, blockingErrors);
+    }
+
+    private static void validateDimensionStackBlend(File packFolder, String context, JSONObject stack,
+                                                    List<String> blockingErrors) {
+        if (!stack.has("blend")) {
+            return;
+        }
+        JSONObject blend = stack.optJSONObject("blend");
+        if (blend == null) {
+            blockingErrors.add(context + ".blend must be an object.");
+            return;
+        }
+        if (blend.has("amplitude")) {
+            validateInteger(blend, "amplitude", 0, 256, context + ".blend", blockingErrors);
+        }
+        if (blend.has("style")) {
+            Object rawStyle = blend.opt("style");
+            boolean styleObject = rawStyle instanceof JSONObject;
+            boolean styleSnippet = rawStyle instanceof String reference
+                    && reference.startsWith("snippet/style/");
+            if (!styleObject && !styleSnippet) {
+                blockingErrors.add(context + ".blend.style must be an object or style snippet reference.");
+                return;
+            }
+            if (styleSnippet) {
+                validateDimensionStackStyleSnippet(
+                        packFolder, context + ".blend.style", (String) rawStyle, blockingErrors);
+            }
+        }
+    }
+
+    private static void validateDimensionStackStyleSnippet(File packFolder, String context, String reference,
+                                                           List<String> blockingErrors) {
+        try {
+            File snippetRoot = new File(packFolder, "snippet/style").getCanonicalFile();
+            File snippetFile = new File(packFolder, reference + ".json").getCanonicalFile();
+            if (!snippetFile.toPath().startsWith(snippetRoot.toPath()) || !snippetFile.isFile()) {
+                blockingErrors.add(context + " references missing style snippet '" + reference + "'.");
+                return;
+            }
+            new JSONObject(Files.readString(snippetFile.toPath(), StandardCharsets.UTF_8));
+        } catch (Throwable error) {
+            blockingErrors.add(context + " references invalid style snippet '" + reference + "': "
+                    + error.getMessage());
+        }
+    }
+
     static void validateStaticObjects(File packFolder, String dimensionKey, JSONObject dimension,
                                       List<String> blockingErrors) {
         if (!dimension.has("staticObjects")) {

core/src/main/java/art/arcane/iris/core/pack/PackExportClosure.java

@@ -18,11 +18,16 @@
 
 package art.arcane.iris.core.pack;
 
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisDimensionStack;
 import art.arcane.iris.engine.object.IrisObjectMarker;
 import art.arcane.iris.engine.object.IrisObjectPlacement;
 import art.arcane.iris.engine.object.IrisStaticObject;
 import art.arcane.volmlib.util.collection.KSet;
 
+import java.util.ArrayDeque;
+import java.util.function.Function;
+
 /**
  * Shared key collection for the pack packagers. Both the Bukkit re-serializing compiler and the
  * modded verbatim-copy compiler must export the complete ambient-spawning graph: object placements
@@ -33,6 +38,48 @@ public final class PackExportClosure {
     private PackExportClosure() {
     }
 
+    public static KSet<String> collectDimensionKeys(IrisDimension dimension) {
+        return collectDimensionKeys(dimension, key -> dimension == null || dimension.getLoader() == null
+                ? null
+                : dimension.getLoader().getDimensionLoader().load(key));
+    }
+
+    static KSet<String> collectDimensionKeys(
+            IrisDimension dimension,
+            Function<String, IrisDimension> resolver
+    ) {
+        KSet<String> dimensionKeys = new KSet<>();
+        if (dimension == null || dimension.getLoadKey() == null || dimension.getLoadKey().isBlank()) {
+            return dimensionKeys;
+        }
+
+        ArrayDeque<IrisDimension> pending = new ArrayDeque<>();
+        pending.add(dimension);
+        while (!pending.isEmpty()) {
+            IrisDimension current = pending.removeFirst();
+            String currentKey = current.getLoadKey();
+            if (currentKey == null || currentKey.isBlank() || !dimensionKeys.add(currentKey)) {
+                continue;
+            }
+            IrisDimensionStack stack = current.getDimensionStack();
+            if (stack == null || stack.getDimensions() == null) {
+                continue;
+            }
+            for (String key : stack.getDimensions()) {
+                if (key == null || key.isBlank() || dimensionKeys.contains(key)) {
+                    continue;
+                }
+                IrisDimension referenced = resolver.apply(key);
+                if (referenced == null) {
+                    dimensionKeys.add(key);
+                } else {
+                    pending.addLast(referenced);
+                }
+            }
+        }
+        return dimensionKeys;
+    }
+
     public static KSet<String> collectMarkerKeys(Iterable<IrisObjectPlacement> placements) {
         KSet<String> markerKeys = new KSet<>();
         if (placements == null) {

core/src/main/java/art/arcane/iris/core/pack/PackHydrologyValidator.java

@@ -80,12 +80,13 @@ final class PackHydrologyValidator {
 
         List<File> sortedDimensions = new ArrayList<>(List.of(dimensionFiles));
         sortedDimensions.sort(Comparator.comparing(File::getPath));
+        File dimensionsFolder = new File(packFolder, PackValidator.DIMENSIONS_FOLDER);
         for (File dimensionFile : sortedDimensions) {
             JSONObject dimension = PackValidationIo.readJson(dimensionFile);
             if (dimension == null) {
                 continue;
             }
-            String dimensionKey = PackValidationIo.stripExtension(dimensionFile.getName());
+            String dimensionKey = PackValidationIo.deriveKey(dimensionsFolder, dimensionFile);
             String dimensionPath = "Dimension '" + dimensionKey + "'";
             JSONObject dimensionPolicy = validatePolicy(
                     dimensionPath,

core/src/main/java/art/arcane/iris/core/pack/PackImageMapValidator.java

@@ -7,6 +7,7 @@ import art.arcane.iris.engine.image.IrisImageMapValidationException;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeGeneratorLink;
 import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisDimensionStack;
 import art.arcane.iris.engine.object.IrisExpression;
 import art.arcane.iris.engine.object.IrisGenerator;
 import art.arcane.iris.engine.object.IrisImage;
@@ -217,11 +218,12 @@ final class PackImageMapValidator {
             Set<String> warnings
     ) {
         File[] sortedFiles = Arrays.copyOf(dimensionFiles, dimensionFiles.length);
-        Arrays.sort(sortedFiles, (File first, File second) -> first.getName().compareTo(second.getName()));
+        Arrays.sort(sortedFiles, (File first, File second) -> first.getPath().compareTo(second.getPath()));
         Map<String, IrisDimension> dimensions = new LinkedHashMap<>();
         Set<String> bindingReadyDimensions = new LinkedHashSet<>();
+        File dimensionsFolder = new File(packFolder, PackValidator.DIMENSIONS_FOLDER);
         for (File dimensionFile : sortedFiles) {
-            String dimensionKey = PackValidationIo.stripExtension(dimensionFile.getName());
+            String dimensionKey = PackValidationIo.deriveKey(dimensionsFolder, dimensionFile);
             JSONObject raw = PackValidationIo.readJson(dimensionFile);
             if (raw == null) {
                 continue;
@@ -247,19 +249,35 @@ final class PackImageMapValidator {
             validateGeneratorStyleReferences(dimensionKey, dimension, data, resources, blockingErrors);
         }
         Map<String, List<Map.Entry<String, IrisDimension>>> upperParents = new TreeMap<>();
+        Map<String, List<Map.Entry<String, IrisDimension>>> stackParents = new TreeMap<>();
         for (Map.Entry<String, IrisDimension> entry : dimensions.entrySet()) {
             String upperKey = normalized(entry.getValue().getUpperDimension());
-            if (upperKey == null || upperKey.equalsIgnoreCase("none")) {
+            if (upperKey != null && !upperKey.equalsIgnoreCase("none")) {
+                upperParents.computeIfAbsent(upperKey, ignored -> new ArrayList<>()).add(entry);
+            }
+            IrisDimensionStack stack = entry.getValue().getDimensionStack();
+            if (stack == null || stack.getDimensions() == null) {
                 continue;
             }
-            upperParents.computeIfAbsent(upperKey, ignored -> new ArrayList<>()).add(entry);
+            Set<String> stackKeys = new LinkedHashSet<>();
+            for (String stackKey : stack.getDimensions()) {
+                String normalizedKey = normalized(stackKey);
+                if (normalizedKey == null || normalizedKey.equals(entry.getKey())
+                        || !stackKeys.add(normalizedKey)) {
+                    continue;
+                }
+                stackParents.computeIfAbsent(normalizedKey, ignored -> new ArrayList<>()).add(entry);
+            }
         }
         for (Map.Entry<String, IrisDimension> entry : dimensions.entrySet()) {
             if (!bindingReadyDimensions.contains(entry.getKey())) {
                 continue;
             }
             List<Map.Entry<String, IrisDimension>> parents = upperParents.get(entry.getKey());
-            if (parents == null || parents.isEmpty()) {
+            List<Map.Entry<String, IrisDimension>> stackHosts = stackParents.get(entry.getKey());
+            boolean hasUpperParents = parents != null && !parents.isEmpty();
+            boolean hasStackHosts = stackHosts != null && !stackHosts.isEmpty();
+            if (!hasUpperParents && !hasStackHosts) {
                 validateBindings(packFolder, "Dimension '" + entry.getKey() + "'", entry.getValue(),
                         entry.getValue().getWorldBoundary(), data, resources, registries, blockingErrors, warnings);
                 continue;
@@ -268,12 +286,23 @@ final class PackImageMapValidator {
                 validateBindings(packFolder, "Dimension '" + entry.getKey() + "'", entry.getValue(),
                         entry.getValue().getWorldBoundary(), data, resources, registries, blockingErrors, warnings);
             }
-            parents.sort(Map.Entry.comparingByKey());
+            if (hasUpperParents) {
-            for (Map.Entry<String, IrisDimension> parent : parents) {
+                parents.sort(Map.Entry.comparingByKey());
-                validateBindings(packFolder,
+                for (Map.Entry<String, IrisDimension> parent : parents) {
-                        "Dimension '" + parent.getKey() + "' upper dimension '" + entry.getKey() + "'",
+                    validateBindings(packFolder,
-                        entry.getValue(), parent.getValue().getWorldBoundary(), data, resources, registries,
+                            "Dimension '" + parent.getKey() + "' upper dimension '" + entry.getKey() + "'",
-                        blockingErrors, warnings);
+                            entry.getValue(), parent.getValue().getWorldBoundary(), data, resources, registries,
+                            blockingErrors, warnings);
+                }
+            }
+            if (hasStackHosts) {
+                stackHosts.sort(Map.Entry.comparingByKey());
+                for (Map.Entry<String, IrisDimension> host : stackHosts) {
+                    validateBindings(packFolder,
+                            "Dimension '" + host.getKey() + "' dimension stack layer '" + entry.getKey() + "'",
+                            entry.getValue(), host.getValue().getWorldBoundary(), data, resources, registries,
+                            blockingErrors, warnings);
+                }
             }
         }
     }

core/src/main/java/art/arcane/iris/core/pack/PackValidator.java

@@ -111,11 +111,13 @@ public final class PackValidator {
             return new PackValidationResult(packName, blockingErrors, warnings, validatedAt);
         }
 
-        File[] dimensionFiles = dimensionsFolder.listFiles(f -> f.isFile() && f.getName().endsWith(".json"));
+        List<File> discoveredDimensions = PackValidationIo.listJsonRecursive(dimensionsFolder);
-        if (dimensionFiles == null || dimensionFiles.length == 0) {
+        if (discoveredDimensions.isEmpty()) {
             blockingErrors.add("No dimension JSON files under dimensions/.");
             return new PackValidationResult(packName, blockingErrors, warnings, validatedAt);
         }
+        discoveredDimensions.sort((first, second) -> first.getPath().compareTo(second.getPath()));
+        File[] dimensionFiles = discoveredDimensions.toArray(File[]::new);
 
         PackDimensionValidator.validateDimensions(packFolder, dimensionFiles, blockingErrors, warnings);
         PackImageMapValidator.Validation imageMaps = PackImageMapValidator.validate(
@@ -195,7 +197,7 @@ public final class PackValidator {
             if (!data.getContentGate().ready()) {
                 return List.of();
             }
-            boolean dimensionExcluded = loadEverything(data, dimensionFiles);
+            boolean dimensionExcluded = loadEverything(packFolder, data, dimensionFiles);
             List<CompatFinding> findings = data.getCompatReport().findings();
             applyCompatFindings(minecraftVersion, dimensionExcluded, findings, blockingErrors, warnings);
             return findings;
@@ -219,11 +221,12 @@ public final class PackValidator {
      *
      * @return true when any dimension in the pack cannot generate on this server
      */
-    private static boolean loadEverything(IrisData data, File[] dimensionFiles) {
+    private static boolean loadEverything(File packFolder, IrisData data, File[] dimensionFiles) {
         boolean dimensionExcluded = false;
+        File dimensionsFolder = new File(packFolder, DIMENSIONS_FOLDER);
         for (File dimensionFile : dimensionFiles) {
             IrisDimension dimension = data.getDimensionLoader().load(
-                    PackValidationIo.stripExtension(dimensionFile.getName()), false);
+                    PackValidationIo.deriveKey(dimensionsFolder, dimensionFile), false);
             if (dimension == null) {
                 continue;
             }

core/src/main/java/art/arcane/iris/core/project/IrisPackageCompiler.java

@@ -30,6 +30,7 @@ import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBlockData;
 import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisEntity;
+import art.arcane.iris.engine.object.IrisExpression;
 import art.arcane.iris.engine.object.IrisGenerator;
 import art.arcane.iris.engine.object.IrisLootTable;
 import art.arcane.iris.core.pack.PackExportClosure;
@@ -106,11 +107,19 @@ public class IrisPackageCompiler {
             throw new IllegalStateException("Failed to create structure package staging folder " + folder.getAbsolutePath());
         }
         IrisLogging.info("Packaging Dimension " + dimension.getName() + " " + (obfuscate ? "(Obfuscated)" : ""));
+        KList<IrisDimension> dimensions = new KList<>();
+        for (String dimensionKey : PackExportClosure.collectDimensionKeys(dimension).stream().sorted().toList()) {
+            IrisDimension exportedDimension = dm.getDimensionLoader().load(dimensionKey);
+            if (exportedDimension != null) {
+                dimensions.add(exportedDimension);
+            }
+        }
         KSet<IrisRegion> regions = new KSet<>();
         KSet<IrisBiome> biomes = new KSet<>();
         KSet<IrisEntity> entities = new KSet<>();
         KSet<IrisSpawner> spawners = new KSet<>();
         KSet<IrisGenerator> generators = new KSet<>();
+        KSet<IrisExpression> expressions = new KSet<>();
         KSet<IrisLootTable> loot = new KSet<>();
         KSet<IrisBlockData> blocks = new KSet<>();
 
@@ -119,22 +128,30 @@ public class IrisPackageCompiler {
         for (String i : dm.getBlockLoader().getPossibleKeys()) {
             addIfPresent(blocks, dm.getBlockLoader().load(i));
         }
+        for (String key : dm.getExpressionLoader().getPossibleKeys()) {
+            addIfPresent(expressions, dm.getExpressionLoader().load(key));
+        }
 
-        dimension.getAllRegions(() -> dm).forEach((region) -> addIfPresent(regions, region));
+        for (IrisDimension exportedDimension : dimensions) {
-        dimension.getLoot().getTables().forEach((i) -> addIfPresent(loot, dm.getLootLoader().load(i)));
+            exportedDimension.getAllRegions(() -> dm).forEach((region) -> addIfPresent(regions, region));
+            exportedDimension.getLoot().getTables().forEach((i) -> addIfPresent(loot, dm.getLootLoader().load(i)));
+            exportedDimension.getReachableBiomes(() -> dm).forEach((biome) -> addIfPresent(biomes, biome));
+            exportedDimension.getEntitySpawners().forEach((sp) ->
+                    addIfPresent(spawners, dm.getSpawnerLoader().load(sp)));
+        }
         regions.forEach((i) -> biomes.addAll(i.getAllBiomes(() -> dm)));
-        dimension.getReachableBiomes(() -> dm).forEach((biome) -> addIfPresent(biomes, biome));
         regions.forEach((r) -> r.getLoot().getTables().forEach((i) -> addIfPresent(loot, dm.getLootLoader().load(i))));
         regions.forEach((r) -> r.getEntitySpawners().forEach((sp) -> addIfPresent(spawners, dm.getSpawnerLoader().load(sp))));
-        dimension.getEntitySpawners().forEach((sp) -> addIfPresent(spawners, dm.getSpawnerLoader().load(sp)));
         biomes.forEach((i) -> i.getGenerators().forEach((j) -> addIfPresent(generators, j.getCachedGenerator(() -> dm))));
         biomes.forEach((r) -> r.getLoot().getTables().forEach((i) -> addIfPresent(loot, dm.getLootLoader().load(i))));
         biomes.forEach((r) -> r.getEntitySpawners().forEach((sp) -> addIfPresent(spawners, dm.getSpawnerLoader().load(sp))));
         KList<IrisObjectPlacement> allPlacements = new KList<>();
         regions.forEach((r) -> allPlacements.addAll(r.getObjects()));
         biomes.forEach((i) -> allPlacements.addAll(i.getObjects()));
-        for (IrisStaticObject placement : dimension.getStaticObjects()) {
+        for (IrisDimension exportedDimension : dimensions) {
-            allPlacements.add(placement.toPlacement());
+            for (IrisStaticObject placement : exportedDimension.getStaticObjects()) {
+                allPlacements.add(placement.toPlacement());
+            }
         }
         KSet<IrisMarker> markers = new KSet<>();
         for (String markerKey : PackExportClosure.collectMarkerKeys(allPlacements)) {
@@ -148,7 +165,9 @@ public class IrisPackageCompiler {
         collectSpawnerEntityKeys(spawners).forEach((i) -> addIfPresent(entities, dm.getEntityLoader().load(i)));
         entities.forEach((e) -> e.getLoot().getTables().forEach((i) -> addIfPresent(loot, dm.getLootLoader().load(i))));
         Set<String> structureKeys = new LinkedHashSet<>();
-        collectStructureKeys(structureKeys, dimension.getStructures());
+        for (IrisDimension exportedDimension : dimensions) {
+            collectStructureKeys(structureKeys, exportedDimension.getStructures());
+        }
         regions.forEach((region) -> collectStructureKeys(structureKeys, region.getStructures()));
         biomes.forEach((biome) -> collectStructureKeys(structureKeys, biome.getStructures()));
         KMap<String, String> renameObjects = new KMap<>();
@@ -176,8 +195,10 @@ public class IrisPackageCompiler {
             j.setPlace(newNames);
         }
 
-        for (IrisStaticObject placement : dimension.getStaticObjects()) {
+        for (IrisDimension exportedDimension : dimensions) {
-            placement.setObject(renameObjects.get(placement.getObject()));
+            for (IrisStaticObject placement : exportedDimension.getStaticObjects()) {
+                placement.setObject(renameObjects.get(placement.getObject()));
+            }
         }
 
         KMap<String, KList<String>> lookupObjects = renameObjects.flip();
@@ -230,9 +251,12 @@ public class IrisPackageCompiler {
             }
             b.append(structureClosure.writeTo(folder, minify));
             b.append(ImageMapPackageClosure.writeAll(dm, folder, minify));
-            a = new JSONObject(new Gson().toJson(dimension)).toString(minify ? 0 : 4);
+            for (IrisDimension exportedDimension : dimensions.stream()
-            IO.writeAll(new File(folder, "dimensions/" + dimension.getLoadKey() + ".json"), a);
+                    .sorted(Comparator.comparing(IrisDimension::getLoadKey)).toList()) {
-            b.append(IO.hash(a));
+                a = new JSONObject(new Gson().toJson(exportedDimension)).toString(minify ? 0 : 4);
+                IO.writeAll(new File(folder, "dimensions/" + exportedDimension.getLoadKey() + ".json"), a);
+                b.append(IO.hash(a));
+            }
 
             for (IrisGenerator i : generators) {
                 a = new JSONObject(new Gson().toJson(i)).toString(minify ? 0 : 4);
@@ -240,6 +264,13 @@ public class IrisPackageCompiler {
                 b.append(IO.hash(a));
             }
 
+            for (IrisExpression i : expressions.stream()
+                    .sorted(Comparator.comparing(IrisExpression::getLoadKey)).toList()) {
+                a = new JSONObject(new Gson().toJson(i)).toString(minify ? 0 : 4);
+                IO.writeAll(new File(folder, "expressions/" + i.getLoadKey() + ".json"), a);
+                b.append(IO.hash(a));
+            }
+
             c.append(IO.hash(b.toString()));
             b = new StringBuilder();
 

core/src/main/java/art/arcane/iris/core/project/IrisProjectCopier.java

@@ -21,6 +21,7 @@ package art.arcane.iris.core.project;
 import art.arcane.iris.core.pack.PackDirectoryResolver;
 import art.arcane.volmlib.util.format.Form;
 import art.arcane.volmlib.util.io.IO;
+import art.arcane.volmlib.util.json.JSONArray;
 import art.arcane.volmlib.util.json.JSONObject;
 
 import java.io.File;
@@ -179,19 +180,58 @@ public final class IrisProjectCopier {
     }
 
     private static void transformDimension(Path stage, String sourceKey, String targetKey) throws IOException {
-        Path oldDimension = stage.resolve("dimensions").resolve(sourceKey + ".json");
+        Path dimensionsRoot = stage.resolve("dimensions");
-        Path newDimension = stage.resolve("dimensions").resolve(targetKey + ".json");
+        Path oldDimension = dimensionsRoot.resolve(sourceKey + ".json");
+        Path newDimension = dimensionsRoot.resolve(targetKey + ".json");
         if (!oldDimension.equals(newDimension)) {
             Files.move(oldDimension, newDimension);
         }
 
-        JSONObject json = new JSONObject(IO.readAll(newDimension.toFile()));
+        try (Stream<Path> files = Files.walk(dimensionsRoot)) {
-        if (json.has("name")) {
+            for (Path dimensionFile : files.filter(path -> Files.isRegularFile(path, LinkOption.NOFOLLOW_LINKS))
-            json.put("name", Form.capitalizeWords(targetKey.replace('-', ' ')));
+                    .filter(path -> path.getFileName().toString().endsWith(".json"))
-            IO.writeAll(newDimension.toFile(), json.toString(4));
+                    .sorted()
+                    .toList()) {
+                JSONObject json = new JSONObject(IO.readAll(dimensionFile.toFile()));
+                boolean changed = false;
+                if (dimensionFile.equals(newDimension) && json.has("name")) {
+                    json.put("name", Form.capitalizeWords(targetKey.replace('-', ' ')));
+                    changed = true;
+                }
+                if (rewriteDimensionStackReferences(json, sourceKey, targetKey)) {
+                    changed = true;
+                }
+                if (changed) {
+                    IO.writeAll(dimensionFile.toFile(), json.toString(4));
+                }
+            }
         }
     }
 
+    private static boolean rewriteDimensionStackReferences(JSONObject json, String sourceKey, String targetKey) {
+        JSONObject stack = json.optJSONObject("dimensionStack");
+        JSONArray dimensions = stack == null ? null : stack.optJSONArray("dimensions");
+        if (dimensions == null) {
+            return false;
+        }
+
+        boolean changed = false;
+        JSONArray rewritten = new JSONArray();
+        for (int index = 0; index < dimensions.length(); index++) {
+            Object value = dimensions.opt(index);
+            if (sourceKey.equals(value)) {
+                rewritten.put(targetKey);
+                changed = true;
+            } else {
+                rewritten.put(value == null ? JSONObject.NULL : value);
+            }
+        }
+        if (changed) {
+            stack.put("dimensions", rewritten);
+        }
+        return changed;
+    }
+
     private static void publish(Path stage, Path target) throws IOException {
         try {
             Files.move(stage, target, StandardCopyOption.ATOMIC_MOVE);

core/src/main/java/art/arcane/iris/core/service/BoardSVC.java

@@ -515,7 +515,7 @@ public class BoardSVC implements IrisService, BoardProvider {
             }
 
             lines.add("&7&m                   ");
-            lines.add(IrisLanguage.text(BukkitUiMessages.SCOREBOARD_REGION, MessageArgument.untrusted("region", engine.getRegion(x, z).getName())));
+            lines.add(IrisLanguage.text(BukkitUiMessages.SCOREBOARD_REGION, MessageArgument.untrusted("region", engine.getRegion(x, y, z).getName())));
             lines.add(IrisLanguage.text(BukkitUiMessages.SCOREBOARD_BIOME, MessageArgument.untrusted("biome", engine.getBiomeOrMantle(x, y, z).getName())));
             lines.add(IrisLanguage.text(BukkitUiMessages.SCOREBOARD_HEIGHT, MessageArgument.trusted("height", Math.round(engine.getHeight(x, z)))));
             lines.add(IrisLanguage.text(BukkitUiMessages.SCOREBOARD_SLOPE, MessageArgument.trusted("slope", Form.f(engine.getSlope(x, z), 2))));

core/src/main/java/art/arcane/iris/core/service/TreeSVC.java

@@ -326,7 +326,10 @@ public class TreeSVC implements IrisService {
 
         // Add more or find any in the region
         if (isUseAll || placements.isEmpty()) {
-            IrisRegion region = worldAccess.getEngine().getRegion(location.getBlockX(), location.getBlockZ());
+            IrisRegion region = worldAccess.getEngine().getRegion(
+                    location.getBlockX(),
+                    location.getBlockY() - worldAccess.getTarget().getWorld().minHeight(),
+                    location.getBlockZ());
             placements.addAll(matchObjectPlacements(region.getObjects(), size, type, worldAccess.getData()));
         }
 

core/src/main/java/art/arcane/iris/engine/DimensionStackContext.java

@@ -0,0 +1,363 @@
+package art.arcane.iris.engine;
+
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisDimensionStack;
+import art.arcane.iris.engine.object.IrisDimensionStackBlend;
+import art.arcane.iris.engine.object.IrisGeneratorStyle;
+import art.arcane.iris.util.project.noise.CNG;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.math.RNG;
+import com.github.benmanes.caffeine.cache.Cache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.function.Supplier;
+
+public final class DimensionStackContext {
+    private static final int LAYOUT_CACHE_SIZE = 65536;
+    private static final long SEAM_SALT = 0x44494D535441434BL;
+    private static final long HASH_OFFSET = 0xCBF29CE484222325L;
+    private static final long HASH_PRIME = 0x100000001B3L;
+
+    private final Engine engine;
+    private final int outputHeight;
+    private final int spacer;
+    private final List<DimensionTerrainContext> layersTopToBottom;
+    private final List<DimensionTerrainContext> layersBottomToTop;
+    private final List<SeamOffsetSampler> seamOffsetSamplersBottomToTop;
+    private final Cache<Long, DimensionStackLayout> layoutCache;
+
+    private DimensionStackContext(ContextState state) {
+        engine = state.engine();
+        outputHeight = state.outputHeight();
+        spacer = state.spacer();
+        layersBottomToTop = List.copyOf(state.layersBottomToTop());
+        ArrayList<DimensionTerrainContext> reversed = new ArrayList<>(layersBottomToTop);
+        Collections.reverse(reversed);
+        layersTopToBottom = List.copyOf(reversed);
+        seamOffsetSamplersBottomToTop = List.copyOf(state.seamOffsetSamplersBottomToTop());
+        layoutCache = Caffeine.newBuilder().maximumSize(LAYOUT_CACHE_SIZE).build();
+    }
+
+    public static DimensionStackContext create(Engine engine, IrisDimensionStack stack) {
+        if (stack == null) {
+            throw new IllegalArgumentException("Dimension stack configuration is required");
+        }
+        KList<String> dimensionKeys = stack.getDimensions();
+        validateRuntimeContract(engine.getDimension(), dimensionKeys);
+        if (stack.getSpacer() < 0 || stack.getSpacer() > 256) {
+            throw new IllegalArgumentException("Dimension stack spacer must be between 0 and 256");
+        }
+
+        ArrayList<DimensionTerrainContext> layersBottomToTop = new ArrayList<>(dimensionKeys.size());
+        for (int keyIndex = dimensionKeys.size() - 1; keyIndex >= 0; keyIndex--) {
+            String dimensionKey = dimensionKeys.get(keyIndex);
+            IrisDimension dimension = dimensionKey.equals(engine.getDimension().getLoadKey())
+                    ? engine.getDimension()
+                    : engine.getData().getDimensionLoader().load(dimensionKey);
+            if (dimension == null) {
+                throw new IllegalArgumentException(
+                        "Dimension stack reference does not resolve in this pack: " + dimensionKey);
+            }
+            layersBottomToTop.add(DimensionTerrainContext.forStack(engine, dimension));
+        }
+
+        List<SeamOffsetSampler> seamSamplers = createSeamSamplers(
+                engine,
+                stack.getBlend(),
+                layersBottomToTop
+        );
+        return new DimensionStackContext(new ContextState(
+                engine,
+                engine.getHeight(),
+                stack.getSpacer(),
+                layersBottomToTop,
+                seamSamplers
+        ));
+    }
+
+    private static void validateRuntimeContract(IrisDimension host, KList<String> dimensionKeys) {
+        if (dimensionKeys == null || dimensionKeys.size() < 2) {
+            throw new IllegalArgumentException("Dimension stack requires at least two dimensions");
+        }
+        String hostKey = host.getLoadKey();
+        if (!hostKey.equals(dimensionKeys.get(dimensionKeys.size() - 1))) {
+            throw new IllegalArgumentException("Dimension stack host must be its final bottom dimension");
+        }
+        int hostReferences = 0;
+        for (String dimensionKey : dimensionKeys) {
+            if (dimensionKey == null || dimensionKey.isBlank()) {
+                throw new IllegalArgumentException("Dimension stack keys must be nonblank");
+            }
+            if (hostKey.equals(dimensionKey)) {
+                hostReferences++;
+            }
+        }
+        if (hostReferences != 1) {
+            throw new IllegalArgumentException("Dimension stack must contain its host exactly once");
+        }
+    }
+
+    private static List<SeamOffsetSampler> createSeamSamplers(
+            Engine engine,
+            IrisDimensionStackBlend blend,
+            List<DimensionTerrainContext> layersBottomToTop
+    ) {
+        int seamCount = layersBottomToTop.size() - 1;
+        ArrayList<SeamOffsetSampler> samplers = new ArrayList<>(seamCount);
+        if (blend == null || blend.getAmplitude() == 0) {
+            for (int seamIndex = 0; seamIndex < seamCount; seamIndex++) {
+                samplers.add((x, z) -> 0);
+            }
+            return samplers;
+        }
+        if (blend.getAmplitude() < 0 || blend.getAmplitude() > 256) {
+            throw new IllegalArgumentException("Dimension stack blend amplitude must be between 0 and 256");
+        }
+        IrisGeneratorStyle style = blend.getStyle();
+        if (style == null) {
+            throw new IllegalArgumentException("Dimension stack blend style is required when amplitude is positive");
+        }
+
+        for (int seamIndex = 0; seamIndex < seamCount; seamIndex++) {
+            DimensionTerrainContext lower = layersBottomToTop.get(seamIndex);
+            DimensionTerrainContext upper = layersBottomToTop.get(seamIndex + 1);
+            long salt = seamSalt(
+                    seamIndex,
+                    lower.getDimension().getLoadKey(),
+                    upper.getDimension().getLoadKey()
+            );
+            RNG seamRng = new RNG(engine.getSeedManager().getTerrain() ^ salt);
+            CNG seamNoise = style.create(seamRng, engine.getData(), engine);
+            int amplitude = blend.getAmplitude();
+            samplers.add((x, z) -> clampSeamOffset(
+                    seamNoise.fitDouble(-amplitude, amplitude, x, z), amplitude));
+        }
+        return samplers;
+    }
+
+    static int clampSeamOffset(double sampledOffset, int amplitude) {
+        if (!Double.isFinite(sampledOffset)) {
+            return 0;
+        }
+        double clamped = Math.max(-amplitude, Math.min(amplitude, sampledOffset));
+        return (int) Math.round(clamped);
+    }
+
+    static long seamSalt(int seamIndex, String lowerDimensionKey, String upperDimensionKey) {
+        long hash = HASH_OFFSET ^ SEAM_SALT;
+        hash = hashString(hash, lowerDimensionKey);
+        hash ^= seamIndex;
+        hash *= HASH_PRIME;
+        return hashString(hash, upperDimensionKey);
+    }
+
+    private static long hashString(long hash, String value) {
+        long result = hash;
+        for (int characterIndex = 0; characterIndex < value.length(); characterIndex++) {
+            result ^= value.charAt(characterIndex);
+            result *= HASH_PRIME;
+        }
+        return result;
+    }
+
+    public DimensionStackLayout sample(int x, int z) {
+        return sample(x, z, usesTemporaryNaturalFallback(x, z));
+    }
+
+    private DimensionStackLayout sample(int x, int z, boolean naturalFallback) {
+        ArrayList<DimensionStackLayout.LayerInput> layerInputs =
+                new ArrayList<>(layersBottomToTop.size());
+        for (DimensionTerrainContext terrainContext : layersBottomToTop) {
+            DimensionTerrainContext.ColumnSample column = terrainContext.sampleColumn(
+                    x, z, naturalFallback);
+            int normalTerrainHeight = localCoordinate(
+                    column.terrainHeight(),
+                    terrainContext.getLocalHeight()
+            );
+            int fluidHeight = localCoordinate(
+                    column.fluidHeight(),
+                    terrainContext.getLocalHeight()
+            );
+            layerInputs.add(new DimensionStackLayout.LayerInput(
+                    terrainContext,
+                    column.biome(),
+                    column.region(),
+                    column.rockBlock(),
+                    column.fluidBlock(),
+                    column.surfaceBlock(),
+                    normalTerrainHeight,
+                    fluidHeight
+            ));
+        }
+
+        int[] seamOffsets = new int[seamOffsetSamplersBottomToTop.size()];
+        for (int seamIndex = 0; seamIndex < seamOffsets.length; seamIndex++) {
+            seamOffsets[seamIndex] = seamOffsetSamplersBottomToTop.get(seamIndex).sample(x, z);
+        }
+        return DimensionStackLayout.create(outputHeight, spacer, layerInputs, seamOffsets);
+    }
+
+    private static int localCoordinate(double sample, int localHeight) {
+        int maximum = Math.max(0, localHeight - 1);
+        long rounded = Math.round(sample);
+        return (int) Math.max(0L, Math.min(maximum, rounded));
+    }
+
+    public int getStackTerrainHeight(int x, int z) {
+        return (int) (sampleTopHeights(x, z) >>> 32);
+    }
+
+    public int getStackTopHeight(int x, int z) {
+        return (int) sampleTopHeights(x, z);
+    }
+
+    public DimensionStackLayout getLayout(int x, int z) {
+        boolean naturalFallback = usesTemporaryNaturalFallback(x, z);
+        long columnKey = ((long) x << 32) ^ (z & 0xFFFFFFFFL);
+        return resolveLayout(
+                layoutCache,
+                columnKey,
+                naturalFallback,
+                () -> sample(x, z, naturalFallback)
+        );
+    }
+
+    public DimensionStackLayout.Layer getLayerAt(int x, int y, int z) {
+        return getLayout(x, z).layerAt(y);
+    }
+
+    static DimensionStackLayout resolveLayout(
+            Cache<Long, DimensionStackLayout> cache,
+            long columnKey,
+            boolean naturalFallback,
+            Supplier<DimensionStackLayout> sampler
+    ) {
+        if (naturalFallback) {
+            return sampler.get();
+        }
+        return cache.get(columnKey, ignored -> sampler.get());
+    }
+
+    private long sampleTopHeights(int x, int z) {
+        boolean naturalFallback = usesTemporaryNaturalFallback(x, z);
+        int baseY = 0;
+        int renderedTerrainTopY = Integer.MIN_VALUE;
+        int renderedContentTopY = Integer.MIN_VALUE;
+        int outputMaxY = outputHeight - 1;
+        int previousContentTopY = Integer.MIN_VALUE;
+        int previousRenderMinY = Integer.MIN_VALUE;
+        int previousRenderMaxY = Integer.MIN_VALUE;
+        int previousSurfaceY = Integer.MIN_VALUE;
+        boolean previousVisible = false;
+        for (int layerIndex = 0; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            DimensionTerrainContext terrainContext = layersBottomToTop.get(layerIndex);
+            if (layerIndex > 0) {
+                int gapMinY = (int) Math.max(0L, (long) previousContentTopY + 1L);
+                int gapMaxY = (int) Math.min((long) outputMaxY, (long) baseY - 1L);
+                int previousTerrainY = previousVisible && previousSurfaceY >= previousRenderMinY
+                        ? Math.min(outputMaxY, previousSurfaceY)
+                        : Integer.MIN_VALUE;
+                renderedTerrainTopY = DimensionStackLayout.eraseHeightThroughGap(
+                        renderedTerrainTopY,
+                        gapMinY,
+                        gapMaxY,
+                        previousTerrainY
+                );
+                renderedContentTopY = DimensionStackLayout.eraseHeightThroughGap(
+                        renderedContentTopY,
+                        gapMinY,
+                        gapMaxY,
+                        previousVisible ? previousRenderMaxY : Integer.MIN_VALUE
+                );
+            }
+            int localTerrainHeight = localCoordinate(
+                    terrainContext.getNormalTerrainHeight(x, z, naturalFallback),
+                    terrainContext.getLocalHeight()
+            );
+            int localFluidHeight = localCoordinate(
+                    terrainContext.getFluidHeight(x, z, naturalFallback),
+                    terrainContext.getLocalHeight()
+            );
+            int surfaceY = saturatedAdd(baseY, localTerrainHeight);
+            int fluidY = saturatedAdd(baseY, localFluidHeight);
+            int layerContentTopY = Math.max(surfaceY, fluidY);
+            int renderMinY = Math.max(0, baseY);
+            int renderMaxY = Math.min(outputMaxY, layerContentTopY);
+            if (renderMinY <= renderMaxY) {
+                renderedContentTopY = Math.max(renderedContentTopY, renderMaxY);
+                renderedTerrainTopY = DimensionStackLayout.mergeVisibleTerrainHeight(
+                        renderedTerrainTopY,
+                        renderMinY,
+                        renderMaxY,
+                        surfaceY,
+                        outputMaxY
+                );
+            }
+            previousContentTopY = layerContentTopY;
+            previousRenderMinY = renderMinY;
+            previousRenderMaxY = renderMaxY;
+            previousSurfaceY = surfaceY;
+            previousVisible = renderMinY <= renderMaxY;
+            if (layerIndex + 1 < layersBottomToTop.size()) {
+                int seamOffset = seamOffsetSamplersBottomToTop.get(layerIndex).sample(x, z);
+                baseY = saturatedAdd(
+                        layerContentTopY,
+                        saturatedAdd(spacer, saturatedAdd(1, seamOffset))
+                );
+            }
+        }
+        int clippedTerrainTopY = Math.max(0, renderedTerrainTopY);
+        int clippedContentTopY = Math.max(0, renderedContentTopY);
+        return ((long) clippedTerrainTopY << 32) | (clippedContentTopY & 0xFFFFFFFFL);
+    }
+
+    private static int saturatedAdd(int left, int right) {
+        long sum = (long) left + right;
+        if (sum > Integer.MAX_VALUE) {
+            return Integer.MAX_VALUE;
+        }
+        if (sum < Integer.MIN_VALUE) {
+            return Integer.MIN_VALUE;
+        }
+        return (int) sum;
+    }
+
+    public int getOutputHeight() {
+        return outputHeight;
+    }
+
+    public int getSpacer() {
+        return spacer;
+    }
+
+    public boolean usesTemporaryNaturalFallback(int x, int z) {
+        return engine.answersFromNaturalTerrain(x, z);
+    }
+
+    public List<DimensionTerrainContext> getLayersTopToBottom() {
+        return layersTopToBottom;
+    }
+
+    public List<DimensionTerrainContext> getLayersBottomToTop() {
+        return layersBottomToTop;
+    }
+
+    private interface SeamOffsetSampler {
+        int sample(double x, double z);
+    }
+
+    private record ContextState(
+            Engine engine,
+            int outputHeight,
+            int spacer,
+            List<DimensionTerrainContext> layersBottomToTop,
+            List<SeamOffsetSampler> seamOffsetSamplersBottomToTop
+    ) {
+    }
+
+}

core/src/main/java/art/arcane/iris/engine/DimensionStackLayout.java

@@ -0,0 +1,349 @@
+package art.arcane.iris.engine;
+
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisRegion;
+import art.arcane.iris.spi.PlatformBlockState;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public final class DimensionStackLayout {
+    private final List<Layer> layersBottomToTop;
+    private final List<Layer> layersTopToBottom;
+    private final int stackTerrainTopY;
+    private final int stackTopY;
+    private final int clippedStackTerrainTopY;
+    private final int clippedStackTopY;
+
+    private DimensionStackLayout(LayoutState state) {
+        layersBottomToTop = List.copyOf(state.layersBottomToTop());
+        ArrayList<Layer> reversed = new ArrayList<>(layersBottomToTop);
+        Collections.reverse(reversed);
+        layersTopToBottom = List.copyOf(reversed);
+        stackTerrainTopY = state.stackTerrainTopY();
+        stackTopY = state.stackTopY();
+        clippedStackTerrainTopY = state.clippedStackTerrainTopY();
+        clippedStackTopY = state.clippedStackTopY();
+    }
+
+    static DimensionStackLayout create(
+            int outputHeight,
+            int spacer,
+            List<LayerInput> layerInputsBottomToTop,
+            int[] seamOffsetsBottomToTop
+    ) {
+        if (outputHeight < 1) {
+            throw new IllegalArgumentException("Output height must be positive");
+        }
+        if (spacer < 0) {
+            throw new IllegalArgumentException("Dimension stack spacer cannot be negative");
+        }
+        if (layerInputsBottomToTop.isEmpty()) {
+            throw new IllegalArgumentException("Dimension stack requires at least one layer");
+        }
+        if (seamOffsetsBottomToTop.length != layerInputsBottomToTop.size() - 1) {
+            throw new IllegalArgumentException("Dimension stack seam offset count does not match its layers");
+        }
+
+        int outputMaxY = outputHeight - 1;
+        int baseY = 0;
+        int stackTerrainTopY = Integer.MIN_VALUE;
+        int stackTopY = Integer.MIN_VALUE;
+        int renderedTerrainTopY = Integer.MIN_VALUE;
+        int renderedContentTopY = Integer.MIN_VALUE;
+        ArrayList<Layer> layers = new ArrayList<>(layerInputsBottomToTop.size());
+        for (int layerIndex = 0; layerIndex < layerInputsBottomToTop.size(); layerIndex++) {
+            LayerInput input = layerInputsBottomToTop.get(layerIndex);
+            int seamOffsetBelow = layerIndex == 0 ? 0 : seamOffsetsBottomToTop[layerIndex - 1];
+            int spacerBelow = layerIndex == 0 ? 0 : spacer;
+            if (layerIndex > 0) {
+                Layer lower = layers.get(layerIndex - 1);
+                int gapMinY = (int) Math.max(0L, (long) lower.contentTopY() + 1L);
+                int gapMaxY = (int) Math.min((long) outputMaxY, (long) baseY - 1L);
+                renderedTerrainTopY = eraseHeightThroughGap(
+                        renderedTerrainTopY,
+                        gapMinY,
+                        gapMaxY,
+                        visibleTerrainY(lower)
+                );
+                renderedContentTopY = eraseHeightThroughGap(
+                        renderedContentTopY,
+                        gapMinY,
+                        gapMaxY,
+                        lower.visible() ? lower.renderMaxY() : Integer.MIN_VALUE
+                );
+            }
+            int surfaceY = saturatedAdd(baseY, input.normalTerrainHeight());
+            int fluidY = saturatedAdd(baseY, input.fluidHeight());
+            int contentTopY = Math.max(surfaceY, fluidY);
+            int renderMinY = Math.max(0, baseY);
+            int renderMaxY = Math.min(outputMaxY, contentTopY);
+            boolean visible = renderMinY <= renderMaxY;
+            Layer layer = new Layer(
+                    input.terrainContext(),
+                    input.biome(),
+                    input.region(),
+                    input.rockBlock(),
+                    input.fluidBlock(),
+                    input.surfaceBlock(),
+                    baseY,
+                    input.normalTerrainHeight(),
+                    input.fluidHeight(),
+                    surfaceY,
+                    fluidY,
+                    contentTopY,
+                    spacerBelow,
+                    seamOffsetBelow,
+                    renderMinY,
+                    renderMaxY,
+                    clip(surfaceY, outputMaxY),
+                    clip(fluidY, outputMaxY),
+                    visible
+            );
+            layers.add(layer);
+            stackTerrainTopY = Math.max(stackTerrainTopY, surfaceY);
+            stackTopY = Math.max(stackTopY, contentTopY);
+            if (visible) {
+                renderedContentTopY = Math.max(renderedContentTopY, renderMaxY);
+                renderedTerrainTopY = mergeVisibleTerrainHeight(
+                        renderedTerrainTopY,
+                        renderMinY,
+                        renderMaxY,
+                        surfaceY,
+                        outputMaxY
+                );
+            }
+            if (layerIndex + 1 < layerInputsBottomToTop.size()) {
+                baseY = saturatedAdd(
+                        contentTopY,
+                        saturatedAdd(spacer, saturatedAdd(1, seamOffsetsBottomToTop[layerIndex]))
+                );
+            }
+        }
+
+        return new DimensionStackLayout(new LayoutState(
+                layers,
+                stackTerrainTopY,
+                stackTopY,
+                Math.max(0, renderedTerrainTopY),
+                Math.max(0, renderedContentTopY)
+        ));
+    }
+
+    private static int saturatedAdd(int left, int right) {
+        long sum = (long) left + right;
+        if (sum > Integer.MAX_VALUE) {
+            return Integer.MAX_VALUE;
+        }
+        if (sum < Integer.MIN_VALUE) {
+            return Integer.MIN_VALUE;
+        }
+        return (int) sum;
+    }
+
+    private static int clip(int value, int outputMaxY) {
+        return Math.max(0, Math.min(outputMaxY, value));
+    }
+
+    static int mergeVisibleTerrainHeight(
+            int currentTerrainTopY,
+            int renderMinY,
+            int renderMaxY,
+            int surfaceY,
+            int outputMaxY
+    ) {
+        int terrainTopY = currentTerrainTopY;
+        if (terrainTopY >= renderMinY && terrainTopY <= renderMaxY) {
+            terrainTopY = Integer.MIN_VALUE;
+        }
+        if (surfaceY >= renderMinY) {
+            terrainTopY = Math.max(terrainTopY, Math.min(outputMaxY, surfaceY));
+        }
+        return terrainTopY;
+    }
+
+    static int eraseHeightThroughGap(
+            int currentTopY,
+            int gapMinY,
+            int gapMaxY,
+            int fallbackTopY
+    ) {
+        if (currentTopY >= gapMinY && currentTopY <= gapMaxY) {
+            return fallbackTopY;
+        }
+        return currentTopY;
+    }
+
+    private static int visibleTerrainY(Layer layer) {
+        if (!layer.visible() || layer.surfaceY() < layer.renderMinY()) {
+            return Integer.MIN_VALUE;
+        }
+        return layer.clippedSurfaceY();
+    }
+
+    public List<Layer> layersBottomToTop() {
+        return layersBottomToTop;
+    }
+
+    public List<Layer> layersTopToBottom() {
+        return layersTopToBottom;
+    }
+
+    public int stackTerrainTopY() {
+        return stackTerrainTopY;
+    }
+
+    public int stackTopY() {
+        return stackTopY;
+    }
+
+    public int clippedStackTerrainTopY() {
+        return clippedStackTerrainTopY;
+    }
+
+    public int clippedStackTopY() {
+        return clippedStackTopY;
+    }
+
+    public Layer topTerrainLayer() {
+        Layer highest = null;
+        int highestSurfaceY = Integer.MIN_VALUE;
+        for (int layerIndex = 0; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            Layer layer = layersBottomToTop.get(layerIndex);
+            if (layerIndex > 0) {
+                Layer lower = layersBottomToTop.get(layerIndex - 1);
+                if ((long) highestSurfaceY > lower.contentTopY()
+                        && (long) highestSurfaceY < layer.localBaseY()) {
+                    highest = visibleTerrainY(lower) == Integer.MIN_VALUE ? null : lower;
+                    highestSurfaceY = visibleTerrainY(lower);
+                }
+            }
+            if (layer.containsRenderedY(highestSurfaceY)) {
+                highest = null;
+                highestSurfaceY = Integer.MIN_VALUE;
+            }
+            if (!layer.visible() || layer.surfaceY() < layer.renderMinY()) {
+                continue;
+            }
+            if (layer.clippedSurfaceY() >= highestSurfaceY) {
+                highest = layer;
+                highestSurfaceY = layer.clippedSurfaceY();
+            }
+        }
+        return highest;
+    }
+
+    public Layer surfaceLayer() {
+        Layer terrainLayer = topTerrainLayer();
+        return terrainLayer == null ? topVisibleLayer() : terrainLayer;
+    }
+
+    private Layer topVisibleLayer() {
+        Layer highest = null;
+        int highestContentY = Integer.MIN_VALUE;
+        for (int layerIndex = 0; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            Layer layer = layersBottomToTop.get(layerIndex);
+            if (layerIndex > 0) {
+                Layer lower = layersBottomToTop.get(layerIndex - 1);
+                if ((long) highestContentY > lower.contentTopY()
+                        && (long) highestContentY < layer.localBaseY()) {
+                    highest = lower.visible() ? lower : null;
+                    highestContentY = lower.visible()
+                            ? lower.renderMaxY()
+                            : Integer.MIN_VALUE;
+                }
+            }
+            if (layer.visible() && layer.renderMaxY() >= highestContentY) {
+                highest = layer;
+                highestContentY = layer.renderMaxY();
+            }
+        }
+        return highest;
+    }
+
+    public Layer layerAt(int y) {
+        Layer bottom = layersBottomToTop.get(0);
+        Layer owner = bottom;
+        for (int layerIndex = 1; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            Layer lower = layersBottomToTop.get(layerIndex - 1);
+            Layer layer = layersBottomToTop.get(layerIndex);
+            if ((long) y > lower.contentTopY() && (long) y < layer.localBaseY()) {
+                owner = bottom;
+            }
+            if (layer.containsRenderedY(y)) {
+                owner = layer;
+            }
+        }
+        return owner;
+    }
+
+    public boolean containsUpperLayerY(int y) {
+        for (int layerIndex = 1; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            if (layersBottomToTop.get(layerIndex).containsRenderedY(y)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public boolean isHostFeatureProtectedY(int y) {
+        for (int layerIndex = 1; layerIndex < layersBottomToTop.size(); layerIndex++) {
+            Layer lower = layersBottomToTop.get(layerIndex - 1);
+            Layer upper = layersBottomToTop.get(layerIndex);
+            if (upper.containsRenderedY(y)
+                    || ((long) y > lower.contentTopY() && (long) y < upper.localBaseY())) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    static record LayerInput(
+            DimensionTerrainContext terrainContext,
+            IrisBiome biome,
+            IrisRegion region,
+            PlatformBlockState rockBlock,
+            PlatformBlockState fluidBlock,
+            PlatformBlockState surfaceBlock,
+            int normalTerrainHeight,
+            int fluidHeight
+    ) {
+    }
+
+    public record Layer(
+            DimensionTerrainContext terrainContext,
+            IrisBiome biome,
+            IrisRegion region,
+            PlatformBlockState rockBlock,
+            PlatformBlockState fluidBlock,
+            PlatformBlockState surfaceBlock,
+            int localBaseY,
+            int normalTerrainHeight,
+            int fluidHeight,
+            int surfaceY,
+            int fluidY,
+            int contentTopY,
+            int spacerBelow,
+            int seamOffsetBelow,
+            int renderMinY,
+            int renderMaxY,
+            int clippedSurfaceY,
+            int clippedFluidY,
+            boolean visible
+    ) {
+        public boolean containsRenderedY(int y) {
+            return visible && y >= renderMinY && y <= renderMaxY;
+        }
+
+    }
+
+    private record LayoutState(
+            List<Layer> layersBottomToTop,
+            int stackTerrainTopY,
+            int stackTopY,
+            int clippedStackTerrainTopY,
+            int clippedStackTopY
+    ) {
+    }
+}

core/src/main/java/art/arcane/iris/engine/DimensionTerrainContext.java

@@ -0,0 +1,764 @@
+package art.arcane.iris.engine;
+
+import art.arcane.iris.core.IrisSettings;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.image.IrisImageMapRuntime;
+import art.arcane.iris.engine.object.InferredType;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisGenerator;
+import art.arcane.iris.engine.object.IrisInterpolator;
+import art.arcane.iris.engine.object.IrisRegion;
+import art.arcane.iris.engine.object.IrisShapedGeneratorStyle;
+import art.arcane.iris.spi.IrisLogging;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.util.common.data.DataProvider;
+import art.arcane.iris.util.project.interpolation.NoiseBounds;
+import art.arcane.iris.util.project.stream.ProceduralStream;
+import art.arcane.iris.util.project.stream.interpolation.Interpolated;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.math.RNG;
+
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+import java.util.Collections;
+import java.util.EnumMap;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.IdentityHashMap;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+
+public final class DimensionTerrainContext implements DataProvider {
+    private static final NoiseBounds ZERO_NOISE_BOUNDS = new NoiseBounds(0D, 0D);
+
+    private final Engine engine;
+    private final IrisDimension dimension;
+    private final IrisData data;
+    private final int localHeight;
+    private final ProceduralStream<Double> heightStream;
+    private final ProceduralStream<Double> slopeStream;
+    private final ProceduralStream<Double> fluidHeightStream;
+    private final ProceduralStream<IrisBiome> biomeStream;
+    private final ProceduralStream<IrisRegion> regionStream;
+    private final ProceduralStream<PlatformBlockState> rockStream;
+    private final FluidBlockSampler fluidBlockSampler;
+    private final IrisImageMapRuntime imageMapRuntime;
+    private final boolean selfReferencing;
+    private final SelfFallback selfFallback;
+
+    private DimensionTerrainContext(ContextState state) {
+        engine = state.engine();
+        dimension = state.dimension();
+        data = state.data();
+        localHeight = state.localHeight();
+        heightStream = state.heightStream();
+        slopeStream = state.slopeStream();
+        fluidHeightStream = state.fluidHeightStream();
+        biomeStream = state.biomeStream();
+        regionStream = state.regionStream();
+        rockStream = state.rockStream();
+        fluidBlockSampler = state.fluidBlockSampler();
+        imageMapRuntime = state.imageMapRuntime();
+        selfReferencing = state.selfReferencing();
+        selfFallback = state.selfFallback();
+    }
+
+    public static DimensionTerrainContext forStack(Engine engine, IrisDimension dimension) {
+        boolean selfReferencing = dimension.getLoadKey().equals(engine.getDimension().getLoadKey());
+        if (selfReferencing) {
+            return createSelfReferencing(engine, false);
+        }
+
+        int localHeight = dimension.getMaxHeight() - dimension.getMinHeight();
+        return createCrossReferencing(
+                engine,
+                dimension,
+                localHeight,
+                dimension.getMinHeight(),
+                "stack-" + dimension.getLoadKey(),
+                true
+        );
+    }
+
+    static DimensionTerrainContext forUpper(Engine engine, IrisDimension dimension) {
+        boolean selfReferencing = dimension.getLoadKey().equals(engine.getDimension().getLoadKey());
+        if (selfReferencing) {
+            return createSelfReferencing(engine, true);
+        }
+
+        return createCrossReferencing(
+                engine,
+                dimension,
+                engine.getHeight(),
+                engine.getMinHeight(),
+                "upper",
+                false
+        );
+    }
+
+    private static DimensionTerrainContext createSelfReferencing(Engine engine, boolean natural) {
+        IrisComplex complex = engine.getComplex();
+        double fluidHeight = engine.getDimension().getFluidHeight();
+        ProceduralStream<Double> fluidHeightStream = natural
+                ? ProceduralStream.ofDouble((x, z) -> fluidHeight)
+                : complex.getRiverWaterSurfaceStream();
+        FluidBlockSampler fluidBlockSampler = natural
+                ? (x, z) -> complex.getFluidStream().get(x, z)
+                : complex::resolveSurfaceFluid;
+        SelfFallback selfFallback = natural
+                ? null
+                : new SelfFallback(
+                        complex.getNaturalHeightStream(),
+                        complex.getNaturalTrueBiomeStream(),
+                        fluidHeight,
+                        (x, z) -> complex.getFluidStream().get(x, z)
+                );
+        return new DimensionTerrainContext(new ContextState(
+                engine,
+                engine.getDimension(),
+                engine.getData(),
+                engine.getHeight(),
+                natural ? complex.getNaturalHeightStream() : complex.getHeightStream(),
+                natural ? complex.getNaturalSlopeStream() : complex.getSlopeStream(),
+                fluidHeightStream,
+                natural ? complex.getNaturalTrueBiomeStream() : complex.getTrueBiomeStream(),
+                complex.getRegionStream(),
+                complex.getRockStream(),
+                fluidBlockSampler,
+                complex.getImageMapRuntime(),
+                true,
+                selfFallback
+        ));
+    }
+
+    private static DimensionTerrainContext createCrossReferencing(
+            Engine engine,
+            IrisDimension dimension,
+            int localHeight,
+            int minimumWorldHeight,
+            String cachePrefix,
+            boolean includeFluid
+    ) {
+        IrisData resolvedData = dimension.getLoader();
+        if (resolvedData == null) {
+            resolvedData = engine.getData();
+        }
+        IrisData dimensionData = resolvedData;
+        IrisImageMapRuntime imageMapRuntime = IrisImageMapRuntime.compile(
+                dimensionData,
+                dimension,
+                minimumWorldHeight
+        );
+        long seedOffset = dimension.getLoadKey().hashCode();
+        RNG rng = new RNG(engine.getSeedManager().getComplex() ^ seedOffset);
+        double fluidHeight = dimension.getFluidHeight();
+        int cacheSize = IrisSettings.get().getPerformance().getNoiseCacheSize();
+        DataProvider dataProvider = () -> dimensionData;
+        IrisBiome configuredFocusBiome = loadFocusBiome(dimension, dimensionData);
+        IrisBiome focusBiome = configuredFocusBiome == null
+                ? null
+                : configuredFocusBiome.withInferredType(InferredType.LAND);
+        IrisRegion focusRegion = focusBiome == null
+                ? loadFocusRegion(dimension, dimensionData)
+                : findFocusRegion(focusBiome, dimension, dataProvider, dimensionData);
+
+        Map<IrisInterpolator, Set<IrisGenerator>> generators = new HashMap<>();
+        Set<IrisBiome> allBiomes = Collections.newSetFromMap(new IdentityHashMap<>());
+        Map<IrisBiome, IrisComplex.ChildSelectionPlan> childSelectionPlans =
+                Collections.synchronizedMap(new IdentityHashMap<>());
+        KList<IrisRegion> preparedRegions = new KList<>();
+        if (focusRegion != null) {
+            preparedRegions.add(focusRegion);
+            focusRegion.getNaturalBiomes(dataProvider).forEach(biome -> registerBiomeGenerators(
+                    biome, dataProvider, allBiomes, generators));
+        } else {
+            for (IrisRegion region : dimension.getAllRegions(dataProvider)) {
+                preparedRegions.add(region);
+                region.getNaturalBiomes(dataProvider).forEach(biome -> registerBiomeGenerators(
+                        biome, dataProvider, allBiomes, generators));
+            }
+            for (IrisBiome mappedBiome : imageMapRuntime.getMappedBiomes()) {
+                if (!mappedBiome.isCompatExcluded()) {
+                    registerBiomeGenerators(mappedBiome, dataProvider, allBiomes, generators);
+                }
+            }
+        }
+        if (focusBiome != null) {
+            allBiomes.add(focusBiome);
+        }
+
+        IrisComplex.GeneratorGroup[] generatorGroups = IrisComplex.freezeGeneratorGroups(generators);
+        Map<IrisInterpolator, IdentityHashMap<IrisBiome, NoiseBounds>> generatorBounds = new HashMap<>();
+        for (IrisComplex.GeneratorGroup group : generatorGroups) {
+            IdentityHashMap<IrisBiome, NoiseBounds> interpolatorBounds =
+                    new IdentityHashMap<>(Math.max(allBiomes.size(), 16));
+            for (IrisBiome biome : allBiomes) {
+                double minimum = 0D;
+                double maximum = 0D;
+                for (IrisGenerator generator : group.generators()) {
+                    String key = generator.getLoadKey();
+                    if (key == null || key.isBlank()) {
+                        continue;
+                    }
+                    maximum += biome.getGenLinkMax(key, engine);
+                    minimum += biome.getGenLinkMin(key, engine);
+                }
+                interpolatorBounds.put(biome, new NoiseBounds(minimum, maximum));
+            }
+            generatorBounds.put(group.interpolator(), interpolatorBounds);
+        }
+
+        ProceduralStream<Double> regionStyleStream = dimension.getRegionStyle()
+                .create(rng.nextParallelRNG(883), dimensionData).stream()
+                .zoom(dimension.getRegionZoom());
+        KList<IrisRegion> regionPool = new KList<>();
+        for (IrisRegion region : dimensionData.getRegionLoader().loadAll(dimension.getRegions())) {
+            if (!region.isCompatExcluded()) {
+                regionPool.add(region);
+            }
+        }
+        ProceduralStream<IrisRegion> proceduralRegionStream = focusRegion == null
+                ? regionStyleStream.selectRarity(regionPool)
+                : ProceduralStream.of(
+                        (x, z) -> focusRegion,
+                        Interpolated.of(value -> 0D, value -> focusRegion)
+                );
+        ProceduralStream<IrisRegion> regionStream = focusRegion == null
+                ? proceduralRegionStream
+                        .convertAware2D((region, x, z) -> mappedRegion(imageMapRuntime, region, x, z))
+                        .cache2D(cachePrefix + "ImageMappedRegionStream", engine, cacheSize)
+                : proceduralRegionStream;
+        Map<IrisRegion, Map<InferredType, ProceduralStream<IrisBiome>>> preparedBiomeStreams =
+                compileInferredBiomeStreams(dimension, dimensionData, rng, preparedRegions);
+
+        ProceduralStream<IrisBiome> landBiomeStream = regionStream
+                .convert(region -> IrisComplex.preparedInferredBiomeStream(
+                        preparedBiomeStreams, region, InferredType.LAND))
+                .convertAware2D(ProceduralStream::get);
+        ProceduralStream<IrisBiome> seaBiomeStream = regionStream
+                .convert(region -> IrisComplex.preparedInferredBiomeStream(
+                        preparedBiomeStreams, region, InferredType.SEA))
+                .convertAware2D(ProceduralStream::get);
+        ProceduralStream<IrisBiome> shoreBiomeStream = regionStream
+                .convert(region -> IrisComplex.preparedInferredBiomeStream(
+                        preparedBiomeStreams, region, InferredType.SHORE))
+                .convertAware2D(ProceduralStream::get);
+
+        Map<InferredType, ProceduralStream<IrisBiome>> inferredStreams = new HashMap<>();
+        inferredStreams.put(InferredType.LAND, landBiomeStream);
+        inferredStreams.put(InferredType.SEA, seaBiomeStream);
+        inferredStreams.put(InferredType.SHORE, shoreBiomeStream);
+
+        ProceduralStream<InferredType> bridgeStream = focusBiome == null
+                ? dimension.getContinentalStyle()
+                        .create(rng.nextParallelRNG(234234565), dimensionData)
+                        .bake().scale(1D / dimension.getContinentZoom()).bake().stream()
+                        .convert(value -> value >= dimension.getLandChance()
+                                ? InferredType.SEA
+                                : InferredType.LAND)
+                : ProceduralStream.of(
+                        (x, z) -> focusBiome.getInferredType(),
+                        Interpolated.of(value -> 0D, value -> focusBiome.getInferredType())
+                );
+
+        ProceduralStream<IrisBiome> proceduralBaseBiomeStream = focusBiome == null
+                ? bridgeStream
+                        .convertAware2D((type, x, z) -> {
+                            ProceduralStream<IrisBiome> stream = inferredStreams.get(type);
+                            return stream != null
+                                    ? stream.get(x, z)
+                                    : inferredStreams.get(InferredType.LAND).get(x, z);
+                        })
+                        .convertAware2D((biome, x, z) -> implode(
+                                biome, x, z, rng, dataProvider, childSelectionPlans, 3))
+                : ProceduralStream.of(
+                        (x, z) -> focusBiome,
+                        Interpolated.of(value -> 0D, value -> focusBiome)
+                );
+        ProceduralStream<IrisBiome> baseBiomeStream = focusBiome == null
+                ? proceduralBaseBiomeStream
+                        .convertAware2D((biome, x, z) -> mappedBiome(imageMapRuntime, biome, x, z))
+                        .cache2D(cachePrefix + "ImageMappedBaseBiomeStream", engine, cacheSize)
+                : proceduralBaseBiomeStream;
+
+        KList<IrisShapedGeneratorStyle> overlayNoise = dimension.getOverlayNoise();
+        ProceduralStream<Double> overlayStream = overlayNoise.isEmpty()
+                ? ProceduralStream.ofDouble((x, z) -> 0D)
+                : ProceduralStream.ofDouble((x, z) -> {
+            double value = 0D;
+            for (IrisShapedGeneratorStyle style : overlayNoise) {
+                value += style.get(rng, dimensionData, x, z);
+            }
+            return value;
+        });
+
+        long heightSeed = engine.getSeedManager().getHeight() ^ seedOffset;
+        ProceduralStream<Double> heightStream = ProceduralStream.of((x, z) -> {
+            IrisBiome biome = baseBiomeStream.get(x, z);
+            if (biome == null) {
+                return mappedTerrainHeight(imageMapRuntime, fluidHeight, x, z);
+            }
+            double interpolatedHeight = 0D;
+            for (IrisComplex.GeneratorGroup group : generatorGroups) {
+                IrisInterpolator interpolator = group.interpolator();
+                IrisGenerator[] groupGenerators = group.generators();
+                if (groupGenerators.length == 0) {
+                    continue;
+                }
+                IdentityHashMap<IrisBiome, NoiseBounds> cachedBounds = generatorBounds.get(interpolator);
+                NoiseBounds sampledBounds = interpolator.interpolateBounds(x, z, (sampleX, sampleZ) -> {
+                    try {
+                        IrisBiome sampledBiome = baseBiomeStream.get(sampleX, sampleZ);
+                        if (sampledBiome == null) {
+                            return ZERO_NOISE_BOUNDS;
+                        }
+                        NoiseBounds bounds = cachedBounds == null ? null : cachedBounds.get(sampledBiome);
+                        if (bounds != null) {
+                            return bounds;
+                        }
+                        double minimum = 0D;
+                        double maximum = 0D;
+                        for (IrisGenerator generator : groupGenerators) {
+                            String key = generator.getLoadKey();
+                            if (key == null || key.isBlank()) {
+                                continue;
+                            }
+                            maximum += sampledBiome.getGenLinkMax(key, engine);
+                            minimum += sampledBiome.getGenLinkMin(key, engine);
+                        }
+                        return new NoiseBounds(minimum, maximum);
+                    } catch (Throwable e) {
+                        IrisLogging.reportError(e);
+                        return ZERO_NOISE_BOUNDS;
+                    }
+                });
+                interpolatedHeight += IrisComplex.averageGeneratorHeights(
+                        groupGenerators,
+                        sampledBounds.min(),
+                        sampledBounds.max(),
+                        x,
+                        z,
+                        heightSeed + 239945
+                );
+            }
+            double proceduralHeight = Math.max(
+                    Math.min(interpolatedHeight + fluidHeight + overlayStream.get(x, z), localHeight),
+                    0D
+            );
+            return mappedTerrainHeight(imageMapRuntime, proceduralHeight, x, z);
+        }, Interpolated.DOUBLE).cache2DDouble(
+                cachePrefix + "ImageMappedHeightStream", engine, cacheSize);
+        ProceduralStream<Double> slopeStream = heightStream.slope(3)
+                .cache2DDouble(cachePrefix + "SlopeStream", engine, cacheSize);
+
+        ProceduralStream<IrisBiome> finalBiomeStream = focusBiome == null
+                ? heightStream.convertAware2D((height, x, z) -> {
+                    IrisBiome mappedBiome = imageMapRuntime.sampleBiome(x, z);
+                    if (mappedBiome != null) {
+                        return mappedBiome;
+                    }
+                    IrisBiome baseBiome = baseBiomeStream.get(x, z);
+                    IrisBiome resolvedBiome = IrisComplex.resolveSurfaceBiome(
+                            height,
+                            baseBiome,
+                            regionStream.get(x, z),
+                            x,
+                            z,
+                            fluidHeight,
+                            landBiomeStream,
+                            seaBiomeStream,
+                            shoreBiomeStream
+                    );
+                    return resolvedBiome == baseBiome
+                            ? baseBiome
+                            : implode(resolvedBiome, x, z, rng, dataProvider, childSelectionPlans, 3);
+                }).cache2D(cachePrefix + "ImageMappedFinalBiomeStream", engine, cacheSize)
+                : ProceduralStream.of(
+                        (x, z) -> focusBiome,
+                        Interpolated.of(value -> 0D, value -> focusBiome)
+                ).cache2D(cachePrefix + "FinalBiomeStreamFocus", engine, cacheSize);
+
+        ProceduralStream<PlatformBlockState> rockStream = dimension.getRockPalette()
+                .getLayerGenerator(rng.nextParallelRNG(45), dimensionData).stream()
+                .select(dimension.getRockPalette().getBlockData(dimensionData));
+        FluidBlockSampler fluidBlockSampler;
+        if (includeFluid) {
+            ProceduralStream<PlatformBlockState> fluidStream = dimension.getFluidPalette()
+                    .getLayerGenerator(rng.nextParallelRNG(78), dimensionData).stream()
+                    .select(dimension.getFluidPalette().getBlockData(dimensionData));
+            fluidBlockSampler = fluidStream::get;
+        } else {
+            fluidBlockSampler = (x, z) -> null;
+        }
+
+        return new DimensionTerrainContext(new ContextState(
+                engine,
+                dimension,
+                dimensionData,
+                localHeight,
+                heightStream,
+                slopeStream,
+                ProceduralStream.ofDouble((x, z) -> fluidHeight),
+                finalBiomeStream,
+                regionStream,
+                rockStream,
+                fluidBlockSampler,
+                imageMapRuntime,
+                false,
+                null
+        ));
+    }
+
+    private static void registerBiomeGenerators(
+            IrisBiome biome,
+            DataProvider dataProvider,
+            Set<IrisBiome> allBiomes,
+            Map<IrisInterpolator, Set<IrisGenerator>> generators
+    ) {
+        allBiomes.add(biome);
+        biome.getGenerators().forEach(link -> {
+            IrisGenerator generator = link.getCachedGenerator(dataProvider);
+            if (generator != null) {
+                generators.computeIfAbsent(generator.getInterpolator(), key -> new HashSet<>()).add(generator);
+            }
+        });
+    }
+
+    static IrisRegion mappedRegion(
+            IrisImageMapRuntime imageMapRuntime,
+            IrisRegion proceduralRegion,
+            double worldX,
+            double worldZ
+    ) {
+        IrisRegion mappedRegion = imageMapRuntime.sampleRegion(worldX, worldZ);
+        return mappedRegion == null ? proceduralRegion : mappedRegion;
+    }
+
+    static double mappedTerrainHeight(
+            IrisImageMapRuntime imageMapRuntime,
+            double proceduralHeight,
+            double worldX,
+            double worldZ
+    ) {
+        return imageMapRuntime.sampleTerrainHeight(worldX, worldZ, proceduralHeight);
+    }
+
+    static IrisBiome mappedBiome(
+            IrisImageMapRuntime imageMapRuntime,
+            IrisBiome proceduralBiome,
+            double worldX,
+            double worldZ
+    ) {
+        IrisBiome mappedBiome = imageMapRuntime.sampleBiome(worldX, worldZ);
+        return mappedBiome == null ? proceduralBiome : mappedBiome;
+    }
+
+    static PlatformBlockState mappedSurfaceBlock(
+            IrisImageMapRuntime imageMapRuntime,
+            PlatformBlockState proceduralBlock,
+            double worldX,
+            double worldZ
+    ) {
+        PlatformBlockState mappedBlock = imageMapRuntime.sampleSurfaceBlock(worldX, worldZ);
+        return mappedBlock == null ? proceduralBlock : mappedBlock;
+    }
+
+    private static Map<IrisRegion, Map<InferredType, ProceduralStream<IrisBiome>>> compileInferredBiomeStreams(
+            IrisDimension dimension,
+            IrisData data,
+            RNG rng,
+            Iterable<IrisRegion> regions
+    ) {
+        IdentityHashMap<IrisRegion, Map<InferredType, ProceduralStream<IrisBiome>>> compiled =
+                new IdentityHashMap<>();
+        for (IrisRegion region : regions) {
+            if (compiled.containsKey(region)) {
+                continue;
+            }
+            EnumMap<InferredType, ProceduralStream<IrisBiome>> streams = new EnumMap<>(InferredType.class);
+            streams.put(InferredType.LAND, createInferredBiomeStream(
+                    dimension, data, rng, region, InferredType.LAND));
+            streams.put(InferredType.SEA, createInferredBiomeStream(
+                    dimension, data, rng, region, InferredType.SEA));
+            streams.put(InferredType.SHORE, createInferredBiomeStream(
+                    dimension, data, rng, region, InferredType.SHORE));
+            compiled.put(region, Collections.unmodifiableMap(streams));
+        }
+        return Collections.unmodifiableMap(compiled);
+    }
+
+    private static ProceduralStream<IrisBiome> createInferredBiomeStream(
+            IrisDimension dimension,
+            IrisData data,
+            RNG rng,
+            IrisRegion region,
+            InferredType type
+    ) {
+        return switch (type) {
+            case LAND -> dimension.getLandBiomeStyle()
+                    .create(rng.nextParallelRNG(InferredType.LAND.ordinal()), data).stream()
+                    .zoom(dimension.getBiomeZoom())
+                    .zoom(dimension.getLandZoom())
+                    .zoom(region.getLandBiomeZoom())
+                    .selectRarity(loadInferredBiomes(data, region.getLandBiomes(), InferredType.LAND));
+            case SEA -> dimension.getSeaBiomeStyle()
+                    .create(rng.nextParallelRNG(InferredType.SEA.ordinal()), data).stream()
+                    .zoom(dimension.getBiomeZoom())
+                    .zoom(dimension.getSeaZoom())
+                    .zoom(region.getSeaBiomeZoom())
+                    .selectRarity(loadInferredBiomes(data, region.getSeaBiomes(), InferredType.SEA));
+            case SHORE -> dimension.getShoreBiomeStyle()
+                    .create(rng.nextParallelRNG(InferredType.SHORE.ordinal()), data).stream()
+                    .zoom(dimension.getBiomeZoom())
+                    .zoom(region.getShoreBiomeZoom())
+                    .selectRarity(loadInferredBiomes(data, region.getShoreBiomes(), InferredType.SHORE));
+            default -> throw new IllegalArgumentException("Unsupported stacked terrain biome type: " + type);
+        };
+    }
+
+    private static KList<IrisBiome> loadInferredBiomes(
+            IrisData data,
+            KList<String> keys,
+            InferredType type
+    ) {
+        KList<IrisBiome> inferredBiomes = new KList<>();
+        for (IrisBiome biome : data.getBiomeLoader().loadAll(keys)) {
+            if (biome.isCompatExcluded()) {
+                continue;
+            }
+            inferredBiomes.add(biome.withInferredType(type));
+        }
+        return inferredBiomes;
+    }
+
+    private static IrisBiome loadFocusBiome(IrisDimension dimension, IrisData data) {
+        String key = dimension.getFocus();
+        if (key == null || key.isBlank()) {
+            return null;
+        }
+        IrisBiome biome = data.getBiomeLoader().load(key);
+        return biome == null || biome.isCompatExcluded() ? null : biome;
+    }
+
+    private static IrisRegion loadFocusRegion(IrisDimension dimension, IrisData data) {
+        String key = dimension.getFocusRegion();
+        if (key == null || key.isBlank()) {
+            return null;
+        }
+        IrisRegion region = data.getRegionLoader().load(key);
+        return region == null || region.isCompatExcluded() ? null : region;
+    }
+
+    private static IrisRegion findFocusRegion(
+            IrisBiome focusBiome,
+            IrisDimension dimension,
+            DataProvider dataProvider,
+            IrisData data
+    ) {
+        for (IrisRegion region : dimension.getAllRegions(dataProvider)) {
+            if (region.getAllBiomeIds().contains(focusBiome.getLoadKey())) {
+                return region;
+            }
+        }
+        String key = UUID.nameUUIDFromBytes((dimension.getLoadKey() + "\u0000" + focusBiome.getLoadKey())
+                .getBytes(StandardCharsets.UTF_8)).toString();
+        IrisRegion region = new IrisRegion();
+        region.getLandBiomes().add(focusBiome.getLoadKey());
+        region.getSeaBiomes().add(focusBiome.getLoadKey());
+        region.getShoreBiomes().add(focusBiome.getLoadKey());
+        region.setLoadKey(key);
+        region.setLoader(data);
+        region.setLoadFile(new File(
+                data.getDataFolder(),
+                data.getRegionLoader().getFolderName() + "/" + key + ".json"
+        ));
+        return region;
+    }
+
+    private static IrisBiome implode(
+            IrisBiome biome,
+            double x,
+            double z,
+            RNG rng,
+            DataProvider dataProvider,
+            Map<IrisBiome, IrisComplex.ChildSelectionPlan> childSelectionPlans,
+            int remainingDepth
+    ) {
+        if (biome == null || remainingDepth < 0 || biome.getChildren().isEmpty()) {
+            return biome;
+        }
+
+        IrisComplex.ChildSelectionPlan selectionPlan = childSelectionPlans.get(biome);
+        if (selectionPlan == null) {
+            synchronized (childSelectionPlans) {
+                selectionPlan = childSelectionPlans.get(biome);
+                if (selectionPlan == null) {
+                    KList<IrisBiome> options = new KList<>();
+                    for (IrisBiome child : biome.getRealChildren(dataProvider)) {
+                        if (child != null) {
+                            options.add(child);
+                        }
+                    }
+                    options.add(biome);
+                    selectionPlan = IrisComplex.ChildSelectionPlan.create(options);
+                    childSelectionPlans.put(biome, selectionPlan);
+                }
+            }
+        }
+
+        IrisBiome selected = selectionPlan.select(
+                biome.getChildrenGenerator(rng, 123, biome.getChildShrinkFactor()), x, z);
+        if (selected == null) {
+            return biome;
+        }
+        return implode(
+                selected.withInferredType(biome.getInferredType()),
+                x,
+                z,
+                rng,
+                dataProvider,
+                childSelectionPlans,
+                remainingDepth - 1
+        );
+    }
+
+    public double getNormalTerrainHeight(double x, double z) {
+        return getNormalTerrainHeight(x, z, usesNaturalFallback(x, z));
+    }
+
+    double getNormalTerrainHeight(double x, double z, boolean naturalFallback) {
+        if (naturalFallback && selfFallback != null) {
+            return selfFallback.heightStream().getDouble(x, z);
+        }
+        return heightStream.getDouble(x, z);
+    }
+
+    public double getFluidHeight(double x, double z) {
+        return getFluidHeight(x, z, usesNaturalFallback(x, z));
+    }
+
+    double getFluidHeight(double x, double z, boolean naturalFallback) {
+        if (naturalFallback && selfFallback != null) {
+            return selfFallback.fluidHeight();
+        }
+        return fluidHeightStream.getDouble(x, z);
+    }
+
+    public ProceduralStream<Double> getSlopeStream() {
+        return slopeStream;
+    }
+
+    public IrisBiome getBiome(double x, double z) {
+        return getBiome(x, z, usesNaturalFallback(x, z));
+    }
+
+    private IrisBiome getBiome(double x, double z, boolean naturalFallback) {
+        if (naturalFallback && selfFallback != null) {
+            return selfFallback.biomeStream().get(x, z);
+        }
+        return biomeStream.get(x, z);
+    }
+
+    public IrisRegion getRegion(double x, double z) {
+        return regionStream == null ? null : regionStream.get(x, z);
+    }
+
+    public PlatformBlockState getRockBlock(double x, double z) {
+        return rockStream.get(x, z);
+    }
+
+    public PlatformBlockState getFluidBlock(double x, double z) {
+        return getFluidBlock(x, z, usesNaturalFallback(x, z));
+    }
+
+    private PlatformBlockState getFluidBlock(double x, double z, boolean naturalFallback) {
+        if (naturalFallback && selfFallback != null) {
+            return selfFallback.fluidBlockSampler().sample(x, z);
+        }
+        return fluidBlockSampler.sample(x, z);
+    }
+
+    public PlatformBlockState getSurfaceBlock(double x, double z) {
+        return imageMapRuntime.sampleSurfaceBlock(x, z);
+    }
+
+    public IrisDimension getDimension() {
+        return dimension;
+    }
+
+    @Override
+    public IrisData getData() {
+        return data;
+    }
+
+    public int getLocalHeight() {
+        return localHeight;
+    }
+
+    public boolean isSelfReferencing() {
+        return selfReferencing;
+    }
+
+    ColumnSample sampleColumn(double x, double z, boolean naturalFallback) {
+        return new ColumnSample(
+                getNormalTerrainHeight(x, z, naturalFallback),
+                getFluidHeight(x, z, naturalFallback),
+                getBiome(x, z, naturalFallback),
+                getRegion(x, z),
+                getRockBlock(x, z),
+                getFluidBlock(x, z, naturalFallback),
+                getSurfaceBlock(x, z)
+        );
+    }
+
+    private boolean usesNaturalFallback(double x, double z) {
+        return selfFallback != null && engine.answersFromNaturalTerrain(
+                (int) Math.floor(x),
+                (int) Math.floor(z)
+        );
+    }
+
+    private interface FluidBlockSampler {
+        PlatformBlockState sample(double x, double z);
+    }
+
+    private record ContextState(
+            Engine engine,
+            IrisDimension dimension,
+            IrisData data,
+            int localHeight,
+            ProceduralStream<Double> heightStream,
+            ProceduralStream<Double> slopeStream,
+            ProceduralStream<Double> fluidHeightStream,
+            ProceduralStream<IrisBiome> biomeStream,
+            ProceduralStream<IrisRegion> regionStream,
+            ProceduralStream<PlatformBlockState> rockStream,
+            FluidBlockSampler fluidBlockSampler,
+            IrisImageMapRuntime imageMapRuntime,
+            boolean selfReferencing,
+            SelfFallback selfFallback
+    ) {
+    }
+
+    private record SelfFallback(
+            ProceduralStream<Double> heightStream,
+            ProceduralStream<IrisBiome> biomeStream,
+            double fluidHeight,
+            FluidBlockSampler fluidBlockSampler
+    ) {
+    }
+
+    record ColumnSample(
+            double terrainHeight,
+            double fluidHeight,
+            IrisBiome biome,
+            IrisRegion region,
+            PlatformBlockState rockBlock,
+            PlatformBlockState fluidBlock,
+            PlatformBlockState surfaceBlock
+    ) {
+    }
+}

core/src/main/java/art/arcane/iris/engine/EngineHotloader.java

@@ -59,15 +59,22 @@ final class EngineHotloader {
             IrisComplex nextComplex = null;
             try {
                 engine.sealForTransition("complex hotload", false);
+                engine.prepareRuntimeHotload();
                 RuntimeAssembly assembly = new RuntimeAssembly(RuntimeAssembly.nextRuntimeId(), previous.target());
                 engine.runtimeAssembly.set(assembly);
                 EngineRuntime next;
                 try (IrisContext.Scope ignored = IrisContext.open(engine, engine.getGenerationSessions().currentSessionId(), null)) {
                     assembly.complex = new IrisComplex(engine);
                     nextComplex = assembly.complex;
+                    assembly.dimensionStackContext = engine.runtimeBuilder.buildDimensionStackContext();
                     assembly.upperContext = engine.runtimeBuilder.buildUpperContext();
                     BiomeMaxes biomeMaxes = engine.runtimeBuilder.computeBiomeMaxes();
-                    next = previous.withComplex(assembly.cacheId, assembly.complex, assembly.upperContext, biomeMaxes);
+                    next = previous.withComplex(
+                            assembly.cacheId,
+                            assembly.complex,
+                            assembly.upperContext,
+                            assembly.dimensionStackContext,
+                            biomeMaxes);
                 } finally {
                     engine.runtimeAssembly.remove();
                 }
@@ -110,6 +117,7 @@ final class EngineHotloader {
             boolean published = false;
             try {
                 engine.sealForTransition("hotload", false);
+                engine.prepareRuntimeHotload();
                 replacementData = IrisData.openRuntime(previousData.getDataFolder());
                 IrisDimension replacement = replacementData.getDimensionLoader().load(previousDimension.getLoadKey());
                 if (replacement == null) {

core/src/main/java/art/arcane/iris/engine/EngineRuntime.java

@@ -34,6 +34,7 @@ record EngineRuntime(
         EngineTarget target,
         IrisComplex complex,
         UpperDimensionContext upperContext,
+        DimensionStackContext dimensionStackContext,
         EngineEffects effects,
         EngineMode mode,
         EngineWorldManager worldManager,
@@ -44,6 +45,7 @@ record EngineRuntime(
             int nextCacheId,
             IrisComplex nextComplex,
             UpperDimensionContext nextUpperContext,
+            DimensionStackContext nextDimensionStackContext,
             BiomeMaxes nextBiomeMaxes
     ) {
         return new EngineRuntime(
@@ -51,6 +53,7 @@ record EngineRuntime(
                 target,
                 nextComplex,
                 nextUpperContext,
+                nextDimensionStackContext,
                 effects,
                 mode,
                 worldManager,

core/src/main/java/art/arcane/iris/engine/EngineRuntimeBuilder.java

@@ -22,12 +22,14 @@ import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.ResourceLoader;
 import art.arcane.iris.engine.EngineRuntime.BiomeMaxes;
 import art.arcane.iris.engine.IrisEngine.LifecycleState;
+import art.arcane.iris.engine.actuator.IrisDimensionStackActuator;
 import art.arcane.iris.engine.framework.EngineEffects;
 import art.arcane.iris.engine.framework.EngineEffectsProvider;
 import art.arcane.iris.engine.framework.EngineMode;
 import art.arcane.iris.engine.framework.EngineTarget;
 import art.arcane.iris.engine.framework.EngineWorldManager;
 import art.arcane.iris.engine.framework.EngineWorldManagerProvider;
+import art.arcane.iris.engine.modifier.IrisCustomModifier;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomePaletteLayer;
 import art.arcane.iris.engine.object.IrisDecorator;
@@ -74,6 +76,9 @@ final class EngineRuntimeBuilder {
             assembly.complex = new IrisComplex(engine);
             IrisLogging.debug("[IrisEngine timing] complex=" + (M.ms() - started) + "ms");
             started = M.ms();
+            assembly.dimensionStackContext = buildDimensionStackContext();
+            IrisLogging.debug("[IrisEngine timing] buildDimensionStackContext=" + (M.ms() - started) + "ms");
+            started = M.ms();
             assembly.upperContext = buildUpperContext();
             IrisLogging.debug("[IrisEngine timing] buildUpperContext=" + (M.ms() - started) + "ms");
             started = M.ms();
@@ -91,6 +96,14 @@ final class EngineRuntimeBuilder {
             IrisStaticObjectLayer staticObjects = engine.getDimension().getStaticObjectLayer(engine.getData());
             assembly.mode.registerStage((x, z, blocks, biomes, multicore, context) ->
                     staticObjects.apply(engine, x, z, blocks));
+            if (assembly.dimensionStackContext != null) {
+                IrisDimensionStackActuator dimensionStack = new IrisDimensionStackActuator(engine);
+                assembly.mode.registerStage((x, z, blocks, biomes, multicore, context) ->
+                        dimensionStack.actuate(x, z, blocks, multicore, context));
+                IrisCustomModifier stackCustom = new IrisCustomModifier(engine);
+                assembly.mode.registerStage((x, z, blocks, biomes, multicore, context) ->
+                        stackCustom.modify(x, z, blocks, multicore, context));
+            }
             IrisLogging.debug("[IrisEngine timing] setupMode=" + (M.ms() - started) + "ms");
             started = M.ms();
             assembly.worldManager = IrisServices.get(EngineWorldManagerProvider.class).create(engine);
@@ -205,6 +218,21 @@ final class EngineRuntimeBuilder {
         return null;
     }
 
+    DimensionStackContext buildDimensionStackContext() {
+        IrisDimension dimension = engine.getDimension();
+        if (!dimension.hasDimensionStack()) {
+            return null;
+        }
+        if (dimension.hasUpperDimension()) {
+            throw new IllegalStateException("Dimension '" + dimension.getLoadKey()
+                    + "' cannot enable both dimensionStack and upperDimension.");
+        }
+        DimensionStackContext context = DimensionStackContext.create(engine, dimension.getDimensionStack());
+        IrisLogging.info("Dimension stack enabled: "
+                + String.join(" -> ", dimension.getDimensionStack().getDimensions()));
+        return context;
+    }
+
     private EngineMode createMode() {
         Throwable configuredFailure = null;
         try {
@@ -300,6 +328,7 @@ final class EngineRuntimeBuilder {
         }
         IrisComplex complex;
         UpperDimensionContext upperContext;
+        DimensionStackContext dimensionStackContext;
         EngineEffects effects;
         EngineMode mode;
         EngineWorldManager worldManager;
@@ -319,6 +348,7 @@ final class EngineRuntimeBuilder {
                     target,
                     complex,
                     upperContext,
+                    dimensionStackContext,
                     effects,
                     mode,
                     worldManager,

core/src/main/java/art/arcane/iris/engine/IrisEngine.java

@@ -501,10 +501,14 @@ public class IrisEngine implements Engine {
     }
 
     public void hotloadSilently() {
-        nativeStructureVolumeMemo.clear();
         hotloader.hotloadSilently();
     }
 
+    void prepareRuntimeHotload() {
+        nativeStructureVolumeMemo.clear();
+        platformHooks.prepareRuntimeHotload(this);
+    }
+
     @Override
     public KList<NativeStructureVolume> getNativeStructureVolumes(int minX, int minZ, int maxX, int maxZ) {
         if (!nativeStructureVolumeQueriesEnabled.get()) {
@@ -632,6 +636,16 @@ public class IrisEngine implements Engine {
         return current == null ? null : current.upperContext();
     }
 
+    @Override
+    public DimensionStackContext getDimensionStackContext() {
+        RuntimeAssembly assembly = runtimeAssembly.get();
+        if (assembly != null) {
+            return assembly.dimensionStackContext;
+        }
+        EngineRuntime current = runtime;
+        return current == null ? null : current.dimensionStackContext();
+    }
+
     @Override
     public CompletableFuture<Long> getHash32() {
         RuntimeAssembly assembly = runtimeAssembly.get();

core/src/main/java/art/arcane/iris/engine/UpperDimensionContext.java

@@ -1,311 +1,46 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2022 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 package art.arcane.iris.engine;
 
-import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.image.IrisImageMapRuntime;
-import art.arcane.iris.engine.object.InferredType;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisDimension;
-import art.arcane.iris.engine.object.IrisGenerator;
-import art.arcane.iris.engine.object.IrisInterpolator;
 import art.arcane.iris.engine.object.IrisRegion;
-import art.arcane.iris.engine.object.IrisShapedGeneratorStyle;
-import art.arcane.iris.spi.IrisLogging;
-import art.arcane.volmlib.util.collection.KList;
-import art.arcane.iris.util.common.data.DataProvider;
-import art.arcane.volmlib.util.math.RNG;
-import art.arcane.iris.util.project.interpolation.NoiseBounds;
 import art.arcane.iris.spi.PlatformBlockState;
-import art.arcane.iris.util.project.stream.ProceduralStream;
+import art.arcane.iris.util.common.data.DataProvider;
-import art.arcane.iris.util.project.stream.interpolation.Interpolated;
-
-import java.util.Collections;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.IdentityHashMap;
-import java.util.Map;
-import java.util.Set;
 
 public class UpperDimensionContext implements DataProvider {
-    private static final NoiseBounds ZERO_NOISE_BOUNDS = new NoiseBounds(0D, 0D);
+    private final DimensionTerrainContext terrainContext;
-    private final IrisDimension dimension;
-    private final IrisData data;
     private final int chunkHeight;
-    private final ProceduralStream<Double> heightStream;
-    private final ProceduralStream<IrisBiome> biomeStream;
-    private final ProceduralStream<IrisRegion> regionStream;
-    private final ProceduralStream<PlatformBlockState> rockStream;
-    private final IrisImageMapRuntime imageMapRuntime;
-    private final boolean selfReferencing;
 
-    private UpperDimensionContext(ContextState state) {
+    private UpperDimensionContext(DimensionTerrainContext terrainContext, int chunkHeight) {
-        this.dimension = state.dimension();
+        this.terrainContext = terrainContext;
-        this.data = state.data();
+        this.chunkHeight = chunkHeight;
-        this.chunkHeight = state.chunkHeight();
-        this.heightStream = state.heightStream();
-        this.biomeStream = state.biomeStream();
-        this.regionStream = state.regionStream();
-        this.rockStream = state.rockStream();
-        this.imageMapRuntime = state.imageMapRuntime();
-        this.selfReferencing = state.selfReferencing();
     }
 
-    public static UpperDimensionContext create(Engine engine, IrisDimension upperDim) {
+    public static UpperDimensionContext create(Engine engine, IrisDimension upperDimension) {
-        boolean selfRef = upperDim.getLoadKey().equals(engine.getDimension().getLoadKey());
+        return new UpperDimensionContext(
-        int chunkHeight = engine.getHeight();
+                DimensionTerrainContext.forUpper(engine, upperDimension),
-        if (selfRef) {
+                engine.getHeight()
-            return createSelfReferencing(engine, chunkHeight);
-        }
-        return createCrossReferencing(engine, upperDim, chunkHeight);
-    }
-
-    private static UpperDimensionContext createSelfReferencing(Engine engine, int chunkHeight) {
-        IrisComplex complex = engine.getComplex();
-        return new UpperDimensionContext(new ContextState(
-                engine.getDimension(),
-                engine.getData(),
-                chunkHeight,
-                complex.getNaturalHeightStream(),
-                complex.getNaturalTrueBiomeStream(),
-                complex.getRegionStream(),
-                complex.getRockStream(),
-                complex.getImageMapRuntime(),
-                true
-        ));
-    }
-
-    private static UpperDimensionContext createCrossReferencing(Engine engine, IrisDimension upperDim, int chunkHeight) {
-        IrisData resolvedData = upperDim.getLoader();
-        if (resolvedData == null) {
-            resolvedData = engine.getData();
-        }
-        IrisData upperData = resolvedData;
-        IrisImageMapRuntime imageMapRuntime = IrisImageMapRuntime.compile(
-                upperData,
-                upperDim,
-                engine.getMinHeight()
         );
-        long seedOffset = upperDim.getLoadKey().hashCode();
-        RNG rng = new RNG(engine.getSeedManager().getComplex() ^ seedOffset);
-        double fluidHeight = upperDim.getFluidHeight();
-        int cacheSize = IrisSettings.get().getPerformance().getNoiseCacheSize();
-        DataProvider dataProvider = () -> upperData;
-
-        Map<IrisInterpolator, Set<IrisGenerator>> generators = new HashMap<>();
-        Set<IrisBiome> allBiomes = Collections.newSetFromMap(new IdentityHashMap<>());
-        Map<IrisBiome, IrisComplex.ChildSelectionPlan> childSelectionPlans =
-                Collections.synchronizedMap(new IdentityHashMap<>());
-        upperDim.getRegions().forEach(regionKey -> {
-            IrisRegion region = upperData.getRegionLoader().load(regionKey);
-            if (region != null) {
-                region.getNaturalBiomes(dataProvider).forEach(biome -> registerBiomeGenerators(
-                        biome, dataProvider, allBiomes, generators));
-            }
-        });
-        for (IrisRegion mappedRegion : imageMapRuntime.getMappedRegions()) {
-            mappedRegion.getNaturalBiomes(dataProvider).forEach(biome -> registerBiomeGenerators(
-                    biome, dataProvider, allBiomes, generators));
-        }
-        for (IrisBiome mappedBiome : imageMapRuntime.getMappedBiomes()) {
-            registerBiomeGenerators(mappedBiome, dataProvider, allBiomes, generators);
-        }
-
-        IrisComplex.GeneratorGroup[] generatorGroups = IrisComplex.freezeGeneratorGroups(generators);
-        Map<IrisInterpolator, IdentityHashMap<IrisBiome, NoiseBounds>> generatorBounds = new HashMap<>();
-        for (IrisComplex.GeneratorGroup group : generatorGroups) {
-            IdentityHashMap<IrisBiome, NoiseBounds> interpolatorBounds = new IdentityHashMap<>(Math.max(allBiomes.size(), 16));
-            for (IrisBiome biome : allBiomes) {
-                double min = 0D;
-                double max = 0D;
-                for (IrisGenerator gen : group.generators()) {
-                    String key = gen.getLoadKey();
-                    if (key == null || key.isBlank()) {
-                        continue;
-                    }
-                    max += biome.getGenLinkMax(key, engine);
-                    min += biome.getGenLinkMin(key, engine);
-                }
-                interpolatorBounds.put(biome, new NoiseBounds(min, max));
-            }
-            generatorBounds.put(group.interpolator(), interpolatorBounds);
-        }
-
-        ProceduralStream<Double> regionStyleStream = upperDim.getRegionStyle()
-                .create(rng.nextParallelRNG(883), upperData).stream()
-                .zoom(upperDim.getRegionZoom());
-        ProceduralStream<IrisRegion> proceduralRegionStream = regionStyleStream
-                .selectRarity(upperData.getRegionLoader().loadAll(upperDim.getRegions()));
-        ProceduralStream<IrisRegion> regionStream = proceduralRegionStream
-                .convertAware2D((region, x, z) -> mappedRegion(imageMapRuntime, region, x, z))
-                .cache2D("upperImageMappedRegionStream", engine, cacheSize);
-
-        ProceduralStream<IrisBiome> landBiomeStream = regionStream
-                .convert(r -> upperDim.getLandBiomeStyle()
-                        .create(rng.nextParallelRNG(InferredType.LAND.ordinal()), upperData).stream()
-                        .zoom(upperDim.getBiomeZoom())
-                        .zoom(upperDim.getLandZoom())
-                        .zoom(r.getLandBiomeZoom())
-                        .selectRarity(loadInferredBiomes(upperData, r.getLandBiomes(), InferredType.LAND)))
-                .convertAware2D(ProceduralStream::get);
-        ProceduralStream<IrisBiome> seaBiomeStream = regionStream
-                .convert(r -> upperDim.getSeaBiomeStyle()
-                        .create(rng.nextParallelRNG(InferredType.SEA.ordinal()), upperData).stream()
-                        .zoom(upperDim.getBiomeZoom())
-                        .zoom(upperDim.getSeaZoom())
-                        .zoom(r.getSeaBiomeZoom())
-                        .selectRarity(loadInferredBiomes(upperData, r.getSeaBiomes(), InferredType.SEA)))
-                .convertAware2D(ProceduralStream::get);
-        ProceduralStream<IrisBiome> shoreBiomeStream = regionStream
-                .convert(r -> upperDim.getShoreBiomeStyle()
-                        .create(rng.nextParallelRNG(InferredType.SHORE.ordinal()), upperData).stream()
-                        .zoom(upperDim.getBiomeZoom())
-                        .zoom(r.getShoreBiomeZoom())
-                        .selectRarity(loadInferredBiomes(upperData, r.getShoreBiomes(), InferredType.SHORE)))
-                .convertAware2D(ProceduralStream::get);
-
-        Map<InferredType, ProceduralStream<IrisBiome>> inferredStreams = new HashMap<>();
-        inferredStreams.put(InferredType.LAND, landBiomeStream);
-        inferredStreams.put(InferredType.SEA, seaBiomeStream);
-        inferredStreams.put(InferredType.SHORE, shoreBiomeStream);
-
-        ProceduralStream<InferredType> bridgeStream = upperDim.getContinentalStyle()
-                .create(rng.nextParallelRNG(234234565), upperData)
-                .bake().scale(1D / upperDim.getContinentZoom()).bake().stream()
-                .convert(v -> v >= upperDim.getLandChance() ? InferredType.SEA : InferredType.LAND);
-
-        ProceduralStream<IrisBiome> proceduralBaseBiomeStream = bridgeStream
-                .convertAware2D((t, x, z) -> {
-                    ProceduralStream<IrisBiome> stream = inferredStreams.get(t);
-                    return stream != null ? stream.get(x, z) : inferredStreams.get(InferredType.LAND).get(x, z);
-                })
-                .convertAware2D((biome, x, z) -> implode(
-                        biome, x, z, rng, dataProvider, childSelectionPlans, 3));
-        ProceduralStream<IrisBiome> baseBiomeStream = proceduralBaseBiomeStream
-                .convertAware2D((biome, x, z) -> mappedBiome(imageMapRuntime, biome, x, z))
-                .cache2D("upperImageMappedBaseBiomeStream", engine, cacheSize);
-
-        KList<IrisShapedGeneratorStyle> overlayNoise = upperDim.getOverlayNoise();
-        ProceduralStream<Double> overlayStream = overlayNoise.isEmpty()
-                ? ProceduralStream.ofDouble((x, z) -> 0.0D)
-                : ProceduralStream.ofDouble((x, z) -> {
-            double value = 0D;
-            for (IrisShapedGeneratorStyle style : overlayNoise) {
-                value += style.get(rng, upperData, x, z);
-            }
-            return value;
-        });
-
-        long heightSeed = engine.getSeedManager().getHeight() ^ seedOffset;
-
-        ProceduralStream<Double> heightStream = ProceduralStream.of((x, z) -> {
-            IrisBiome b = baseBiomeStream.get(x, z);
-            if (b == null) {
-                return mappedTerrainHeight(imageMapRuntime, fluidHeight, x, z);
-            }
-            double interpolatedHeight = 0;
-            for (IrisComplex.GeneratorGroup group : generatorGroups) {
-                IrisInterpolator interpolator = group.interpolator();
-                IrisGenerator[] groupGenerators = group.generators();
-                if (groupGenerators.length == 0) {
-                    continue;
-                }
-                IdentityHashMap<IrisBiome, NoiseBounds> cachedBounds = generatorBounds.get(interpolator);
-                NoiseBounds sampledBounds = interpolator.interpolateBounds(x, z, (xx, zz) -> {
-                    try {
-                        IrisBiome bx = baseBiomeStream.get(xx, zz);
-                        if (bx == null) {
-                            return ZERO_NOISE_BOUNDS;
-                        }
-                        NoiseBounds bounds = cachedBounds != null ? cachedBounds.get(bx) : null;
-                        if (bounds != null) {
-                            return bounds;
-                        }
-                        double bMin = 0D;
-                        double bMax = 0D;
-                        for (IrisGenerator gen : groupGenerators) {
-                            String key = gen.getLoadKey();
-                            if (key == null || key.isBlank()) {
-                                continue;
-                            }
-                            bMax += bx.getGenLinkMax(key, engine);
-                            bMin += bx.getGenLinkMin(key, engine);
-                        }
-                        return new NoiseBounds(bMin, bMax);
-                    } catch (Throwable e) {
-                        IrisLogging.reportError(e);
-                        return ZERO_NOISE_BOUNDS;
-                    }
-                });
-                double hi = sampledBounds.max();
-                double lo = sampledBounds.min();
-                interpolatedHeight += IrisComplex.averageGeneratorHeights(
-                        groupGenerators,
-                        lo,
-                        hi,
-                        x,
-                        z,
-                        heightSeed + 239945
-                );
-            }
-            double proceduralHeight = Math.max(
-                    Math.min(interpolatedHeight + fluidHeight + overlayStream.get(x, z), chunkHeight),
-                    0D
-            );
-            return mappedTerrainHeight(imageMapRuntime, proceduralHeight, x, z);
-        }, Interpolated.DOUBLE).cache2DDouble("upperImageMappedHeightStream", engine, cacheSize);
-
-        ProceduralStream<IrisBiome> finalBiomeStream = heightStream.convertAware2D((height, x, z) -> {
-            IrisBiome mappedBiome = imageMapRuntime.sampleBiome(x, z);
-            if (mappedBiome != null) {
-                return mappedBiome;
-            }
-            IrisBiome baseBiome = baseBiomeStream.get(x, z);
-            IrisBiome resolved = IrisComplex.resolveSurfaceBiome(
-                    height,
-                    baseBiome,
-                    regionStream.get(x, z),
-                    x,
-                    z,
-                    fluidHeight,
-                    landBiomeStream,
-                    seaBiomeStream,
-                    shoreBiomeStream);
-            return resolved == baseBiome
-                    ? baseBiome
-                    : implode(resolved, x, z, rng, dataProvider, childSelectionPlans, 3);
-        }).cache2D("upperImageMappedFinalBiomeStream", engine, cacheSize);
-
-        ProceduralStream<PlatformBlockState> rockStream = upperDim.getRockPalette()
-                .getLayerGenerator(rng.nextParallelRNG(45), upperData).stream()
-                .select(upperDim.getRockPalette().getBlockData(upperData));
-
-        return new UpperDimensionContext(new ContextState(
-                upperDim,
-                upperData,
-                chunkHeight,
-                heightStream,
-                finalBiomeStream,
-                regionStream,
-                rockStream,
-                imageMapRuntime,
-                false
-        ));
-    }
-
-    private static void registerBiomeGenerators(
-            IrisBiome biome,
-            DataProvider dataProvider,
-            Set<IrisBiome> allBiomes,
-            Map<IrisInterpolator, Set<IrisGenerator>> generators
-    ) {
-        allBiomes.add(biome);
-        biome.getGenerators().forEach(link -> {
-            IrisGenerator generator = link.getCachedGenerator(dataProvider);
-            if (generator != null) {
-                generators.computeIfAbsent(generator.getInterpolator(), key -> new HashSet<>()).add(generator);
-            }
-        });
     }
 
     static IrisRegion mappedRegion(
@@ -314,8 +49,8 @@ public class UpperDimensionContext implements DataProvider {
             double worldX,
             double worldZ
     ) {
-        IrisRegion mappedRegion = imageMapRuntime.sampleRegion(worldX, worldZ);
+        return DimensionTerrainContext.mappedRegion(
-        return mappedRegion == null ? proceduralRegion : mappedRegion;
+                imageMapRuntime, proceduralRegion, worldX, worldZ);
     }
 
     static double mappedTerrainHeight(
@@ -324,7 +59,8 @@ public class UpperDimensionContext implements DataProvider {
             double worldX,
             double worldZ
     ) {
-        return imageMapRuntime.sampleTerrainHeight(worldX, worldZ, proceduralHeight);
+        return DimensionTerrainContext.mappedTerrainHeight(
+                imageMapRuntime, proceduralHeight, worldX, worldZ);
     }
 
     static IrisBiome mappedBiome(
@@ -333,8 +69,8 @@ public class UpperDimensionContext implements DataProvider {
             double worldX,
             double worldZ
     ) {
-        IrisBiome mappedBiome = imageMapRuntime.sampleBiome(worldX, worldZ);
+        return DimensionTerrainContext.mappedBiome(
-        return mappedBiome == null ? proceduralBiome : mappedBiome;
+                imageMapRuntime, proceduralBiome, worldX, worldZ);
     }
 
     static PlatformBlockState mappedSurfaceBlock(
@@ -343,108 +79,40 @@ public class UpperDimensionContext implements DataProvider {
             double worldX,
             double worldZ
     ) {
-        PlatformBlockState mappedBlock = imageMapRuntime.sampleSurfaceBlock(worldX, worldZ);
+        return DimensionTerrainContext.mappedSurfaceBlock(
-        return mappedBlock == null ? proceduralBlock : mappedBlock;
+                imageMapRuntime, proceduralBlock, worldX, worldZ);
-    }
-
-    private static KList<IrisBiome> loadInferredBiomes(IrisData data, KList<String> keys, InferredType type) {
-        KList<IrisBiome> inferred = new KList<>();
-        for (IrisBiome biome : data.getBiomeLoader().loadAll(keys)) {
-            inferred.add(biome.withInferredType(type));
-        }
-        return inferred;
-    }
-
-    private static IrisBiome implode(
-            IrisBiome biome,
-            double x,
-            double z,
-            RNG rng,
-            DataProvider dataProvider,
-            Map<IrisBiome, IrisComplex.ChildSelectionPlan> childSelectionPlans,
-            int remainingDepth
-    ) {
-        if (biome == null || remainingDepth < 0 || biome.getChildren().isEmpty()) {
-            return biome;
-        }
-
-        IrisComplex.ChildSelectionPlan selectionPlan = childSelectionPlans.get(biome);
-        if (selectionPlan == null) {
-            synchronized (childSelectionPlans) {
-                selectionPlan = childSelectionPlans.get(biome);
-                if (selectionPlan == null) {
-                    KList<IrisBiome> options = new KList<>();
-                    for (IrisBiome child : biome.getRealChildren(dataProvider)) {
-                        if (child != null) {
-                            options.add(child);
-                        }
-                    }
-                    options.add(biome);
-                    selectionPlan = IrisComplex.ChildSelectionPlan.create(options);
-                    childSelectionPlans.put(biome, selectionPlan);
-                }
-            }
-        }
-
-        IrisBiome selected = selectionPlan.select(
-                biome.getChildrenGenerator(rng, 123, biome.getChildShrinkFactor()), x, z);
-        if (selected == null) {
-            return biome;
-        }
-        return implode(
-                selected.withInferredType(biome.getInferredType()),
-                x,
-                z,
-                rng,
-                dataProvider,
-                childSelectionPlans,
-                remainingDepth - 1);
     }
 
     public int getUpperSurfaceY(int x, int z) {
-        double rawHeight = heightStream.get((double) x, (double) z);
+        return chunkHeight - 1 - (int) Math.round(terrainContext.getNormalTerrainHeight(x, z));
-        return chunkHeight - 1 - (int) Math.round(rawHeight);
     }
 
     public IrisBiome getUpperBiome(int x, int z) {
-        return biomeStream.get((double) x, (double) z);
+        return terrainContext.getBiome(x, z);
     }
 
     public IrisRegion getUpperRegion(int x, int z) {
-        return regionStream == null ? null : regionStream.get((double) x, (double) z);
+        return terrainContext.getRegion(x, z);
     }
 
     public PlatformBlockState getRockBlock(int x, int z) {
-        return rockStream.get((double) x, (double) z);
+        return terrainContext.getRockBlock(x, z);
     }
 
     public PlatformBlockState getSurfaceBlock(int x, int z) {
-        return imageMapRuntime.sampleSurfaceBlock(x, z);
+        return terrainContext.getSurfaceBlock(x, z);
     }
 
     public IrisDimension getDimension() {
-        return dimension;
+        return terrainContext.getDimension();
     }
 
     @Override
     public IrisData getData() {
-        return data;
+        return terrainContext.getData();
     }
 
     public boolean isSelfReferencing() {
-        return selfReferencing;
+        return terrainContext.isSelfReferencing();
-    }
-
-    private record ContextState(
-            IrisDimension dimension,
-            IrisData data,
-            int chunkHeight,
-            ProceduralStream<Double> heightStream,
-            ProceduralStream<IrisBiome> biomeStream,
-            ProceduralStream<IrisRegion> regionStream,
-            ProceduralStream<PlatformBlockState> rockStream,
-            IrisImageMapRuntime imageMapRuntime,
-            boolean selfReferencing
-    ) {
     }
 }

core/src/main/java/art/arcane/iris/engine/actuator/IrisBiomeActuator.java

@@ -18,10 +18,13 @@
 
 package art.arcane.iris.engine.actuator;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.EngineAssignedActuator;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisBiomeCustom;
+import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.util.project.context.ChunkContext;
 import art.arcane.iris.util.project.context.ChunkedDataCache;
 import art.arcane.volmlib.util.collection.KMap;
@@ -38,6 +41,8 @@ import art.arcane.iris.spi.IrisPlatform;
 import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.spi.PlatformBiome;
 
+import java.util.List;
+
 public class IrisBiomeActuator extends EngineAssignedActuator<PlatformBiome> {
     private final RNG rng;
     private final ChronoLatch cl = new ChronoLatch(5000);
@@ -60,33 +65,167 @@ public class IrisBiomeActuator extends EngineAssignedActuator<PlatformBiome> {
         Engine engine = getEngine();
         Mantle<Matter> mantle = engine.getMantle().getMantle();
         ChunkedDataCache<IrisBiome> biomeCache = context.getBiome();
+        DimensionStackContext dimensionStackContext = engine.getDimensionStackContext();
 
         for (int xf = 0; xf < width; xf++) {
-            IrisBiome ib;
             for (int zf = 0; zf < depth; zf++) {
-                ib = biomeCache.get(xf, zf);
+                int worldX = x + xf;
-                String key;
+                int worldZ = z + zf;
+                IrisBiome biome = biomeCache.get(xf, zf);
+                ResolvedBiome resolved = resolve(biomeKey(
+                        biome,
+                        getDimension(),
+                        engine,
+                        worldX,
+                        0,
+                        worldZ
+                ));
+                PlatformBiome platformBiome = resolved.biome();
 
-                if (ib.isCustom()) {
+                if (platformBiome != null) {
-                    IrisBiomeCustom custom = ib.getCustomBiome(rng, engine, x + xf, 0, z + zf);
+                    h.set(xf, 0, zf, xf, height - 1, zf, platformBiome);
-                    key = getDimension().getLoadKey() + ":" + custom.getId();
-                } else {
-                    key = ib.getSkyBiomeKey(rng, engine, x + xf, 0, z + zf);
                 }
 
-                ResolvedBiome resolved = resolve(key);
+                mantle.set(worldX, 0, worldZ, resolved.matter());
-                PlatformBiome biome = resolved.biome();
+                if (dimensionStackContext != null) {
-
+                    PlatformBiome bottomBiome = platformBiome == null
-                if (biome != null) {
+                            ? h.getRaw(xf, 0, zf)
-                    h.set(xf, 0, zf, xf, height - 1, zf, biome);
+                            : platformBiome;
+                    applyDimensionStackBiomes(
+                            xf,
+                            zf,
+                            worldX,
+                            worldZ,
+                            h,
+                            mantle,
+                            engine,
+                            new ResolvedBiome(bottomBiome, resolved.matter()),
+                            context.getDimensionStackLayout(xf, zf)
+                    );
                 }
-
-                mantle.set(x + xf, 0, z + zf, resolved.matter());
             }
         }
         engine.getMetrics().getBiome().put(p.getMilliseconds());
     }
 
+    private void applyDimensionStackBiomes(
+            int localX,
+            int localZ,
+            int worldX,
+            int worldZ,
+            Hunk<PlatformBiome> output,
+            Mantle<Matter> mantle,
+            Engine engine,
+            ResolvedBiome bottom,
+            DimensionStackLayout layout
+    ) {
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        for (int layerIndex = 1; layerIndex < layers.size(); layerIndex++) {
+            DimensionStackLayout.Layer lower = layers.get(layerIndex - 1);
+            DimensionStackLayout.Layer layer = layers.get(layerIndex);
+            int gapMinY = (int) Math.max(0L, (long) lower.contentTopY() + 1L);
+            int gapMaxY = (int) Math.min(
+                    (long) output.getHeight() - 1L,
+                    (long) layer.localBaseY() - 1L
+            );
+            applyBiomeRange(
+                    localX,
+                    localZ,
+                    worldX,
+                    worldZ,
+                    gapMinY,
+                    gapMaxY,
+                    output,
+                    mantle,
+                    bottom
+            );
+            if (!layer.visible()) {
+                continue;
+            }
+            ResolvedBiome resolved = bottom;
+            if (layer.biome() != null) {
+                IrisDimension dimension = layer.terrainContext().getDimension();
+                resolved = resolve(biomeKey(
+                        layer.biome(),
+                        dimension,
+                        engine,
+                        worldX,
+                        layer.clippedSurfaceY(),
+                        worldZ
+                ));
+            }
+            applyBiomeRange(
+                    localX,
+                    localZ,
+                    worldX,
+                    worldZ,
+                    layer.renderMinY(),
+                    layer.renderMaxY(),
+                    output,
+                    mantle,
+                    resolved
+            );
+        }
+    }
+
+    private void applyBiomeRange(
+            int localX,
+            int localZ,
+            int worldX,
+            int worldZ,
+            int minimumY,
+            int maximumY,
+            Hunk<PlatformBiome> output,
+            Mantle<Matter> mantle,
+            ResolvedBiome resolved
+    ) {
+        if (minimumY > maximumY) {
+            return;
+        }
+        clearBiomeMatterRange(mantle, worldX, worldZ, minimumY, maximumY);
+        if (resolved.biome() != null) {
+            output.set(
+                    localX,
+                    minimumY,
+                    localZ,
+                    localX,
+                    maximumY,
+                    localZ,
+                    resolved.biome()
+            );
+        }
+        if (resolved.matter() != null) {
+            mantle.set(worldX, minimumY, worldZ, resolved.matter());
+        }
+    }
+
+    static void clearBiomeMatterRange(
+            Mantle<Matter> mantle,
+            int worldX,
+            int worldZ,
+            int minimumY,
+            int maximumY
+    ) {
+        for (int y = minimumY; y <= maximumY; y++) {
+            mantle.remove(worldX, y, worldZ, MatterBiomeInject.class);
+        }
+    }
+
+    private String biomeKey(
+            IrisBiome biome,
+            IrisDimension dimension,
+            Engine engine,
+            int x,
+            int y,
+            int z
+    ) {
+        if (biome.isCustom()) {
+            IrisBiomeCustom custom = biome.getCustomBiome(rng, engine, x, y, z);
+            return dimension.getCustomBiomeKey(custom.getId());
+        }
+        return biome.getSkyBiomeKey(rng, engine, x, y, z);
+    }
+
     /**
      * The registry lookup and the biome id only depend on the scatter resolved key, so cache the pair
      * instead of paying two string keyed lookups per column. Unresolved biomes are never cached, so a

core/src/main/java/art/arcane/iris/engine/actuator/IrisDimensionStackActuator.java

@@ -0,0 +1,258 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.actuator;
+
+import art.arcane.iris.core.link.Identifier;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.DimensionStackLayout;
+import art.arcane.iris.engine.DimensionTerrainContext;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.framework.EngineAssignedActuator;
+import art.arcane.iris.engine.framework.TreeBlockMaterial;
+import art.arcane.iris.engine.hydrology.cave.HydrologyCaveCell;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.util.common.data.B;
+import art.arcane.iris.util.project.context.ChunkContext;
+import art.arcane.iris.util.project.hunk.Hunk;
+import art.arcane.iris.util.project.matter.TileWrapper;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.documentation.BlockCoordinates;
+import art.arcane.volmlib.util.mantle.runtime.MantleChunk;
+import art.arcane.volmlib.util.math.RNG;
+import art.arcane.volmlib.util.matter.Matter;
+import art.arcane.volmlib.util.matter.MatterCavern;
+import art.arcane.volmlib.util.matter.MatterMarker;
+import art.arcane.volmlib.util.matter.MatterSlice;
+import art.arcane.volmlib.util.matter.MatterStructurePOI;
+import art.arcane.volmlib.util.matter.MatterUpdate;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public final class IrisDimensionStackActuator extends EngineAssignedActuator<PlatformBlockState> {
+    private static final PlatformBlockState AIR = B.getState("AIR");
+    private static final PlatformBlockState BEDROCK = B.getState("BEDROCK");
+    private static final List<Class<?>> REPLACED_METADATA = List.of(
+            TileWrapper.class,
+            Identifier.class,
+            String.class,
+            TreeBlockMaterial.class,
+            MatterCavern.class,
+            HydrologyCaveCell.class,
+            MatterMarker.class,
+            MatterStructurePOI.class,
+            MatterUpdate.class
+    );
+
+    private final RNG rng;
+
+    public IrisDimensionStackActuator(Engine engine) {
+        super(engine, "Dimension Stack");
+        rng = new RNG(engine.getSeedManager().getTerrain());
+    }
+
+    @BlockCoordinates
+    @Override
+    public void onActuate(
+            int x,
+            int z,
+            Hunk<PlatformBlockState> blocks,
+            boolean multicore,
+            ChunkContext context
+    ) {
+        if (getEngine().getDimensionStackContext() == null) {
+            return;
+        }
+        MantleChunk<Matter> mantleChunk = getEngine().getMantle().getMantle()
+                .getChunk(x >> 4, z >> 4).use();
+        try {
+            MetadataCleaner metadata = new MetadataCleaner(mantleChunk, blocks.getHeight());
+            for (int localX = 0; localX < blocks.getWidth(); localX++) {
+                int worldX = x + localX;
+                for (int localZ = 0; localZ < blocks.getDepth(); localZ++) {
+                    renderLayers(
+                            localX,
+                            localZ,
+                            worldX,
+                            z + localZ,
+                            blocks,
+                            context.getDimensionStackLayout(localX, localZ),
+                            metadata
+                    );
+                }
+            }
+        } finally {
+            mantleChunk.release();
+        }
+    }
+
+    private void renderLayers(
+            int localX,
+            int localZ,
+            int worldX,
+            int worldZ,
+            Hunk<PlatformBlockState> blocks,
+            DimensionStackLayout layout,
+            MetadataCleaner metadata
+    ) {
+        if (layout == null) {
+            return;
+        }
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        for (int layerIndex = 1; layerIndex < layers.size(); layerIndex++) {
+            DimensionStackLayout.Layer layer = layers.get(layerIndex);
+            clearGap(localX, localZ, blocks, layers.get(layerIndex - 1), layer, metadata);
+            if (layer.visible()) {
+                renderLayer(localX, localZ, worldX, worldZ, blocks, layer, metadata);
+            }
+        }
+    }
+
+    private void clearGap(
+            int localX,
+            int localZ,
+            Hunk<PlatformBlockState> output,
+            DimensionStackLayout.Layer lower,
+            DimensionStackLayout.Layer upper,
+            MetadataCleaner metadata
+    ) {
+        int minimumY = (int) Math.max(0L, (long) lower.contentTopY() + 1L);
+        int maximumY = (int) Math.min(
+                (long) output.getHeight() - 1L,
+                (long) upper.localBaseY() - 1L
+        );
+        for (int y = minimumY; y <= maximumY; y++) {
+            metadata.clear(localX, y, localZ);
+            output.setRaw(localX, y, localZ, AIR);
+        }
+    }
+
+    private void renderLayer(
+            int localX,
+            int localZ,
+            int worldX,
+            int worldZ,
+            Hunk<PlatformBlockState> output,
+            DimensionStackLayout.Layer layer,
+            MetadataCleaner metadata
+    ) {
+        DimensionTerrainContext terrainContext = layer.terrainContext();
+        IrisDimension dimension = terrainContext.getDimension();
+        IrisData data = terrainContext.getData();
+        IrisBiome biome = layer.biome();
+        int surfaceDepth = Math.max(0, layer.normalTerrainHeight());
+        int fluidDepth = Math.max(0, layer.fluidHeight() - layer.normalTerrainHeight());
+        KList<PlatformBlockState> surfaceBlocks = biome == null
+                ? null
+                : biome.generateLayersWithSlope(
+                        dimension,
+                        worldX,
+                        worldZ,
+                        rng,
+                        surfaceDepth,
+                        layer.normalTerrainHeight(),
+                        data,
+                        terrainContext.getSlopeStream()
+                );
+        KList<PlatformBlockState> seaBlocks = biome == null || fluidDepth == 0
+                ? null
+                : biome.generateSeaLayers(worldX, worldZ, rng, fluidDepth, data);
+
+        for (int y = layer.renderMaxY(); y >= layer.renderMinY(); y--) {
+            int sourceY = y - layer.localBaseY();
+            if (sourceY == 0 && dimension.isBedrock()) {
+                writeBlock(output, metadata, localX, y, localZ, BEDROCK);
+                continue;
+            }
+            if (sourceY > layer.normalTerrainHeight() && sourceY <= layer.fluidHeight()) {
+                int fluidLayerDepth = layer.fluidY() - y;
+                writeBlock(output, metadata, localX, y, localZ, HydrologyFluidLayerSelector.select(
+                        seaBlocks,
+                        fluidLayerDepth,
+                        layer.fluidBlock(),
+                        false
+                ));
+                continue;
+            }
+            if (sourceY > layer.normalTerrainHeight()) {
+                continue;
+            }
+            int depth = layer.surfaceY() - y;
+            if (depth == 0 && layer.surfaceBlock() != null) {
+                writeBlock(output, metadata, localX, y, localZ, layer.surfaceBlock());
+            } else if (surfaceBlocks != null && surfaceBlocks.hasIndex(depth)) {
+                writeBlock(output, metadata, localX, y, localZ, surfaceBlocks.get(depth));
+            } else {
+                writeBlock(output, metadata, localX, y, localZ, layer.rockBlock());
+            }
+        }
+    }
+
+    private void writeBlock(
+            Hunk<PlatformBlockState> output,
+            MetadataCleaner metadata,
+            int x,
+            int y,
+            int z,
+            PlatformBlockState block
+    ) {
+        metadata.clear(x, y, z);
+        output.setRaw(x, y, z, block);
+    }
+
+    static final class MetadataCleaner {
+        private final MantleChunk<Matter> chunk;
+        private final List<MatterSlice<?>>[] slicesBySection;
+
+        @SuppressWarnings("unchecked")
+        MetadataCleaner(MantleChunk<Matter> chunk, int height) {
+            this.chunk = chunk;
+            slicesBySection = (List<MatterSlice<?>>[]) new List<?>[(height + 15) >> 4];
+        }
+
+        void clear(int x, int y, int z) {
+            int sectionIndex = y >> 4;
+            List<MatterSlice<?>> slices = slicesBySection[sectionIndex];
+            if (slices == null) {
+                slices = resolveSlices(sectionIndex);
+                slicesBySection[sectionIndex] = slices;
+            }
+            for (MatterSlice<?> slice : slices) {
+                slice.set(x, y & 15, z, null);
+            }
+        }
+
+        private List<MatterSlice<?>> resolveSlices(int sectionIndex) {
+            Matter section = chunk.get(sectionIndex);
+            if (section == null) {
+                return List.of();
+            }
+            ArrayList<MatterSlice<?>> slices = new ArrayList<>(REPLACED_METADATA.size());
+            for (Class<?> type : REPLACED_METADATA) {
+                MatterSlice<?> slice = section.getSlice(type);
+                if (slice != null) {
+                    slices.add(slice);
+                }
+            }
+            return List.copyOf(slices);
+        }
+    }
+}

core/src/main/java/art/arcane/iris/engine/framework/Engine.java

@@ -28,6 +28,9 @@ import art.arcane.iris.core.nms.container.BlockPos;
 import art.arcane.iris.core.nms.container.Pair;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.engine.UpperDimensionContext;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.data.chunk.TerrainChunk;
 import art.arcane.iris.engine.mantle.EngineMantle;
 import art.arcane.iris.engine.object.IrisBiome;
@@ -90,6 +93,10 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         return null;
     }
 
+    default @Nullable DimensionStackContext getDimensionStackContext() {
+        return null;
+    }
+
     EngineMode getMode();
 
     EnginePlatformHooks getPlatformHooks();
@@ -255,6 +262,25 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     default IrisRegion getRegion(int x, int z) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            DimensionStackLayout.Layer layer = dimensionStackContext.getLayout(x, z).surfaceLayer();
+            if (layer != null && layer.region() != null) {
+                return layer.region();
+            }
+        }
+        return getComplex().getRegionStream().get(x, z);
+    }
+
+    @BlockCoordinates
+    default IrisRegion getRegion(int x, int y, int z) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            DimensionStackLayout.Layer layer = dimensionStackContext.getLayout(x, z).layerAt(y);
+            if (layer != null && layer.region() != null) {
+                return layer.region();
+            }
+        }
         return getComplex().getRegionStream().get(x, z);
     }
 
@@ -301,7 +327,45 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     default IrisBiome getCaveBiome(int x, int y, int z, IrisDimensionCarvingResolver.State state) {
-        IrisBiome surfaceBiome = getSurfaceBiome(x, z);
+        IrisBiome configuredBiome = resolveConfiguredCaveBiome(x, y, z, state);
+        if (configuredBiome != null) {
+            return configuredBiome;
+        }
+        boolean naturalFallback = answersFromNaturalTerrain(x, z);
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        IrisBiome surfaceBiome = naturalFallback || dimensionStackContext == null
+                ? naturalFallback
+                        ? getComplex().naturalSurfaceBiome(x, z)
+                        : getSurfaceBiome(x, z)
+                : getComplex().getTrueBiomeStream().get(x, z);
+        int surfaceY = naturalFallback
+                ? getComplex().naturalTrueHeight(x, z)
+                : getComplex().getHeightStream().get(x, z).intValue();
+        return resolveDepthCaveBiome(x, y, z, surfaceBiome, surfaceY);
+    }
+
+    @BlockCoordinates
+    default IrisBiome getCaveBiome(
+            int x,
+            int y,
+            int z,
+            IrisDimensionCarvingResolver.State state,
+            IrisBiome surfaceBiome,
+            int surfaceY
+    ) {
+        IrisBiome configuredBiome = resolveConfiguredCaveBiome(x, y, z, state);
+        if (configuredBiome != null) {
+            return configuredBiome;
+        }
+        return resolveDepthCaveBiome(x, y, z, surfaceBiome, surfaceY);
+    }
+
+    private IrisBiome resolveConfiguredCaveBiome(
+            int x,
+            int y,
+            int z,
+            IrisDimensionCarvingResolver.State state
+    ) {
         int worldY = y + getWorld().minHeight();
         IrisDimensionCarvingEntry rootCarvingEntry = IrisDimensionCarvingResolver.resolveRootEntry(this, worldY, state);
         if (rootCarvingEntry != null) {
@@ -311,13 +375,21 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
                 return resolvedCarvingBiome;
             }
         }
+        return null;
+    }
 
+    private IrisBiome resolveDepthCaveBiome(
+            int x,
+            int y,
+            int z,
+            IrisBiome surfaceBiome,
+            int surfaceY
+    ) {
         IrisBiome caveBiome = getCaveBiome(x, z);
         if (caveBiome == null) {
             return surfaceBiome;
         }
 
-        int surfaceY = getComplex().getHeightStream().get(x, z).intValue();
         int depthBelowSurface = surfaceY - y;
         if (depthBelowSurface <= 0) {
             return surfaceBiome;
@@ -333,6 +405,18 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     default IrisBiome getSurfaceBiome(int x, int z) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            DimensionStackLayout.Layer layer = dimensionStackContext.getLayout(x, z).surfaceLayer();
+            if (layer != null && layer.biome() != null) {
+                return layer.biome();
+            }
+        }
+        return getHostSurfaceBiome(x, z);
+    }
+
+    @BlockCoordinates
+    default IrisBiome getHostSurfaceBiome(int x, int z) {
         if (answersFromNaturalTerrain(x, z)) {
             return getComplex().naturalSurfaceBiome(x, z);
         }
@@ -364,6 +448,12 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     default int getHeight(int x, int z, boolean ignoreFluid) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            return ignoreFluid
+                    ? dimensionStackContext.getStackTerrainHeight(x, z)
+                    : dimensionStackContext.getStackTopHeight(x, z);
+        }
         if (answersFromNaturalTerrain(x, z)) {
             int natural = getComplex().naturalTrueHeight(x, z);
             return ignoreFluid ? natural : Math.max(natural, getMantle().getFluidHeight());
@@ -371,6 +461,24 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         return getMantle().getHighest(x, z, getData(), ignoreFluid);
     }
 
+    @BlockCoordinates
+    static int hostHeight(Engine engine, int x, int z, boolean ignoreFluid) {
+        EngineMantle mantle = engine.getMantle();
+        if (mantle == null) {
+            return engine.getHeight(x, z, ignoreFluid);
+        }
+        if (engine.answersFromNaturalTerrain(x, z)) {
+            int naturalHeight = engine.getComplex().naturalTrueHeight(x, z);
+            return ignoreFluid
+                    ? naturalHeight
+                    : Math.max(naturalHeight, mantle.getFluidHeight());
+        }
+        int terrainHeight = mantle.trueHeight(x, z);
+        return ignoreFluid
+                ? terrainHeight
+                : Math.max(terrainHeight, mantle.getFluidHeight(x, z));
+    }
+
     @BlockCoordinates
     @Override
     default void catchBlockUpdates(int x, int y, int z, PlatformBlockState data) {
@@ -410,6 +518,15 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
         IrisDimension dim = getDimension();
         dim.getReachableBiomes(this).forEach((i) -> v.put(i.getLoadKey(), i));
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            for (DimensionTerrainContext terrainContext : dimensionStackContext.getLayersBottomToTop()) {
+                if (!terrainContext.isSelfReferencing()) {
+                    terrainContext.getDimension().getReachableBiomes(terrainContext)
+                            .forEach((i) -> v.put(i.getLoadKey(), i));
+                }
+            }
+        }
 
         return v.v();
     }
@@ -454,6 +571,22 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
     boolean isStudio();
 
     default IrisBiome getBiome(int x, int y, int z) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            DimensionStackLayout layout = dimensionStackContext.getLayout(x, z);
+            IrisBiome stackedBiome = dimensionStackBiome(layout, y);
+            if (stackedBiome != null) {
+                return stackedBiome;
+            }
+            DimensionStackLayout.Layer bottomLayer = layout.layersBottomToTop().get(0);
+            if (y > bottomLayer.renderMaxY()) {
+                return bottomLayer.biome();
+            }
+            if (y <= bottomLayer.clippedSurfaceY() - 2) {
+                return getCaveBiome(x, y, z);
+            }
+            return bottomLayer.biome();
+        }
         if (y <= getHeight(x, z) - 2) {
             return getCaveBiome(x, y, z);
         }
@@ -462,6 +595,22 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
     }
 
     default IrisBiome getBiomeOrMantle(int x, int y, int z) {
+        DimensionStackContext dimensionStackContext = getDimensionStackContext();
+        if (dimensionStackContext != null) {
+            DimensionStackLayout layout = dimensionStackContext.getLayout(x, z);
+            IrisBiome stackedBiome = dimensionStackBiome(layout, y);
+            if (stackedBiome != null) {
+                return stackedBiome;
+            }
+            DimensionStackLayout.Layer bottomLayer = layout.layersBottomToTop().get(0);
+            if (y > bottomLayer.renderMaxY()) {
+                return bottomLayer.biome();
+            }
+            if (y <= bottomLayer.clippedSurfaceY() - 2) {
+                return getCaveOrMantleBiome(x, y, z);
+            }
+            return bottomLayer.biome();
+        }
         if (y <= getHeight(x, z) - 2) {
             return getCaveOrMantleBiome(x, y, z);
         }
@@ -469,6 +618,12 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         return getSurfaceBiome(x, z);
     }
 
+    private static IrisBiome dimensionStackBiome(DimensionStackLayout layout, int y) {
+        DimensionStackLayout.Layer bottom = layout.layersBottomToTop().get(0);
+        DimensionStackLayout.Layer owner = layout.layerAt(y);
+        return owner == bottom ? null : owner.biome();
+    }
+
     default String getObjectPlacementKey(int x, int y, int z) {
         PlacedObject o = getObjectPlacement(x, y, z);
 
@@ -524,7 +679,7 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         }
 
 
-        IrisRegion region = getRegion(x, z);
+        IrisRegion region = getComplex().getRegionStream().get(x, z);
 
         for (IrisObjectPlacement i : region.getObjects()) {
             if (i.compatPlace(getData()).contains(object)) {
@@ -532,7 +687,7 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
             }
         }
 
-        IrisBiome biome = getSurfaceBiome(x, z);
+        IrisBiome biome = getComplex().getTrueBiomeStream().get(x, z);
 
         for (IrisObjectPlacement i : biome.getObjects()) {
             if (i.compatPlace(getData()).contains(object)) {

core/src/main/java/art/arcane/iris/engine/framework/EngineMode.java

@@ -151,7 +151,16 @@ public interface EngineMode extends Staged {
     default void generate(int x, int z, Hunk<PlatformBlockState> blocks, Hunk<PlatformBiome> biomes, boolean multicore, long generationSessionId) {
         boolean cacheContext = !getEngine().getPlatformHooks().shouldDisableChunkContextCache(getEngine());
         ChunkContext.PrefillPlan prefillPlan = cacheContext ? ChunkContext.PrefillPlan.NO_CAVE : ChunkContext.PrefillPlan.NONE;
-        ChunkContext ctx = new ChunkContext(x, z, getComplex(), generationSessionId, cacheContext, prefillPlan, getEngine().getMetrics());
+        ChunkContext ctx = new ChunkContext(
+                x,
+                z,
+                getComplex(),
+                generationSessionId,
+                cacheContext,
+                prefillPlan,
+                getEngine().getMetrics(),
+                getEngine().getDimensionStackContext()
+        );
 
         EngineStage[] stages = getStages().toArray(new EngineStage[0]);
         try (IrisContext.Scope chunkScope = IrisContext.open(getEngine(), generationSessionId, ctx)) {

core/src/main/java/art/arcane/iris/engine/framework/EnginePlatformHooks.java

@@ -46,6 +46,9 @@ public interface EnginePlatformHooks {
     default void validateDimensionHotload(Engine engine, IrisDimension replacement) {
     }
 
+    default void prepareRuntimeHotload(Engine engine) {
+    }
+
     default void applyWorldBoundary(Engine engine) {
     }
 

core/src/main/java/art/arcane/iris/engine/framework/HintedLocator.java

@@ -19,6 +19,8 @@
 package art.arcane.iris.engine.framework;
 
 import art.arcane.iris.core.IrisSettings;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.object.IrisBiome;
@@ -282,6 +284,13 @@ public final class HintedLocator<T> implements Locator<T> {
         if (engine.getFocus() != null || engine.getFocusRegion() != null) {
             return SearchPlan.unpruned();
         }
+        if (engine.getDimensionStackContext() != null) {
+            for (IrisBiome biome : engine.getAllBiomes()) {
+                if (biome != null && biomeKey.equals(biome.getLoadKey())) {
+                    return SearchPlan.unpruned();
+                }
+            }
+        }
 
         IrisComplex complex = engine.getComplex();
         BiomeSource source = new BiomeSource(engine);
@@ -380,6 +389,17 @@ public final class HintedLocator<T> implements Locator<T> {
             return SearchPlan.unpruned();
         }
 
+        DimensionStackContext stackContext = engine.getDimensionStackContext();
+        if (stackContext != null) {
+            for (DimensionTerrainContext terrainContext : stackContext.getLayersTopToBottom()) {
+                for (IrisRegion region : terrainContext.getDimension().getAllRegions(terrainContext)) {
+                    if (region != null && regionKey.equals(region.getLoadKey())) {
+                        return SearchPlan.unpruned();
+                    }
+                }
+            }
+        }
+
         if (!engine.getDimension().getRegions().contains(regionKey)) {
             return SearchPlan.impossible();
         }

core/src/main/java/art/arcane/iris/engine/framework/IrisStructureLocator.java

@@ -655,7 +655,7 @@ public final class IrisStructureLocator {
             return null;
         }
         if (placement.resolvedAnchor() == IrisStructureAnchorMode.SURFACE) {
-            int surfaceY = engine.getHeight(originX, originZ, true) + engine.getMinHeight();
+            int surfaceY = Engine.hostHeight(engine, originX, originZ, true) + engine.getMinHeight();
             return surfaceY < placement.getMinHeight() || surfaceY > placement.getMaxHeight() ? null : surfaceY;
         }
         int bandMin = Math.max(worldMin, Math.min(placement.getMinHeight(), placement.getMaxHeight()));
@@ -766,7 +766,7 @@ public final class IrisStructureLocator {
                     if (surfaceHeights.size() >= MAX_BURIAL_COLUMNS) {
                         return Integer.MIN_VALUE;
                     }
-                    surfaceY = engine.getHeight(x, z, true) + engine.getMinHeight();
+                    surfaceY = Engine.hostHeight(engine, x, z, true) + engine.getMinHeight();
                     surfaceHeights.put(columnKey, surfaceY);
                 }
                 int allowedTopY = surfaceY - UNDERGROUND_SURFACE_CLEARANCE;

core/src/main/java/art/arcane/iris/engine/framework/NativeStructurePlacementPlanner.java

@@ -142,7 +142,7 @@ public final class NativeStructurePlacementPlanner {
             return null;
         }
         if (!placement.isUnderground()) {
-            int surfaceY = engine.getHeight(blockX, blockZ, true) + engine.getMinHeight();
+            int surfaceY = Engine.hostHeight(engine, blockX, blockZ, true) + engine.getMinHeight();
             if (surfaceY < placement.getMinHeight() || surfaceY > placement.getMaxHeight()) {
                 return null;
             }
@@ -160,7 +160,7 @@ public final class NativeStructurePlacementPlanner {
         int localFluidHeight = engine.getComplex() == null
                 ? engine.getDimension().getFluidHeight()
                 : (int) Math.round(engine.getComplex().getRiverWaterSurfaceStream().get(blockX, blockZ));
-        return engine.getHeight(blockX, blockZ, true) < localFluidHeight;
+        return Engine.hostHeight(engine, blockX, blockZ, true) < localFluidHeight;
     }
 
     private static int comparePlacementPriority(IrisStructurePlacement left, IrisStructurePlacement right) {

core/src/main/java/art/arcane/iris/engine/mantle/EngineMantle.java

@@ -21,6 +21,8 @@ package art.arcane.iris.engine.mantle;
 import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.IrisComplex;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.UpperDimensionContext;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.EngineTarget;
@@ -36,6 +38,7 @@ import art.arcane.iris.util.common.data.B;
 import art.arcane.volmlib.util.documentation.BlockCoordinates;
 import art.arcane.volmlib.util.documentation.ChunkCoordinates;
 import art.arcane.iris.util.project.hunk.Hunk;
+import art.arcane.iris.util.project.context.ChunkContext;
 import art.arcane.volmlib.util.mantle.runtime.Mantle;
 import art.arcane.volmlib.util.mantle.runtime.MantleChunk;
 import art.arcane.volmlib.util.mantle.flag.MantleFlag;
@@ -196,17 +199,37 @@ public interface EngineMantle extends MatterGenerator {
     }
 
     @ChunkCoordinates
-    default void insertMatter(int x, int z, Hunk<PlatformBlockState> blocks, boolean multicore) {
+    default void insertMatter(
+            int x,
+            int z,
+            Hunk<PlatformBlockState> blocks,
+            boolean multicore,
+            ChunkContext context
+    ) {
         if (!getEngine().getDimension().isUseMantle()) {
             return;
         }
 
         UpperDimensionContext upperCtx = getEngine().getUpperContext();
         boolean protectUpper = upperCtx != null;
+        DimensionStackContext stackContext = getEngine().getDimensionStackContext();
 
         MantleChunk<Matter> chunk = getMantle().getChunk(x, z).use();
         try {
-            if (protectUpper) {
+            if (stackContext != null) {
+                DimensionStackLayout[] layouts = new DimensionStackLayout[256];
+                for (int i = 0; i < layouts.length; i++) {
+                    int localX = i >> 4;
+                    int localZ = i & 15;
+                    layouts[i] = context.getDimensionStackLayout(localX, localZ);
+                }
+                chunk.iterate(PlatformBlockState.class, (localX, y, localZ, value) -> {
+                    DimensionStackLayout layout = layouts[(localX << 4) | (localZ & 15)];
+                    if (!layout.isHostFeatureProtectedY(y)) {
+                        blocks.set(localX, y, localZ, value);
+                    }
+                });
+            } else if (protectUpper) {
                 int chunkBlockX = x << 4;
                 int chunkBlockZ = z << 4;
                 int gap = getEngine().getDimension().getUpperDimensionGap();

core/src/main/java/art/arcane/iris/engine/mantle/components/CaveObjectPlacementTransaction.java

@@ -227,7 +227,7 @@ final class CaveObjectPlacementTransaction implements IObjectPlacer {
         if (cached != CACHE_MISS) {
             return cached;
         }
-        int surfaceY = engine.getHeight(x, z, true);
+        int surfaceY = Engine.hostHeight(engine, x, z, true);
         surfaceHeights.put(key, surfaceY);
         return surfaceY;
     }

core/src/main/java/art/arcane/iris/engine/mantle/components/IrisCaveCarver3D.java

@@ -286,7 +286,7 @@ public class IrisCaveCarver3D {
                     if (precomputedSurfaceHeights != null && precomputedSurfaceHeights.length > index) {
                         columnSurfaceY = precomputedSurfaceHeights[index];
                     } else {
-                        columnSurfaceY = engine.getHeight(x, z);
+                        columnSurfaceY = Engine.hostHeight(engine, x, z, true);
                     }
                     int unclampedClearanceTopY = columnSurfaceY - surfaceClearance;
                     int clearanceTopY = Math.min(maxY, Math.max(minY, unclampedClearanceTopY));

core/src/main/java/art/arcane/iris/engine/mantle/components/MantleObjectComponent.java

@@ -189,7 +189,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
         int zzz = 8 + (z << 4);
         IrisRegion region = complex.getRegionStream().get(xxx, zzz);
         IrisBiome surfaceBiome = complex.getTrueBiomeStream().get(xxx, zzz);
-        int surfaceY = getEngineMantle().getEngine().getHeight(xxx, zzz, true);
+        int surfaceY = complex.getRoundedHeighteightStream().get(xxx, zzz);
         IrisBiome caveBiome = resolveCaveObjectBiome(xxx, zzz, surfaceY, surfaceBiome);
         if (IrisSettings.get().getGeneral().isDebug() && (x & 31) == 0 && (z & 31) == 0) {
             int carvedBlocks = 0;
@@ -948,7 +948,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
         }
         Engine engine = getEngineMantle().getEngine();
         IrisBiome at = engine.getCaveBiome(x, y, z);
-        IrisBiome surface = engine.getSurfaceBiome(x, z);
+        IrisBiome surface = engine.getComplex().getTrueBiomeStream().get(x, z);
         return caveAnchorBiomeConflicts(at, surface, expectedCaveBiomeKey);
     }
 
@@ -1631,8 +1631,10 @@ public class MantleObjectComponent extends IrisMantleComponent {
     public Set<String> guess(int x, int z) {
         // todo The guess doesnt bring into account that the placer may return -1
         RNG rng = applyNoise(x, z, Cache.key(x, z) + seed());
-        IrisBiome biome = getEngineMantle().getEngine().getSurfaceBiome((x << 4) + 8, (z << 4) + 8);
+        int blockX = (x << 4) + 8;
-        IrisRegion region = getEngineMantle().getEngine().getRegion((x << 4) + 8, (z << 4) + 8);
+        int blockZ = (z << 4) + 8;
+        IrisBiome biome = getComplex().getTrueBiomeStream().get(blockX, blockZ);
+        IrisRegion region = getComplex().getRegionStream().get(blockX, blockZ);
         Set<String> v = new KSet<>();
         for (IrisObjectPlacement i : biome.getSurfaceObjects()) {
             if (rng.chance(i.getChance() + rng.d(-0.005, 0.005)) && !i.isCompatExcluded(getData())) {
@@ -1879,7 +1881,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
                 return surfaceY;
             }
 
-            surfaceY = getEngineMantle().getEngine().getHeight(x, z);
+            surfaceY = getEngineMantle().trueHeight(x, z);
             surfaceHeights.put(columnKey, surfaceY);
             return surfaceY;
         }

core/src/main/java/art/arcane/iris/engine/mode/ModeEnclosure.java

@@ -27,8 +27,8 @@ import art.arcane.iris.engine.framework.IrisEngineMode;
 public class ModeEnclosure extends IrisEngineMode implements EngineMode {
     public ModeEnclosure(Engine engine) {
         super(engine);
-        var terrain = new IrisTerrainNormalActuator(getEngine());
+        IrisTerrainNormalActuator terrain = new IrisTerrainNormalActuator(getEngine());
-        var biome = new IrisBiomeActuator(getEngine());
+        IrisBiomeActuator biome = new IrisBiomeActuator(getEngine());
 
         registerStage(burst(
                 (x, z, k, p, m, c) -> terrain.actuate(x, z, k, m, c),

core/src/main/java/art/arcane/iris/engine/mode/ModeIslands.java

@@ -27,8 +27,8 @@ import art.arcane.iris.engine.framework.IrisEngineMode;
 public class ModeIslands extends IrisEngineMode implements EngineMode {
     public ModeIslands(Engine engine) {
         super(engine);
-        var terrain = new IrisTerrainNormalActuator(getEngine());
+        IrisTerrainNormalActuator terrain = new IrisTerrainNormalActuator(getEngine());
-        var biome = new IrisBiomeActuator(getEngine());
+        IrisBiomeActuator biome = new IrisBiomeActuator(getEngine());
 
         registerStage(burst(
                 (x, z, k, p, m, c) -> terrain.actuate(x, z, k, m, c),

core/src/main/java/art/arcane/iris/engine/mode/ModeOverworld.java

@@ -87,7 +87,7 @@ public class ModeOverworld extends IrisEngineMode implements EngineMode {
             if (shouldBypassMantleStages()) {
                 return;
             }
-            getMantle().insertMatter(x >> 4, z >> 4, K, m);
+            getMantle().insertMatter(x >> 4, z >> 4, K, m, c);
         };
         EngineStage sFloatingTerrainSolid = (x, z, k, p, m, c) -> floatingChildBiomes.modify(x, z, k, m, c);
         EngineStage sFloatingDecorate = (x, z, k, p, m, c) -> floatingChildBiomes.decorateColumns(x, z, k, m, c);
@@ -118,7 +118,9 @@ public class ModeOverworld extends IrisEngineMode implements EngineMode {
         registerStage(sDecorant);
         registerStage(sFloatingDecorate);
         registerStage(sPerfection);
-        registerStage(sCustom);
+        if (getEngine().getDimensionStackContext() == null) {
+            registerStage(sCustom);
+        }
     }
 
     private boolean shouldBypassMantleStages() {

core/src/main/java/art/arcane/iris/engine/mode/ModeSuperFlat.java

@@ -27,8 +27,8 @@ import art.arcane.iris.engine.framework.IrisEngineMode;
 public class ModeSuperFlat extends IrisEngineMode implements EngineMode {
     public ModeSuperFlat(Engine engine) {
         super(engine);
-        var terrain = new IrisTerrainNormalActuator(getEngine());
+        IrisTerrainNormalActuator terrain = new IrisTerrainNormalActuator(getEngine());
-        var biome = new IrisBiomeActuator(getEngine());
+        IrisBiomeActuator biome = new IrisBiomeActuator(getEngine());
 
         registerStage(burst(
                 (x, z, k, p, m, c) -> terrain.actuate(x, z, k, m, c),

core/src/main/java/art/arcane/iris/engine/modifier/IrisCarveModifier.java

@@ -19,6 +19,8 @@
 package art.arcane.iris.engine.modifier;
 
 import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.UpperDimensionContext;
 import art.arcane.iris.engine.actuator.IrisDecorantActuator;
 import art.arcane.iris.engine.framework.Engine;
@@ -91,6 +93,8 @@ public class IrisCarveModifier extends EngineAssignedModifier<PlatformBlockState
         UpperDimensionContext upperCtx = getEngine().getUpperContext();
         boolean protectUpper = upperCtx != null && !getEngine().getDimension().isUpperDimensionCarving();
         int[] upperSurfaceHeights = protectUpper ? scratch.getOrCreateUpperSurfaceHeights() : null;
+        DimensionStackContext stackContext = getEngine().getDimensionStackContext();
+        DimensionStackLayout[] stackLayouts = stackContext == null ? null : new DimensionStackLayout[256];
         int chunkBlockX = PowerOfTwoCoordinates.chunkToBlock(x);
         int chunkBlockZ = PowerOfTwoCoordinates.chunkToBlock(z);
         for (int columnIndex = 0; columnIndex < 256; columnIndex++) {
@@ -104,6 +108,9 @@ public class IrisCarveModifier extends EngineAssignedModifier<PlatformBlockState
                 int gap = getEngine().getDimension().getUpperDimensionGap();
                 upperSurfaceHeights[columnIndex] = Math.max(rawUpper, surfaceHeights[columnIndex] + gap);
             }
+            if (stackLayouts != null) {
+                stackLayouts[columnIndex] = context.getDimensionStackLayout(localX, localZ);
+            }
         }
 
         MantleChunk<Matter> mantleChunk = mantle.getChunk(x, z).use();
@@ -117,6 +124,7 @@ public class IrisCarveModifier extends EngineAssignedModifier<PlatformBlockState
                     scratch,
                     columnMasks,
                     upperSurfaceHeights,
+                    stackLayouts,
                     worldHeightSpan,
                     caveLavaHeight,
                     chunkBlockX,
@@ -348,6 +356,10 @@ public class IrisCarveModifier extends EngineAssignedModifier<PlatformBlockState
             if (context.upperSurfaceHeights() != null && y >= context.upperSurfaceHeights()[columnIndex]) {
                 return;
             }
+            if (context.stackLayouts() != null
+                    && context.stackLayouts()[columnIndex].isHostFeatureProtectedY(y)) {
+                return;
+            }
 
             PlatformBlockState current = context.output().getRaw(localX, y, localZ);
             if (hydrology != null && hydrology.action() == HydrologyCaveAction.SEAL_GUARD) {
@@ -405,6 +417,7 @@ public class IrisCarveModifier extends EngineAssignedModifier<PlatformBlockState
             IrisCarveScratch scratch,
             CarveColumnMask[] columnMasks,
             int[] upperSurfaceHeights,
+            DimensionStackLayout[] stackLayouts,
             int worldHeightSpan,
             int caveLavaHeight,
             int chunkBlockX,

core/src/main/java/art/arcane/iris/engine/modifier/IrisDepositModifier.java

@@ -20,6 +20,7 @@ package art.arcane.iris.engine.modifier;
 
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.framework.EngineAssignedModifier;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisDepositGenerator;
 import art.arcane.iris.engine.object.IrisDepositHeightDistribution;
@@ -186,6 +187,11 @@ public class IrisDepositModifier extends EngineAssignedModifier<PlatformBlockSta
                     continue;
                 }
 
+                DimensionStackLayout stackLayout = context.getDimensionStackLayout(nx, nz);
+                if (stackLayout != null && stackLayout.isHostFeatureProtectedY(ny)) {
+                    continue;
+                }
+
                 PlatformBlockState current = data.get(nx, ny, nz);
                 if (!canReplaceDepositTarget(current)) {
                     continue;

core/src/main/java/art/arcane/iris/engine/modifier/IrisFloatingChildBiomeModifier.java

@@ -20,6 +20,7 @@ package art.arcane.iris.engine.modifier;
 
 
 import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.engine.decorator.FloatingDecorator;
 import art.arcane.iris.engine.decorator.IrisSeaSurfaceDecorator;
@@ -285,7 +286,15 @@ public class IrisFloatingChildBiomeModifier extends EngineAssignedModifier<Platf
                     }
                 }
 
-                writeIslandSkyBiomes(parent, wx, wz, sample, chunkHeight, data);
+                writeIslandSkyBiomes(
+                        parent,
+                        wx,
+                        wz,
+                        sample,
+                        chunkHeight,
+                        data,
+                        context.getDimensionStackLayout(xf, zf)
+                );
             }
         }
 
@@ -367,7 +376,15 @@ public class IrisFloatingChildBiomeModifier extends EngineAssignedModifier<Platf
         }
     }
 
-    private void writeIslandSkyBiomes(IrisBiome parent, int wx, int wz, FloatingIslandSample sample, int chunkHeight, IrisData data) {
+    private void writeIslandSkyBiomes(
+            IrisBiome parent,
+            int wx,
+            int wz,
+            FloatingIslandSample sample,
+            int chunkHeight,
+            IrisData data,
+            DimensionStackLayout stackLayout
+    ) {
         try {
             IdentityHashMap<IrisFloatingChildBiomes, MatterBiomeInject> matterByEntry = new IdentityHashMap<>();
             for (int k = 0; k <= sample.topIdx; k++) {
@@ -378,6 +395,9 @@ public class IrisFloatingChildBiomeModifier extends EngineAssignedModifier<Platf
                 if (y < 0 || y >= chunkHeight) {
                     continue;
                 }
+                if (!shouldWriteHostBiomeMarker(stackLayout, y)) {
+                    continue;
+                }
                 IrisFloatingChildBiomes entry = sample.entryAt(k);
                 MatterBiomeInject matter = matterByEntry.get(entry);
                 if (matter == null) {
@@ -395,10 +415,15 @@ public class IrisFloatingChildBiomeModifier extends EngineAssignedModifier<Platf
         }
     }
 
+    static boolean shouldWriteHostBiomeMarker(DimensionStackLayout stackLayout, int y) {
+        return stackLayout == null || !stackLayout.isHostFeatureProtectedY(y);
+    }
+
     private MatterBiomeInject createSkyBiomeMatter(IrisBiome target, int wx, int wz) {
         if (target.isCustom()) {
             IrisBiomeCustom custom = target.getCustomBiome(rng, getEngine(), wx, 0, wz);
-            return BiomeInjectMatter.get(IrisPlatforms.get().biomeWriter().biomeIdFor(getDimension().getLoadKey() + ":" + custom.getId()));
+            return BiomeInjectMatter.get(IrisPlatforms.get().biomeWriter().biomeIdFor(
+                    getDimension().getCustomBiomeKey(custom.getId())));
         }
 
         return BiomeInjectMatter.get(IrisPlatforms.get().biomeWriter().biomeIdFor(target.getSkyBiomeKey(rng, getEngine(), wx, 0, wz)));

core/src/main/java/art/arcane/iris/engine/object/IrisBiome.java

@@ -40,6 +40,7 @@ import art.arcane.iris.util.common.data.DataProvider;
 import art.arcane.volmlib.util.math.RNG;
 import art.arcane.iris.util.project.noise.CNG;
 import art.arcane.iris.util.project.context.IrisContext;
+import art.arcane.iris.util.project.stream.ProceduralStream;
 import lombok.AccessLevel;
 import lombok.Data;
 import lombok.EqualsAndHashCode;
@@ -495,6 +496,10 @@ public class IrisBiome extends IrisRegistrant implements IRare {
         return IrisBiomeLayerGenerator.generateLayers(this, dim, wx, wz, random, maxDepth, height, rdata, complex);
     }
 
+    public KList<PlatformBlockState> generateLayersWithSlope(IrisDimension dim, double wx, double wz, RNG random, int maxDepth, int height, IrisData rdata, ProceduralStream<Double> slopeStream) {
+        return IrisBiomeLayerGenerator.generateLayers(this, dim, wx, wz, random, maxDepth, height, rdata, slopeStream);
+    }
+
     public KList<PlatformBlockState> generateCeilingLayers(IrisDimension dim, double wx, double wz, RNG random, int maxDepth, int height, IrisData rdata, IrisComplex complex) {
         return IrisBiomeLayerGenerator.generateCeilingLayers(this, dim, wx, wz, random, maxDepth, height, rdata, complex);
     }

core/src/main/java/art/arcane/iris/engine/object/IrisBiomeLayerGenerator.java

@@ -26,6 +26,7 @@ import art.arcane.iris.spi.IrisLogging;
 import art.arcane.iris.spi.PlatformBlockState;
 import art.arcane.iris.util.common.data.B;
 import art.arcane.iris.util.project.noise.CNG;
+import art.arcane.iris.util.project.stream.ProceduralStream;
 import art.arcane.volmlib.util.collection.KList;
 import art.arcane.volmlib.util.math.RNG;
 
@@ -42,8 +43,16 @@ final class IrisBiomeLayerGenerator {
     }
 
     static KList<PlatformBlockState> generateLayers(IrisBiome biome, IrisDimension dim, double wx, double wz, RNG random, int maxDepth, int height, IrisData rdata, IrisComplex complex) {
+        return generateLayers(biome, dim, wx, wz, random, maxDepth, height, rdata, complex, null);
+    }
+
+    static KList<PlatformBlockState> generateLayers(IrisBiome biome, IrisDimension dim, double wx, double wz, RNG random, int maxDepth, int height, IrisData rdata, ProceduralStream<Double> slopeStream) {
+        return generateLayers(biome, dim, wx, wz, random, maxDepth, height, rdata, null, slopeStream);
+    }
+
+    private static KList<PlatformBlockState> generateLayers(IrisBiome biome, IrisDimension dim, double wx, double wz, RNG random, int maxDepth, int height, IrisData rdata, IrisComplex complex, ProceduralStream<Double> slopeStream) {
         if (biome.isLockLayers()) {
-            return generateLockedLayers(biome, wx, wz, random, maxDepth, height, rdata, complex);
+            return generateLockedLayers(biome, wx, wz, random, maxDepth, height, rdata, complex, slopeStream);
         }
 
         KList<PlatformBlockState> data = new KList<>();
@@ -70,7 +79,7 @@ final class IrisBiomeLayerGenerator {
             IrisSlopeClip sc = layer.getSlopeCondition();
 
             if (!sc.isDefault()) {
-                if (!sc.isValid(complex.getSlopeStream().get(wx, wz))) {
+                if (!sc.isValid(resolveSlopeStream(complex, slopeStream).getDouble(wx, wz))) {
                     d = 0;
                 }
             }
@@ -168,6 +177,10 @@ final class IrisBiomeLayerGenerator {
     }
 
     static KList<PlatformBlockState> generateLockedLayers(IrisBiome biome, double wx, double wz, RNG random, int maxDepthf, int height, IrisData rdata, IrisComplex complex) {
+        return generateLockedLayers(biome, wx, wz, random, maxDepthf, height, rdata, complex, null);
+    }
+
+    private static KList<PlatformBlockState> generateLockedLayers(IrisBiome biome, double wx, double wz, RNG random, int maxDepthf, int height, IrisData rdata, IrisComplex complex, ProceduralStream<Double> slopeStream) {
         KList<PlatformBlockState> data = new KList<>();
         KList<PlatformBlockState> real = new KList<>();
         int maxDepth = Math.min(maxDepthf, biome.getLockLayersMax());
@@ -190,7 +203,7 @@ final class IrisBiomeLayerGenerator {
                 IrisSlopeClip sc = layer.getSlopeCondition();
 
                 if (!sc.isDefault()) {
-                    if (!sc.isValid(complex.getSlopeStream().get(wx, wz))) {
+                    if (!sc.isValid(resolveSlopeStream(complex, slopeStream).getDouble(wx, wz))) {
                         d = 0;
                     }
                 }
@@ -222,6 +235,13 @@ final class IrisBiomeLayerGenerator {
         return real;
     }
 
+    private static ProceduralStream<Double> resolveSlopeStream(
+            IrisComplex complex,
+            ProceduralStream<Double> slopeStream
+    ) {
+        return slopeStream == null ? complex.getSlopeStream() : slopeStream;
+    }
+
     static KList<PlatformBlockState> generateSeaLayers(IrisBiome biome, double wx, double wz, RNG random, int maxDepth, IrisData rdata) {
         KList<PlatformBlockState> data = new KList<>();
 

core/src/main/java/art/arcane/iris/engine/object/IrisDimension.java

@@ -222,6 +222,8 @@ public class IrisDimension extends IrisRegistrant {
     private boolean upperDimensionObjects = false;
     @Desc("When true, upper-dimension objects force-place regardless of placement restrictions (slope, underwater, clamp, collisions, carving). Normal dimension objects always place first; upper objects place second and may clip or occlude lower-dimension placements when this is enabled.")
     private boolean upperObjectsForcePlace = false;
+    @Desc("Stack dimension terrain upright in declared top-to-bottom order. The final dimension key must reference this dimension.")
+    private IrisDimensionStack dimensionStack = null;
     @RegistryListResource(IrisBiome.class)
     @Desc("Keep this either undefined or empty. Setting any biome name into this will force iris to only generate the specified biome. Great for testing.")
     private String focus = "";
@@ -528,6 +530,12 @@ public class IrisDimension extends IrisRegistrant {
                 }
             }
         }
+        if (hasFocusRegion()) {
+            IrisRegion focus = data.getRegionLoader().load(getFocusRegion());
+            if (focus != null && !focus.isCompatExcluded()) {
+                regions.put(getFocusRegion(), focus);
+            }
+        }
         return regions.v();
     }
 
@@ -564,6 +572,7 @@ public class IrisDimension extends IrisRegistrant {
         }
 
         Deque<String> pending = new ArrayDeque<>();
+        addReachableBiomeKey(pending, getFocus());
         IrisHydrology configuredHydrology = getHydrology();
         IrisRiverHydrology configuredRivers = configuredHydrology == null ? null : configuredHydrology.getRivers();
         boolean hydrologyPoliciesActive = configuredRivers != null && configuredRivers.isEnabled();
@@ -721,8 +730,14 @@ public class IrisDimension extends IrisRegistrant {
     }
 
     public void installBiomes(IDataFixer fixer, DataProvider data, KList<File> datapackRoots, KSet<String> biomes) throws IOException {
-        String namespace = getLoadKey().toLowerCase(Locale.ROOT);
+        String keyPrefix = getCustomBiomeKeyPrefix();
-        installBiomes(fixer, data, datapackRoots, namespace, "", biomes);
+        int separator = keyPrefix.indexOf(':');
+        String namespace = keyPrefix.substring(0, separator);
+        String pathPrefix = keyPrefix.substring(separator + 1);
+        if (pathPrefix.endsWith("/")) {
+            pathPrefix = pathPrefix.substring(0, pathPrefix.length() - 1);
+        }
+        installBiomes(fixer, data, datapackRoots, namespace, pathPrefix, biomes);
     }
 
     public void installBiomes(IDataFixer fixer, DataProvider data, KList<File> datapackRoots,
@@ -747,18 +762,19 @@ public class IrisDimension extends IrisRegistrant {
 
             for (IrisBiomeCustom customBiome : irisBiome.getCustomDerivitives()) {
                 String customBiomeId = customBiome.getId();
+                String customBiomePath = sanitizeRegistryPath(customBiomeId, "biome");
                 String json = customBiome.generateJson(fixer, contentGate);
 
                 synchronized (biomes) {
-                    if (!biomes.add(customBiomeId)) {
+                    if (!biomes.add(customBiomePath)) {
                         IrisLogging.debug("Duplicate Data Pack Biome: " + getLoadKey() + "/" + customBiomeId);
                         continue;
                     }
                 }
 
                 String biomePath = pathPrefix.isBlank()
-                        ? customBiomeId
+                        ? customBiomePath
-                        : pathPrefix + "/" + customBiomeId;
+                        : pathPrefix + "/" + customBiomePath;
                 for (File datapackRoot : datapackRoots) {
                     File output = new File(datapackRoot, "data/" + namespace + "/worldgen/biome/" + biomePath + ".json");
 
@@ -900,6 +916,28 @@ public class IrisDimension extends IrisRegistrant {
         return sanitizeDimensionTypeKeyValue(getLoadKey());
     }
 
+    public String getCustomBiomeKey(String customBiomeId) {
+        return customBiomeKey(getLoadKey(), customBiomeId);
+    }
+
+    public String getCustomBiomeKeyPrefix() {
+        return customBiomeKeyPrefix(getLoadKey());
+    }
+
+    public static String customBiomeKey(String dimensionLoadKey, String customBiomeId) {
+        return customBiomeKeyPrefix(dimensionLoadKey) + sanitizeRegistryPath(customBiomeId, "biome");
+    }
+
+    private static String customBiomeKeyPrefix(String dimensionLoadKey) {
+        String dimensionPath = sanitizeRegistryPath(dimensionLoadKey, "dimension");
+        int separator = dimensionPath.indexOf('/');
+        if (separator < 0) {
+            return sanitizeRegistryNamespace(dimensionPath) + ":";
+        }
+        String namespace = sanitizeRegistryNamespace(dimensionPath.substring(0, separator));
+        return namespace + ":" + dimensionPath.substring(separator + 1) + "/";
+    }
+
     public static String sanitizeDimensionTypeKeyValue(String value) {
         if (value == null || value.isBlank()) {
             return "dimension";
@@ -918,6 +956,26 @@ public class IrisDimension extends IrisRegistrant {
         return sanitized.isBlank() ? "dimension" : sanitized;
     }
 
+    private static String sanitizeRegistryNamespace(String value) {
+        String sanitized = value.toLowerCase(Locale.ROOT).replaceAll("[^a-z0-9_.-]", "_");
+        return sanitized.isBlank() ? "dimension" : sanitized;
+    }
+
+    private static String sanitizeRegistryPath(String value, String fallback) {
+        if (value == null || value.isBlank()) {
+            return fallback;
+        }
+        String sanitized = value.trim().toLowerCase(Locale.ROOT).replace("\\", "/");
+        sanitized = sanitized.replaceAll("[^a-z0-9/._-]", "_");
+        sanitized = sanitized.replaceAll("/+", "/");
+        sanitized = sanitized.replaceAll("^/+", "");
+        sanitized = sanitized.replaceAll("/+$", "");
+        while (sanitized.contains("..")) {
+            sanitized = sanitized.replace("..", "_");
+        }
+        return sanitized.isBlank() ? fallback : sanitized;
+    }
+
     public IrisDimensionType getDimensionType() {
         IrisDimensionTypeOptions options = getDimensionOptions();
         if (fullbright) {
@@ -930,6 +988,10 @@ public class IrisDimension extends IrisRegistrant {
         return upperDimension != null && !upperDimension.isEmpty() && !upperDimension.equalsIgnoreCase("none");
     }
 
+    public boolean hasDimensionStack() {
+        return dimensionStack != null;
+    }
+
     public void installDimensionType(IDataFixer fixer, KList<File> datapackRoots) throws IOException {
         IrisDimensionType type = getDimensionType();
         String json = type.toJson(fixer);

core/src/main/java/art/arcane/iris/engine/object/IrisDimensionStack.java

@@ -0,0 +1,50 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.engine.object.annotations.ArrayType;
+import art.arcane.iris.engine.object.annotations.Desc;
+import art.arcane.iris.engine.object.annotations.MaxNumber;
+import art.arcane.iris.engine.object.annotations.MinNumber;
+import art.arcane.iris.engine.object.annotations.RegistryListResource;
+import art.arcane.iris.engine.object.annotations.Required;
+import art.arcane.volmlib.util.collection.KList;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import lombok.experimental.Accessors;
+
+@Accessors(chain = true)
+@NoArgsConstructor
+@Desc("Stacks dimension terrain upright in declared top-to-bottom order")
+@Data
+public class IrisDimensionStack {
+    @Required
+    @ArrayType(min = 2, type = String.class)
+    @RegistryListResource(IrisDimension.class)
+    @Desc("Dimension load keys ordered from the highest layer to the lowest layer. The final key must be this dimension.")
+    private KList<String> dimensions = new KList<>();
+
+    @MinNumber(0)
+    @MaxNumber(256)
+    @Desc("Nominal air gap in blocks between adjacent dimension layers")
+    private int spacer = 32;
+
+    @Desc("Optional noise that blends adjacent layer boundaries. Omit this object for a constant configured gap.")
+    private IrisDimensionStackBlend blend = null;
+}

core/src/main/java/art/arcane/iris/engine/object/IrisDimensionStackBlend.java

@@ -0,0 +1,40 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.engine.object.annotations.Desc;
+import art.arcane.iris.engine.object.annotations.MaxNumber;
+import art.arcane.iris.engine.object.annotations.MinNumber;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import lombok.experimental.Accessors;
+
+@Accessors(chain = true)
+@NoArgsConstructor
+@Desc("Controls noise blending at each dimension stack boundary")
+@Data
+public class IrisDimensionStackBlend {
+    @MinNumber(0)
+    @MaxNumber(256)
+    @Desc("Maximum vertical boundary displacement in blocks")
+    private int amplitude = 8;
+
+    @Desc("Noise style sampled independently for each adjacent layer boundary")
+    private IrisGeneratorStyle style = NoiseStyle.SIMPLEX.style();
+}

core/src/main/java/art/arcane/iris/engine/platform/EngineBukkitOps.java

@@ -553,11 +553,15 @@ public final class EngineBukkitOps {
     }
 
     public static IrisRegion getRegion(Engine engine, Location l) {
-        return engine.getRegion(l.getBlockX(), l.getBlockZ());
+        return engine.getRegion(
+                l.getBlockX(), l.getBlockY() - engine.getWorld().minHeight(), l.getBlockZ());
     }
 
     public static IrisBiome getBiomeOrMantle(Engine engine, Location l) {
-        return engine.getBiomeOrMantle(l.getBlockX(), l.getBlockY(), l.getBlockZ());
+        return engine.getBiomeOrMantle(
+                l.getBlockX(),
+                l.getBlockY() - engine.getWorld().minHeight(),
+                l.getBlockZ());
     }
 
     public static IrisBiome getSurfaceBiome(Engine engine, Chunk c) {

core/src/main/java/art/arcane/iris/util/project/context/ChunkContext.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.util.project.context;
 
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
 import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.engine.framework.EngineMetrics;
@@ -27,6 +29,8 @@ public class ChunkContext {
     private final ChunkedDataCache<PlatformBlockState> fluid;
     private final ChunkedDataCache<IrisRegion> region;
     private final FloatingIslandBoundarySampler floatingIslandBoundarySampler;
+    private final DimensionStackContext dimensionStackContext;
+    private final DimensionStackLayout[] dimensionStackLayouts;
 
     public ChunkContext(int x, int z, IrisComplex complex) {
         this(x, z, complex, 0L, true, PrefillPlan.NO_CAVE, null);
@@ -45,6 +49,19 @@ public class ChunkContext {
     }
 
     public ChunkContext(int x, int z, IrisComplex complex, long generationSessionId, boolean cache, PrefillPlan prefillPlan, EngineMetrics metrics) {
+        this(x, z, complex, generationSessionId, cache, prefillPlan, metrics, null);
+    }
+
+    public ChunkContext(
+            int x,
+            int z,
+            IrisComplex complex,
+            long generationSessionId,
+            boolean cache,
+            PrefillPlan prefillPlan,
+            EngineMetrics metrics,
+            DimensionStackContext dimensionStackContext
+    ) {
         this.x = x;
         this.z = z;
         this.complex = complex;
@@ -57,6 +74,8 @@ public class ChunkContext {
         this.fluid = new ChunkedDataCache<>(complex.getFluidStream(), x, z, cache);
         this.region = new ChunkedDataCache<>(complex.getRegionStream(), x, z, cache);
         this.floatingIslandBoundarySampler = new FloatingIslandBoundarySampler((wx, wz) -> complex.getTrueBiomeStream().get(wx, wz));
+        this.dimensionStackContext = dimensionStackContext;
+        this.dimensionStackLayouts = dimensionStackContext == null ? null : new DimensionStackLayout[256];
 
         if (cache) {
             PrefillPlan resolvedPlan = prefillPlan == null ? PrefillPlan.NO_CAVE : prefillPlan;
@@ -165,6 +184,19 @@ public class ChunkContext {
         return region;
     }
 
+    public DimensionStackLayout getDimensionStackLayout(int localX, int localZ) {
+        if (dimensionStackContext == null) {
+            return null;
+        }
+        int index = (localZ << 4) | localX;
+        DimensionStackLayout layout = dimensionStackLayouts[index];
+        if (layout == null) {
+            layout = dimensionStackContext.sample(x + localX, z + localZ);
+            dimensionStackLayouts[index] = layout;
+        }
+        return layout;
+    }
+
     public enum PrefillPlan {
         ALL(true, true, true, true, true, true),
         NO_CAVE(true, true, false, true, true, true),

core/src/test/java/art/arcane/iris/core/IrisDatapackCompilerTest.java

@@ -1,8 +1,10 @@
 package art.arcane.iris.core;
 
 import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.IrisGeneratorBinding;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.nms.datapack.DataVersion;
 import art.arcane.iris.core.nms.datapack.v1217.DataFixerV1217;
+import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.volmlib.util.collection.KList;
 import art.arcane.volmlib.util.json.JSONObject;
 import org.junit.Rule;
@@ -88,6 +90,26 @@ public class IrisDatapackCompilerTest {
         assertTrue(Files.isRegularFile(datapackRoot.resolve("pack.mcmeta")));
     }
 
+    @Test
+    public void compilesRecursiveDimensionBiomeToCanonicalRegistryPath() throws Exception {
+        Path packRoot = temporaryFolder.newFolder("recursive-dimension-pack").toPath();
+        Path datapackRoot = temporaryFolder.newFolder("recursive-dimension-datapack").toPath();
+        createPack(packRoot, "layers/sky", "Aurora");
+
+        IrisDatapackCompiler.compile(
+                List.of(packRoot.toFile()),
+                new KList<File>().qadd(datapackRoot.toFile()),
+                List.of(),
+                new DataFixerV1217(),
+                false
+        );
+
+        assertTrue(Files.isRegularFile(
+                datapackRoot.resolve("data/layers/worldgen/biome/sky/aurora.json")));
+        assertTrue(Files.isRegularFile(
+                datapackRoot.resolve("data/iris/dimension_type/layers_sky.json")));
+    }
+
     @Test
     public void collectsInstalledPacksThroughSymbolicLinkWorkspace() throws Exception {
         Path dataDirectory = temporaryFolder.newFolder("linked-data").toPath();
@@ -252,6 +274,57 @@ public class IrisDatapackCompilerTest {
         assertFalse(ServerConfigurator.loadedRegistrySatisfies(loaded, changedBiomeTags));
     }
 
+    @Test
+    public void stackRuntimeRequirementsIncludeSourceDimensionRegistries() throws Exception {
+        Path pack = temporaryFolder.newFolder("registry-stack-pack").toPath();
+        createPack(pack, "host", "stack_custom", "NORMAL");
+        Files.writeString(
+                pack.resolve("dimensions/host.json"),
+                """
+                        {
+                          "name": "Host",
+                          "environment": "NORMAL",
+                          "logicalHeight": 256,
+                          "dimensionHeight": {"min": -64, "max": 320},
+                          "dimensionStack": {
+                            "dimensions": ["layers/source", "host"],
+                            "spacer": 8
+                          }
+                        }
+                        """,
+                StandardCharsets.UTF_8
+        );
+        Files.createDirectories(pack.resolve("dimensions/layers"));
+        Files.writeString(
+                pack.resolve("dimensions/layers/source.json"),
+                """
+                        {
+                          "name": "Source",
+                          "environment": "NORMAL",
+                          "logicalHeight": 256,
+                          "dimensionHeight": {"min": -64, "max": 320}
+                        }
+                        """,
+                StandardCharsets.UTF_8
+        );
+
+        IrisData data = IrisData.openDatapackCompiler(pack.toFile());
+        try {
+            IrisDimension host = data.getDimensionLoader().load("host");
+            Map<String, String> requirements = IrisDatapackCompiler.computeRegistryRequirements(
+                    host,
+                    new DataFixerV1217()
+            );
+
+            assertTrue(requirements.containsKey("dimension_type/iris:host"));
+            assertTrue(requirements.containsKey("dimension_type/iris:layers_source"));
+            assertTrue(requirements.containsKey("worldgen/biome/host:stack_custom"));
+            assertTrue(requirements.containsKey("worldgen/biome/layers:source/stack_custom"));
+        } finally {
+            data.close();
+        }
+    }
+
     @Test
     public void rejectsBindingToMissingDimension() throws Exception {
         Path packRoot = temporaryFolder.newFolder("missing-binding-pack").toPath();
@@ -325,6 +398,7 @@ public class IrisDatapackCompilerTest {
     ) throws Exception {
         Files.createDirectories(root.resolve("dimensions"));
         Files.createDirectories(root.resolve("biomes"));
+        Files.createDirectories(root.resolve("dimensions").resolve(dimensionKey).getParent());
         Files.writeString(
                 root.resolve("dimensions").resolve(dimensionKey + ".json"),
                 """

core/src/test/java/art/arcane/iris/core/ServerConfiguratorRegistryVerificationTest.java

@@ -80,6 +80,7 @@ public class ServerConfiguratorRegistryVerificationTest {
         IrisDimension dimension = mock(IrisDimension.class);
         when(dimension.getAllBiomes(any())).thenReturn(new KList<>(biome));
         when(dimension.getLoadKey()).thenReturn("overworld");
+        when(dimension.getCustomBiomeKey("missing")).thenReturn("overworld:missing");
         when(dimension.getLoadFile()).thenReturn(
                 new File("packs/overworld/dimensions/overworld.json"));
         when(dimension.getDimensionTypeKey()).thenReturn("iris:overworld");

core/src/test/java/art/arcane/iris/core/pack/PackExportClosureTest.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.core.pack;
 
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisDimensionStack;
 import art.arcane.iris.engine.object.IrisObjectMarker;
 import art.arcane.iris.engine.object.IrisObjectPlacement;
 import art.arcane.iris.engine.object.IrisStaticObject;
@@ -9,6 +11,7 @@ import org.junit.Test;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.List;
+import java.util.Map;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
@@ -63,6 +66,38 @@ public class PackExportClosureTest {
                 PackExportClosure.collectStaticObjectKeys(placements).stream().sorted().toList());
     }
 
+    @Test
+    public void collectsRootAndStackedDimensionKeys() {
+        IrisDimension dimension = new IrisDimension();
+        dimension.setLoadKey("main");
+        dimension.setDimensionStack(new IrisDimensionStack()
+                .setDimensions(new KList<>("sky", "middle", "main", "sky")));
+
+        assertEquals(List.of("main", "middle", "sky"),
+                PackExportClosure.collectDimensionKeys(dimension).stream().sorted().toList());
+    }
+
+    @Test
+    public void collectsTransitiveStackDimensionsWithoutLooping() {
+        IrisDimension main = dimension("main", "sky", "main");
+        IrisDimension sky = dimension("sky", "cloud", "sky");
+        IrisDimension cloud = dimension("cloud", "main", "cloud");
+        Map<String, IrisDimension> dimensions = Map.of(
+                "main", main,
+                "sky", sky,
+                "cloud", cloud);
+
+        assertEquals(List.of("cloud", "main", "sky"),
+                PackExportClosure.collectDimensionKeys(main, dimensions::get).stream().sorted().toList());
+    }
+
+    private static IrisDimension dimension(String key, String... stackKeys) {
+        IrisDimension dimension = new IrisDimension();
+        dimension.setLoadKey(key);
+        dimension.setDimensionStack(new IrisDimensionStack().setDimensions(new KList<>(stackKeys)));
+        return dimension;
+    }
+
     /**
      * Source guard: both packagers must export the ambient-spawning graph. Spawner and marker
      * folders were silently omitted from exports, leaving dangling entitySpawners references.
@@ -75,10 +110,16 @@ public class PackExportClosureTest {
         assertTrue("Bukkit packager must write markers/", bukkit.contains("\"markers/\""));
         assertTrue("Bukkit packager must export region objects alongside biome objects",
                 bukkit.contains("regions.forEach((r) -> allPlacements.addAll(r.getObjects()))"));
-        assertTrue("Bukkit packager must export static objects", bukkit.contains("dimension.getStaticObjects()"));
+        assertTrue("Bukkit packager must export static objects",
+                bukkit.contains("exportedDimension.getStaticObjects()"));
         assertTrue("Bukkit obfuscation must rewrite static object references",
                 bukkit.contains("placement.setObject(renameObjects.get(placement.getObject()))"));
         assertTrue("Bukkit packager must export entity loot tables", bukkit.contains("getLoot().getTables()"));
+        assertTrue("Bukkit packager must include dimension-stack resources",
+                bukkit.contains("PackExportClosure.collectDimensionKeys(dimension)"));
+        assertTrue("Bukkit packager must export expression-backed stack blend styles",
+                bukkit.contains("dm.getExpressionLoader().getPossibleKeys()")
+                        && bukkit.contains("\"expressions/\""));
         int bukkitValidation = bukkit.indexOf("PackValidator.validateForPackaging(project.getPath())");
         assertTrue("Bukkit packager must validate before opening or mutating package state",
                 bukkitValidation >= 0
@@ -92,7 +133,14 @@ public class PackExportClosureTest {
         assertTrue("modded packager must include initial spawns", modded.contains("getInitialSpawns"));
         assertTrue("modded packager must export region objects", modded.contains("region.getObjects()"));
         assertTrue("modded packager must export static objects",
-                modded.contains("PackExportClosure.collectStaticObjectKeys(dimension.getStaticObjects())"));
+                modded.contains("PackExportClosure.collectStaticObjectKeys(exportedDimension.getStaticObjects())"));
+        assertTrue("modded packager must include dimension-stack resources",
+                modded.contains("PackExportClosure.collectDimensionKeys(dimension)"));
+        assertTrue("modded packager must export expression-backed stack blend styles",
+                modded.contains("dm.getExpressionLoader().getPossibleKeys()")
+                        && modded.contains("\"expressions\""));
+        assertTrue("modded packager must retain referenced style snippets",
+                modded.contains("copyStyleSnippets(packFolder, folder)"));
         int moddedValidation = modded.indexOf("PackValidator.validateForPackaging(packFolder)");
         assertTrue("modded packager must validate before mutating package state",
                 moddedValidation >= 0 && moddedValidation < modded.indexOf("IO.delete(folder)"));

core/src/test/java/art/arcane/iris/core/pack/PackImageMapValidatorTest.java

@@ -317,6 +317,48 @@ public class PackImageMapValidatorTest {
                 "Dimension 'upper' image-map 'terrain' source footprint does not cover"));
     }
 
+    @Test
+    public void validatesStackLayerCoverageAgainstHostBoundary() throws Exception {
+        File pack = createPack("stack-host-boundary", """
+                {
+                  "regions": ["region"],
+                  "worldBoundary": {"center": {"x": 0, "z": 0}, "size": 16},
+                  "dimensionStack": {
+                    "dimensions": ["upper", "upper", "main"],
+                    "spacer": 24
+                  }
+                }
+                """);
+        write(pack, "dimensions/upper.json", """
+                {
+                  "regions": ["region"],
+                  "worldBoundary": {"center": {"x": 0, "z": 0}, "size": 2},
+                  "imageMaps": [
+                    {"key": "terrain", "map": "upper-terrain", "application": "TERRAIN_HEIGHT"}
+                  ]
+                }
+                """);
+        writeGray(pack, "upper-terrain", 4, 4);
+        write(pack, "image-maps/upper-terrain.json", """
+                {
+                  "source": "upper-terrain",
+                  "type": "GRAYSCALE_HEIGHT",
+                  "origin": {"x": -2, "z": -2},
+                  "outOfBounds": "ERROR"
+                }
+                """);
+
+        PackImageMapValidator.Validation validation = validate(pack, false);
+
+        assertTrue(validation.errors().toString(), contains(validation.errors(),
+                "Dimension 'main' dimension stack layer 'upper' image-map 'terrain' source footprint does not cover"));
+        assertEquals(1L, validation.errors().stream()
+                .filter(error -> error.contains("Dimension 'main' dimension stack layer 'upper'"))
+                .count());
+        assertFalse(validation.errors().toString(), contains(validation.errors(),
+                "Dimension 'upper' image-map 'terrain' source footprint does not cover"));
+    }
+
     @Test
     public void acceptsUpperDimensionWithoutStandaloneBoundaryWhenParentIsCovered() throws Exception {
         File pack = createPack("upper-parent-only-boundary", """

core/src/test/java/art/arcane/iris/core/pack/PackValidatorDimensionStackTest.java

@@ -0,0 +1,260 @@
+package art.arcane.iris.core.pack;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class PackValidatorDimensionStackTest {
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    @Test
+    public void acceptsCanonicalTopToBottomStack() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "spacer": 24,
+                    "blend": {
+                      "amplitude": 8,
+                      "style": {"style": "SIMPLEX", "zoom": 128}
+                    }
+                  }
+                }
+                """);
+
+        assertTrue(result.getBlockingErrors().toString(), result.isLoadable());
+    }
+
+    @Test
+    public void rejectsMissingBlendStyleSnippet() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "blend": {"style": "snippet/style/missing"}
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "references missing style snippet 'snippet/style/missing'"));
+    }
+
+    @Test
+    public void rejectsWrongBlendSnippetType() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "blend": {"style": "snippet/generator/stack-blend"}
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "blend.style must be an object or style snippet reference"));
+    }
+
+    @Test
+    public void rejectsMissingLayerAndWrongLowestDimension() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["missing", "sky"],
+                    "spacer": 24
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "dimensions[0] references missing dimension 'missing'"));
+        assertTrue(contains(result, "dimensions must end with its host dimension key 'main'"));
+    }
+
+    @Test
+    public void rejectsUpperDimensionAndUnsafeNumericValues() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "upperDimension": "sky",
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "spacer": 24.5,
+                    "blend": {
+                      "amplitude": 257,
+                      "style": {}
+                    }
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "dimensionStack cannot be used with upperDimension"));
+        assertTrue(contains(result, "spacer must be an integer between 0 and 256"));
+        assertTrue(contains(result, "amplitude must be an integer between 0 and 256"));
+    }
+
+    @Test
+    public void rejectsShortStackAndMalformedBlendStyle() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["main"],
+                    "blend": {"style": 42}
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "dimensions must contain at least two dimension keys"));
+        assertTrue(contains(result, "blend.style must be an object or style snippet reference"));
+    }
+
+    @Test
+    public void acceptsBlendStyleSnippetReference() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "blend": {"style": "snippet/style/stack-blend"}
+                  }
+                }
+                """, "{\"style\":\"SIMPLEX\",\"zoom\":128}");
+
+        assertTrue(result.getBlockingErrors().toString(), result.isLoadable());
+    }
+
+    @Test
+    public void rejectsMalformedBlendStyleSnippet() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "blend": {"style": "snippet/style/stack-blend"}
+                  }
+                }
+                """, "{");
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "references invalid style snippet 'snippet/style/stack-blend'"));
+    }
+
+    @Test
+    public void acceptsNestedDimensionKeys() throws Exception {
+        File pack = temporaryFolder.newFolder("nested-pack");
+        write(pack, "dimensions/root/main.json", """
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky/clouds", "root/main"]
+                  }
+                }
+                """);
+        write(pack, "dimensions/sky/clouds.json", "{\"regions\":[\"region\"]}");
+        write(pack, "regions/region.json", "{\"landBiomes\":[\"biome\"]}");
+        write(pack, "biomes/biome.json", "{\"name\":\"Biome\"}");
+
+        PackValidationResult result = PackValidator.validateForPackaging(pack);
+
+        assertTrue(result.getBlockingErrors().toString(), result.isLoadable());
+    }
+
+    @Test
+    public void acceptsEmptyBlendWithDefaults() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "main"],
+                    "blend": {}
+                  }
+                }
+                """);
+
+        assertTrue(result.getBlockingErrors().toString(), result.isLoadable());
+    }
+
+    @Test
+    public void acceptsRepeatedNonHostDimensions() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["sky", "sky", "main"]
+                  }
+                }
+                """);
+
+        assertTrue(result.getBlockingErrors().toString(), result.isLoadable());
+    }
+
+    @Test
+    public void rejectsDuplicateHostDimension() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": {
+                    "dimensions": ["main", "sky", "main"]
+                  }
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "host dimension key 'main' exactly once"));
+    }
+
+    @Test
+    public void rejectsExplicitNullConfiguration() throws Exception {
+        PackValidationResult result = validate("""
+                {
+                  "regions": ["region"],
+                  "dimensionStack": null
+                }
+                """);
+
+        assertFalse(result.isLoadable());
+        assertTrue(contains(result, "dimensionStack must be an object"));
+    }
+
+    private PackValidationResult validate(String mainDimension) throws Exception {
+        return validate(mainDimension, null);
+    }
+
+    private PackValidationResult validate(String mainDimension, String styleSnippet) throws Exception {
+        File pack = temporaryFolder.newFolder("pack-" + System.nanoTime());
+        write(pack, "dimensions/main.json", mainDimension);
+        write(pack, "dimensions/sky.json", "{\"regions\":[\"region\"]}");
+        write(pack, "regions/region.json", "{\"landBiomes\":[\"biome\"]}");
+        write(pack, "biomes/biome.json", "{\"name\":\"Biome\"}");
+        if (styleSnippet != null) {
+            write(pack, "snippet/style/stack-blend.json", styleSnippet);
+        }
+        return PackValidator.validateForPackaging(pack);
+    }
+
+    private static boolean contains(PackValidationResult result, String fragment) {
+        return result.getBlockingErrors().stream().anyMatch(error -> error.contains(fragment));
+    }
+
+    private static void write(File root, String relative, String content) throws Exception {
+        Path target = new File(root, relative).toPath();
+        Files.createDirectories(target.getParent());
+        Files.writeString(target, content, StandardCharsets.UTF_8);
+    }
+}

core/src/test/java/art/arcane/iris/core/project/IrisDimensionStackSchemaTest.java

@@ -0,0 +1,108 @@
+package art.arcane.iris.core.project;
+
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.IrisRegistrant;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisDimensionStack;
+import art.arcane.iris.engine.object.IrisDimensionStackBlend;
+import art.arcane.iris.engine.object.annotations.ArrayType;
+import art.arcane.iris.engine.object.annotations.RegistryListResource;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.collection.KMap;
+import art.arcane.volmlib.util.json.JSONArray;
+import art.arcane.volmlib.util.json.JSONObject;
+import com.google.gson.Gson;
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisDimensionStackSchemaTest {
+    @Test
+    public void dimensionExposesNullableStackConfiguration() throws Exception {
+        IrisDimension dimension = new IrisDimension();
+        Field stackField = IrisDimension.class.getDeclaredField("dimensionStack");
+        Field dimensionsField = IrisDimensionStack.class.getDeclaredField("dimensions");
+
+        assertNull(dimension.getDimensionStack());
+        assertFalse(dimension.hasDimensionStack());
+        dimension.setDimensionStack(new IrisDimensionStack());
+        assertTrue(dimension.hasDimensionStack());
+        assertEquals(IrisDimensionStack.class, stackField.getType());
+        assertEquals(2, dimensionsField.getAnnotation(ArrayType.class).min());
+        assertEquals(IrisDimension.class,
+                dimensionsField.getAnnotation(RegistryListResource.class).value());
+    }
+
+    @Test
+    public void schemaCarriesStackOrderAndNumericBounds() {
+        JSONObject schema = new SchemaBuilder(IrisDimensionStack.class, schemaData()).construct();
+        JSONObject properties = schema.getJSONObject("properties");
+        JSONObject dimensions = properties.getJSONObject("dimensions");
+        JSONObject spacer = properties.getJSONObject("spacer");
+        JSONObject blend = referencedSchema(schema, properties.getJSONObject("blend"));
+        JSONObject blendProperties = blend.getJSONObject("properties");
+        JSONObject amplitude = blendProperties.getJSONObject("amplitude");
+
+        assertEquals("array", dimensions.getString("type"));
+        assertEquals(2, dimensions.getInt("minItems"));
+        assertEquals(0, spacer.getInt("minimum"));
+        assertEquals(256, spacer.getInt("maximum"));
+        assertEquals(0, amplitude.getInt("minimum"));
+        assertEquals(256, amplitude.getInt("maximum"));
+        assertTrue(contains(schema.getJSONArray("required"), "dimensions"));
+        assertFalse(blend.has("required"));
+    }
+
+    @Test
+    public void gsonUsesTheCanonicalNestedShape() {
+        IrisDimensionStack stack = new IrisDimensionStack()
+                .setDimensions(new KList<>("sky", "main"))
+                .setSpacer(24)
+                .setBlend(new IrisDimensionStackBlend().setAmplitude(8));
+
+        JSONObject json = new JSONObject(new Gson().toJson(stack));
+
+        assertEquals(2, json.getJSONArray("dimensions").length());
+        assertEquals("main", json.getJSONArray("dimensions").getString(1));
+        assertEquals(24, json.getInt("spacer"));
+        assertEquals(8, json.getJSONObject("blend").getInt("amplitude"));
+        assertTrue(json.getJSONObject("blend").getJSONObject("style").has("style"));
+    }
+
+    private static JSONObject referencedSchema(JSONObject schema, JSONObject property) {
+        String key = property.getString("$ref").substring("#/definitions/".length());
+        return schema.getJSONObject("definitions").getJSONObject(key);
+    }
+
+    private static boolean contains(JSONArray values, String expected) {
+        for (int index = 0; index < values.length(); index++) {
+            if (expected.equals(values.optString(index, null))) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    @SuppressWarnings("unchecked")
+    private static IrisData schemaData() {
+        IrisData data = mock(IrisData.class);
+        ResourceLoader<IrisDimension> dimensionLoader = mock(ResourceLoader.class);
+        KMap<Class<? extends IrisRegistrant>, ResourceLoader<? extends IrisRegistrant>> loaders = new KMap<>();
+        loaders.put(IrisDimension.class, dimensionLoader);
+        when(data.getLoaders()).thenReturn(loaders);
+        when(dimensionLoader.getFolderName()).thenReturn("dimensions");
+        when(dimensionLoader.getResourceTypeName()).thenReturn("Dimension");
+        when(dimensionLoader.getPossibleKeys()).thenReturn(new String[]{"sky", "main"});
+        when(data.getPossibleSnippets(anyString())).thenReturn(new KList<>());
+        return data;
+    }
+}

core/src/test/java/art/arcane/iris/core/project/IrisProjectCopierTest.java

@@ -86,6 +86,46 @@ public class IrisProjectCopierTest {
         assertTrue(Files.isRegularFile(target.resolve("dimensions/the_nether.json")));
     }
 
+    @Test
+    public void rewritesDimensionStackReferencesAcrossCopiedDimensionsWhenRenaming() throws Exception {
+        Path workspace = temporaryFolder.newFolder("dimension-stack-reference").toPath();
+        Path source = workspace.resolve("template-pack");
+        Files.createDirectories(source.resolve("dimensions"));
+        Files.writeString(source.resolve("dimensions/overworld.json"), """
+                {
+                  "name": "Source",
+                  "dimensionStack": {
+                    "dimensions": ["layers/sky", "overworld"],
+                    "spacer": 24
+                  }
+                }
+                """);
+        Files.createDirectories(source.resolve("dimensions/layers"));
+        Files.writeString(source.resolve("dimensions/layers/sky.json"), """
+                {
+                  "name": "Sky",
+                  "dimensionStack": {
+                    "dimensions": ["overworld", "layers/sky"]
+                  }
+                }
+                """);
+
+        IrisProjectCopier.copyProject(source.toFile(), workspace.toFile(), "overworld", "new-project");
+
+        JSONObject dimension = new JSONObject(Files.readString(
+                workspace.resolve("new-project/dimensions/new-project.json")));
+        assertEquals("layers/sky", dimension.getJSONObject("dimensionStack")
+                .getJSONArray("dimensions").getString(0));
+        assertEquals("new-project", dimension.getJSONObject("dimensionStack")
+                .getJSONArray("dimensions").getString(1));
+        JSONObject supportingDimension = new JSONObject(Files.readString(
+                workspace.resolve("new-project/dimensions/layers/sky.json")));
+        assertEquals("new-project", supportingDimension.getJSONObject("dimensionStack")
+                .getJSONArray("dimensions").getString(0));
+        assertEquals("layers/sky", supportingDimension.getJSONObject("dimensionStack")
+                .getJSONArray("dimensions").getString(1));
+    }
+
     @Test
     public void existingTargetRemainsUnchanged() throws Exception {
         Path workspace = temporaryFolder.newFolder("packs").toPath();

core/src/test/java/art/arcane/iris/engine/DimensionStackLayoutTest.java

@@ -0,0 +1,337 @@
+package art.arcane.iris.engine;
+
+import com.github.benmanes.caffeine.cache.Cache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+
+public class DimensionStackLayoutTest {
+    @Test
+    public void spacerCountsExactAirBlocksBetweenInclusiveLayers() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                8,
+                List.of(input(15, 15), input(15, 15), input(15, 15)),
+                new int[]{0, 0}
+        );
+
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        assertEquals(0, layers.get(0).localBaseY());
+        assertEquals(15, layers.get(0).surfaceY());
+        assertEquals(24, layers.get(1).localBaseY());
+        assertEquals(39, layers.get(1).surfaceY());
+        assertEquals(48, layers.get(2).localBaseY());
+        assertEquals(63, layers.get(2).surfaceY());
+        assertEquals(8, layers.get(1).localBaseY() - layers.get(0).contentTopY() - 1);
+        assertEquals(8, layers.get(2).localBaseY() - layers.get(1).contentTopY() - 1);
+        assertEquals(63, layout.stackTerrainTopY());
+        assertEquals(63, layout.stackTopY());
+    }
+
+    @Test
+    public void independentlySignedSeamOffsetsShiftOnlyTheirUpperLayer() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                4,
+                List.of(input(10, 7), input(10, 7), input(10, 7)),
+                new int[]{3, -2}
+        );
+
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        assertEquals(0, layers.get(0).localBaseY());
+        assertEquals(18, layers.get(1).localBaseY());
+        assertEquals(31, layers.get(2).localBaseY());
+        assertEquals(3, layers.get(1).seamOffsetBelow());
+        assertEquals(-2, layers.get(2).seamOffsetBelow());
+        assertEquals(41, layout.stackTerrainTopY());
+        assertEquals(41, layout.stackTopY());
+    }
+
+    @Test
+    public void renderingBoundsClipWithoutChangingRawLayout() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                32,
+                8,
+                List.of(input(15, 15), input(15, 15), input(15, 15)),
+                new int[]{0, 0}
+        );
+
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        DimensionStackLayout.Layer middle = layers.get(1);
+        DimensionStackLayout.Layer top = layers.get(2);
+        assertEquals(39, middle.contentTopY());
+        assertEquals(24, middle.renderMinY());
+        assertEquals(31, middle.renderMaxY());
+        assertEquals(31, middle.clippedSurfaceY());
+        assertTrue(middle.visible());
+        assertTrue(middle.containsRenderedY(31));
+        assertFalse(middle.containsRenderedY(32));
+        assertTrue(layout.containsUpperLayerY(31));
+        assertFalse(layout.containsUpperLayerY(23));
+        assertEquals(48, top.localBaseY());
+        assertFalse(top.visible());
+        assertFalse(top.containsRenderedY(31));
+        assertEquals(63, layout.stackTerrainTopY());
+        assertEquals(63, layout.stackTopY());
+        assertEquals(31, layout.clippedStackTerrainTopY());
+        assertEquals(31, layout.clippedStackTopY());
+    }
+
+    @Test
+    public void invisibleLayersDoNotRaiseRenderedHeightToTheCeiling() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                64,
+                List.of(input(15, 15), input(15, 15)),
+                new int[]{0}
+        );
+
+        assertFalse(layout.layersBottomToTop().get(1).visible());
+        assertEquals(15, layout.clippedStackTerrainTopY());
+        assertEquals(15, layout.clippedStackTopY());
+    }
+
+    @Test
+    public void layerLookupReturnsRenderedOwnerAndBottomOwnerForAir() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                8,
+                List.of(input(15, 15), input(15, 15)),
+                new int[]{0}
+        );
+
+        assertSame(layout.layersBottomToTop().get(0), layout.layerAt(8));
+        assertSame(layout.layersBottomToTop().get(0), layout.layerAt(20));
+        assertSame(layout.layersBottomToTop().get(1), layout.layerAt(24));
+    }
+
+    @Test
+    public void laterGapsEraseEarlierUpperLayerOwnership() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                128,
+                0,
+                List.of(input(100, 100), input(60, 60), input(10, 10), input(10, 10)),
+                new int[]{-81, -41, 39}
+        );
+
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        assertSame(layers.get(1), layout.layerAt(30));
+        assertSame(layers.get(2), layout.layerAt(45));
+        assertSame(layers.get(0), layout.layerAt(60));
+        assertSame(layers.get(3), layout.layerAt(95));
+    }
+
+    @Test
+    public void hostFeaturesCannotFillStackGapsOrOverwriteUpperLayers() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                8,
+                List.of(input(15, 15), input(15, 15)),
+                new int[]{0}
+        );
+
+        assertFalse(layout.isHostFeatureProtectedY(15));
+        assertTrue(layout.isHostFeatureProtectedY(20));
+        assertTrue(layout.isHostFeatureProtectedY(24));
+    }
+
+    @Test
+    public void fluidExtendsContentWithoutChangingTerrainTop() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                2,
+                List.of(input(5, 9), input(4, 7)),
+                new int[]{0}
+        );
+
+        List<DimensionStackLayout.Layer> layers = layout.layersBottomToTop();
+        assertEquals(9, layers.get(0).contentTopY());
+        assertEquals(12, layers.get(1).localBaseY());
+        assertEquals(16, layers.get(1).surfaceY());
+        assertEquals(19, layers.get(1).contentTopY());
+        assertEquals(16, layout.stackTerrainTopY());
+        assertEquals(19, layout.stackTopY());
+    }
+
+    @Test
+    public void physicallyHighestTerrainLayerWinsWhenBlendEmbedsAnUpperLayer() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                0,
+                List.of(input(20, 20), input(5, 5)),
+                new int[]{-15}
+        );
+
+        assertSame(layout.layersBottomToTop().get(0), layout.topTerrainLayer());
+    }
+
+    @Test
+    public void declaredUpperLayerWinsWhenRenderedSurfacesTie() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                0,
+                List.of(input(10, 10), input(5, 5)),
+                new int[]{-6}
+        );
+
+        assertSame(layout.layersBottomToTop().get(1), layout.topTerrainLayer());
+    }
+
+    @Test
+    public void upperFluidOccludesAPhysicallyHigherLowerSurface() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                0,
+                List.of(input(20, 20), input(5, 15)),
+                new int[]{-11}
+        );
+
+        assertEquals(15, layout.clippedStackTerrainTopY());
+        assertEquals(25, layout.clippedStackTopY());
+        assertSame(layout.layersBottomToTop().get(1), layout.topTerrainLayer());
+    }
+
+    @Test
+    public void laterGapErasesProtrudingTerrainWhenItsUpperLayerIsClipped() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                128,
+                0,
+                List.of(input(100, 100), input(10, 10), input(10, 10)),
+                new int[]{-51, 89}
+        );
+
+        assertEquals(50, layout.layersBottomToTop().get(1).localBaseY());
+        assertEquals(150, layout.layersBottomToTop().get(2).localBaseY());
+        assertFalse(layout.layersBottomToTop().get(2).visible());
+        assertEquals(60, layout.clippedStackTerrainTopY());
+        assertEquals(60, layout.clippedStackTopY());
+        assertSame(layout.layersBottomToTop().get(1), layout.topTerrainLayer());
+    }
+
+    @Test
+    public void visibleFluidOwnsSurfaceQueriesWhenAllTerrainIsOccludedBelowTheFloor() {
+        DimensionStackLayout layout = DimensionStackLayout.create(
+                64,
+                0,
+                List.of(input(5, 5), input(2, 30)),
+                new int[]{-20}
+        );
+
+        assertEquals(-14, layout.layersBottomToTop().get(1).localBaseY());
+        assertEquals(0, layout.clippedStackTerrainTopY());
+        assertEquals(16, layout.clippedStackTopY());
+        assertNull(layout.topTerrainLayer());
+        assertSame(layout.layersBottomToTop().get(1), layout.surfaceLayer());
+    }
+
+    @Test
+    public void layoutListsAreImmutableAndExposeBothOrders() {
+        ArrayList<DimensionStackLayout.LayerInput> inputs = new ArrayList<>();
+        DimensionStackLayout.LayerInput bottom = input(3, 3);
+        DimensionStackLayout.LayerInput top = input(4, 4);
+        inputs.add(bottom);
+        inputs.add(top);
+        DimensionStackLayout layout = DimensionStackLayout.create(32, 1, inputs, new int[]{0});
+        inputs.clear();
+
+        assertEquals(2, layout.layersBottomToTop().size());
+        assertEquals(
+                layout.layersBottomToTop().get(0),
+                layout.layersTopToBottom().get(1)
+        );
+        assertThrows(
+                UnsupportedOperationException.class,
+                () -> layout.layersBottomToTop().clear()
+        );
+    }
+
+    @Test
+    public void seamSaltsAreStableAndBoundarySpecific() {
+        long first = DimensionStackContext.seamSalt(0, "root", "middle");
+        long repeat = DimensionStackContext.seamSalt(0, "root", "middle");
+        long differentIndex = DimensionStackContext.seamSalt(1, "root", "middle");
+        long differentUpper = DimensionStackContext.seamSalt(0, "root", "top");
+
+        assertEquals(first, repeat);
+        assertNotEquals(first, differentIndex);
+        assertNotEquals(first, differentUpper);
+    }
+
+    @Test
+    public void seamOffsetsClampUnboundedAndNonFiniteNoise() {
+        assertEquals(4, DimensionStackContext.clampSeamOffset(12D, 4));
+        assertEquals(-4, DimensionStackContext.clampSeamOffset(-12D, 4));
+        assertEquals(2, DimensionStackContext.clampSeamOffset(1.6D, 4));
+        assertEquals(0, DimensionStackContext.clampSeamOffset(Double.NaN, 4));
+        assertEquals(0, DimensionStackContext.clampSeamOffset(Double.POSITIVE_INFINITY, 4));
+    }
+
+    @Test
+    public void queryLayoutsCacheStableColumnsAndBypassTemporaryNaturalFallbacks() {
+        Cache<Long, DimensionStackLayout> cache = Caffeine.newBuilder().maximumSize(4).build();
+        DimensionStackLayout stable = DimensionStackLayout.create(
+                32, 0, List.of(input(10, 10), input(10, 10)), new int[]{0});
+        DimensionStackLayout unusedStable = DimensionStackLayout.create(
+                32, 0, List.of(input(11, 11), input(10, 10)), new int[]{0});
+        AtomicInteger stableSamples = new AtomicInteger();
+
+        DimensionStackLayout first = DimensionStackContext.resolveLayout(
+                cache, 7L, false, () -> {
+                    stableSamples.incrementAndGet();
+                    return stable;
+                });
+        DimensionStackLayout second = DimensionStackContext.resolveLayout(
+                cache, 7L, false, () -> {
+                    stableSamples.incrementAndGet();
+                    return unusedStable;
+                });
+
+        assertSame(stable, first);
+        assertSame(stable, second);
+        assertEquals(1, stableSamples.get());
+
+        DimensionStackLayout naturalFirst = DimensionStackLayout.create(
+                32, 0, List.of(input(12, 12), input(10, 10)), new int[]{0});
+        DimensionStackLayout naturalSecond = DimensionStackLayout.create(
+                32, 0, List.of(input(13, 13), input(10, 10)), new int[]{0});
+        AtomicInteger naturalSamples = new AtomicInteger();
+
+        assertSame(naturalFirst, DimensionStackContext.resolveLayout(
+                cache, 7L, true, () -> {
+                    naturalSamples.incrementAndGet();
+                    return naturalFirst;
+                }));
+        assertSame(naturalSecond, DimensionStackContext.resolveLayout(
+                cache, 7L, true, () -> {
+                    naturalSamples.incrementAndGet();
+                    return naturalSecond;
+                }));
+        assertEquals(2, naturalSamples.get());
+        assertSame(stable, cache.getIfPresent(7L));
+    }
+
+    private static DimensionStackLayout.LayerInput input(int terrainHeight, int fluidHeight) {
+        return new DimensionStackLayout.LayerInput(
+                mock(DimensionTerrainContext.class),
+                null,
+                null,
+                null,
+                null,
+                null,
+                terrainHeight,
+                fluidHeight
+        );
+    }
+}

core/src/test/java/art/arcane/iris/engine/DimensionTerrainContextFallbackTest.java

@@ -0,0 +1,73 @@
+package art.arcane.iris.engine;
+
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.image.IrisImageMapRuntime;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisRegion;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.util.project.stream.ProceduralStream;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class DimensionTerrainContextFallbackTest {
+    @Test
+    @SuppressWarnings("unchecked")
+    public void selfSamplerUsesNaturalValuesOnlyForNonblockingColumns() {
+        Engine engine = mock(Engine.class);
+        IrisDimension dimension = mock(IrisDimension.class);
+        IrisComplex complex = mock(IrisComplex.class);
+        ProceduralStream<Double> naturalHeight = mock(ProceduralStream.class);
+        ProceduralStream<Double> resolvedHeight = mock(ProceduralStream.class);
+        ProceduralStream<Double> resolvedFluidHeight = mock(ProceduralStream.class);
+        ProceduralStream<IrisBiome> naturalBiome = mock(ProceduralStream.class);
+        ProceduralStream<IrisBiome> resolvedBiome = mock(ProceduralStream.class);
+        ProceduralStream<IrisRegion> region = mock(ProceduralStream.class);
+        ProceduralStream<PlatformBlockState> rock = mock(ProceduralStream.class);
+        ProceduralStream<PlatformBlockState> configuredFluid = mock(ProceduralStream.class);
+        IrisImageMapRuntime imageMapRuntime = mock(IrisImageMapRuntime.class);
+        IrisBiome naturalBiomeValue = mock(IrisBiome.class);
+        IrisBiome resolvedBiomeValue = mock(IrisBiome.class);
+        PlatformBlockState naturalFluid = mock(PlatformBlockState.class);
+        PlatformBlockState resolvedFluid = mock(PlatformBlockState.class);
+
+        when(engine.getDimension()).thenReturn(dimension);
+        when(engine.getComplex()).thenReturn(complex);
+        when(engine.getHeight()).thenReturn(128);
+        when(dimension.getLoadKey()).thenReturn("root");
+        when(dimension.getFluidHeight()).thenReturn(12);
+        when(complex.getNaturalHeightStream()).thenReturn(naturalHeight);
+        when(complex.getHeightStream()).thenReturn(resolvedHeight);
+        when(complex.getRiverWaterSurfaceStream()).thenReturn(resolvedFluidHeight);
+        when(complex.getNaturalTrueBiomeStream()).thenReturn(naturalBiome);
+        when(complex.getTrueBiomeStream()).thenReturn(resolvedBiome);
+        when(complex.getRegionStream()).thenReturn(region);
+        when(complex.getRockStream()).thenReturn(rock);
+        when(complex.getFluidStream()).thenReturn(configuredFluid);
+        when(complex.getImageMapRuntime()).thenReturn(imageMapRuntime);
+        when(naturalHeight.getDouble(12D, -8D)).thenReturn(15D);
+        when(resolvedHeight.getDouble(12D, -8D)).thenReturn(18D);
+        when(resolvedFluidHeight.getDouble(12D, -8D)).thenReturn(14D);
+        when(naturalBiome.get(12D, -8D)).thenReturn(naturalBiomeValue);
+        when(resolvedBiome.get(12D, -8D)).thenReturn(resolvedBiomeValue);
+        when(configuredFluid.get(12D, -8D)).thenReturn(naturalFluid);
+        when(complex.resolveSurfaceFluid(12D, -8D)).thenReturn(resolvedFluid);
+
+        DimensionTerrainContext context = DimensionTerrainContext.forStack(engine, dimension);
+        when(engine.answersFromNaturalTerrain(12, -8)).thenReturn(true);
+        assertEquals(15D, context.getNormalTerrainHeight(12D, -8D), 0D);
+        assertEquals(12D, context.getFluidHeight(12D, -8D), 0D);
+        assertSame(naturalBiomeValue, context.getBiome(12D, -8D));
+        assertSame(naturalFluid, context.getFluidBlock(12D, -8D));
+
+        when(engine.answersFromNaturalTerrain(12, -8)).thenReturn(false);
+        assertEquals(18D, context.getNormalTerrainHeight(12D, -8D), 0D);
+        assertEquals(14D, context.getFluidHeight(12D, -8D), 0D);
+        assertSame(resolvedBiomeValue, context.getBiome(12D, -8D));
+        assertSame(resolvedFluid, context.getFluidBlock(12D, -8D));
+    }
+}

core/src/test/java/art/arcane/iris/engine/EngineRuntimePublicationContractTest.java

@@ -37,6 +37,20 @@ public class EngineRuntimePublicationContractTest {
         assertBefore(publish, "sealAndAwait(", "closeRuntime(next, e)");
     }
 
+    @Test
+    public void dimensionStackRunsAfterHostStaticObjects() throws IOException {
+        String source = Files.readString(Path.of(
+                "src/main/java/art/arcane/iris/engine/EngineRuntimeBuilder.java")).replace("\r\n", "\n");
+        int buildStart = source.indexOf("EngineRuntime buildRuntime(EngineTarget runtimeTarget)");
+        int buildEnd = source.indexOf("DimensionStackContext buildDimensionStackContext", buildStart);
+        String build = source.substring(buildStart, buildEnd);
+
+        assertBefore(build, "staticObjects.apply(engine, x, z, blocks)",
+                "dimensionStack.actuate(x, z, blocks, multicore, context)");
+        assertBefore(build, "dimensionStack.actuate(x, z, blocks, multicore, context)",
+                "stackCustom.modify(x, z, blocks, multicore, context)");
+    }
+
     private static void assertBefore(String source, String first, String second) {
         int firstIndex = source.indexOf(first);
         int secondIndex = source.indexOf(second);

core/src/test/java/art/arcane/iris/engine/actuator/IrisBiomeActuatorCoordinateTest.java

@@ -17,6 +17,7 @@ import art.arcane.iris.util.project.hunk.Hunk;
 import art.arcane.volmlib.util.mantle.runtime.Mantle;
 import art.arcane.volmlib.util.math.RNG;
 import art.arcane.volmlib.util.matter.Matter;
+import art.arcane.volmlib.util.matter.MatterBiomeInject;
 import org.junit.After;
 import org.junit.Test;
 
@@ -30,6 +31,8 @@ import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.same;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
 import static org.mockito.Mockito.when;
 
 public class IrisBiomeActuatorCoordinateTest {
@@ -69,6 +72,19 @@ public class IrisBiomeActuatorCoordinateTest {
         assertEquals(List.of("100,-200", "100,-199", "101,-200", "101,-199"), samples);
     }
 
+    @Test
+    @SuppressWarnings("unchecked")
+    public void stackedBiomeReplacementRemovesEveryExistingMarkerWithItsTypedSlice() {
+        Mantle<Matter> mantle = mock(Mantle.class);
+
+        IrisBiomeActuator.clearBiomeMatterRange(mantle, 100, -200, 4, 6);
+
+        verify(mantle).remove(100, 4, -200, MatterBiomeInject.class);
+        verify(mantle).remove(100, 5, -200, MatterBiomeInject.class);
+        verify(mantle).remove(100, 6, -200, MatterBiomeInject.class);
+        verifyNoMoreInteractions(mantle);
+    }
+
     @SuppressWarnings("unchecked")
     private Engine engine() {
         Mantle<Matter> mantle = mock(Mantle.class);

core/src/test/java/art/arcane/iris/engine/actuator/IrisDimensionStackActuatorMetadataTest.java

@@ -0,0 +1,30 @@
+package art.arcane.iris.engine.actuator;
+
+import art.arcane.iris.engine.hydrology.cave.HydrologyCaveCell;
+import art.arcane.volmlib.util.mantle.runtime.MantleChunk;
+import art.arcane.volmlib.util.matter.Matter;
+import art.arcane.volmlib.util.matter.MatterSlice;
+import org.junit.Test;
+
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class IrisDimensionStackActuatorMetadataTest {
+    @Test
+    @SuppressWarnings("unchecked")
+    public void overwrittenStackCellsClearHostHydrologyCaveOwnership() {
+        MantleChunk<Matter> chunk = mock(MantleChunk.class);
+        Matter section = mock(Matter.class);
+        MatterSlice<HydrologyCaveCell> hydrology = mock(MatterSlice.class);
+        when(chunk.get(1)).thenReturn(section);
+        doReturn(hydrology).when(section).getSlice(HydrologyCaveCell.class);
+
+        IrisDimensionStackActuator.MetadataCleaner cleaner =
+                new IrisDimensionStackActuator.MetadataCleaner(chunk, 64);
+        cleaner.clear(3, 20, 7);
+
+        verify(hydrology).set(3, 4, 7, null);
+    }
+}

core/src/test/java/art/arcane/iris/engine/framework/EngineHostHeightTest.java

@@ -0,0 +1,123 @@
+package art.arcane.iris.engine.framework;
+
+import art.arcane.iris.engine.IrisComplex;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionStackLayout;
+import art.arcane.iris.engine.mantle.EngineMantle;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisRegion;
+import art.arcane.iris.spi.IrisPlatform;
+import art.arcane.iris.spi.IrisPlatforms;
+import art.arcane.iris.spi.PlatformBiome;
+import art.arcane.iris.spi.PlatformBiomeWriter;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.spi.PlatformRegistries;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
+import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class EngineHostHeightTest {
+    @Before
+    public void bindPlatform() {
+        IrisPlatforms.unbind();
+        PlatformBiome biome = mock(PlatformBiome.class);
+        PlatformBlockState block = mock(PlatformBlockState.class);
+        PlatformRegistries registries = mock(PlatformRegistries.class);
+        when(registries.biome(anyString())).thenReturn(biome);
+        when(registries.block(anyString())).thenReturn(block);
+        PlatformBiomeWriter biomeWriter = mock(PlatformBiomeWriter.class);
+        when(biomeWriter.biomeIdFor(anyString())).thenReturn(1);
+        IrisPlatform platform = mock(IrisPlatform.class);
+        when(platform.registries()).thenReturn(registries);
+        when(platform.biomeWriter()).thenReturn(biomeWriter);
+        IrisPlatforms.bind(platform);
+    }
+
+    @After
+    public void unbindPlatform() {
+        IrisPlatforms.unbind();
+    }
+
+    @Test
+    public void coldMainThreadQueryUsesNaturalHostTerrain() {
+        Engine engine = mock(Engine.class);
+        EngineMantle mantle = mock(EngineMantle.class);
+        IrisComplex complex = mock(IrisComplex.class);
+        when(engine.getMantle()).thenReturn(mantle);
+        when(engine.getComplex()).thenReturn(complex);
+        when(engine.answersFromNaturalTerrain(12, -8)).thenReturn(true);
+        when(complex.naturalTrueHeight(12, -8)).thenReturn(42);
+        when(mantle.getFluidHeight()).thenReturn(50);
+
+        assertEquals(42, Engine.hostHeight(engine, 12, -8, true));
+        assertEquals(50, Engine.hostHeight(engine, 12, -8, false));
+        verify(mantle, never()).trueHeight(12, -8);
+        verify(mantle, never()).getFluidHeight(12, -8);
+    }
+
+    @Test
+    public void plannedQueryUsesHostColumnTerrainAndFluid() {
+        Engine engine = mock(Engine.class);
+        EngineMantle mantle = mock(EngineMantle.class);
+        when(engine.getMantle()).thenReturn(mantle);
+        when(mantle.trueHeight(3, 7)).thenReturn(45);
+        when(mantle.getFluidHeight(3, 7)).thenReturn(48);
+
+        assertEquals(45, Engine.hostHeight(engine, 3, 7, true));
+        assertEquals(48, Engine.hostHeight(engine, 3, 7, false));
+    }
+
+    @Test
+    public void yAwareRegionUsesTheOwningStackLayer() {
+        Engine engine = mock(Engine.class, CALLS_REAL_METHODS);
+        DimensionStackContext stack = mock(DimensionStackContext.class);
+        DimensionStackLayout.Layer layer = mock(DimensionStackLayout.Layer.class);
+        DimensionStackLayout layout = mock(DimensionStackLayout.class);
+        IrisRegion expected = mock(IrisRegion.class);
+        when(engine.getDimensionStackContext()).thenReturn(stack);
+        when(stack.getLayout(12, -8)).thenReturn(layout);
+        when(layout.layerAt(96)).thenReturn(layer);
+        when(layer.region()).thenReturn(expected);
+
+        assertEquals(expected, engine.getRegion(12, 96, -8));
+    }
+
+    @Test
+    public void stackQueryConsumersUseCacheAwareLayouts() {
+        Engine engine = mock(Engine.class, CALLS_REAL_METHODS);
+        DimensionStackContext stack = mock(DimensionStackContext.class);
+        DimensionStackLayout layout = mock(DimensionStackLayout.class);
+        DimensionStackLayout.Layer bottom = mock(DimensionStackLayout.Layer.class);
+        DimensionStackLayout.Layer upper = mock(DimensionStackLayout.Layer.class);
+        IrisRegion region = mock(IrisRegion.class);
+        IrisBiome biome = mock(IrisBiome.class);
+        when(engine.getDimensionStackContext()).thenReturn(stack);
+        when(stack.getLayout(12, -8)).thenReturn(layout);
+        when(layout.layersBottomToTop()).thenReturn(List.of(bottom, upper));
+        when(layout.surfaceLayer()).thenReturn(upper);
+        when(layout.layerAt(96)).thenReturn(upper);
+        when(upper.region()).thenReturn(region);
+        when(upper.biome()).thenReturn(biome);
+
+        assertSame(region, engine.getRegion(12, -8));
+        assertSame(biome, engine.getSurfaceBiome(12, -8));
+        assertSame(biome, engine.getBiome(12, 96, -8));
+        assertSame(biome, engine.getBiomeOrMantle(12, 96, -8));
+
+        verify(stack, times(4)).getLayout(12, -8);
+        verify(stack, never()).sample(anyInt(), anyInt());
+    }
+}

core/src/test/java/art/arcane/iris/engine/framework/HintedLocatorTest.java

@@ -2,6 +2,8 @@ package art.arcane.iris.engine.framework;
 
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.iris.engine.DimensionStackContext;
+import art.arcane.iris.engine.DimensionTerrainContext;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisDimension;
@@ -22,6 +24,7 @@ import org.junit.Test;
 import java.io.File;
 import java.nio.file.Files;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Future;
@@ -301,6 +304,20 @@ public class HintedLocatorTest {
         assertEquals(1, plan.getStrideChunks());
     }
 
+    @Test
+    public void biomePlanIsUnprunedForStackLayerBiome() {
+        DimensionStackContext stackContext = mock(DimensionStackContext.class);
+        IrisBiome stacked = biome("stacked");
+        when(engine.getDimensionStackContext()).thenReturn(stackContext);
+        when(engine.getAllBiomes()).thenReturn(new KList<>(stacked));
+
+        HintedLocator.SearchPlan plan = HintedLocator.biomePlan(engine, "stacked");
+
+        assertTrue(plan.isPossible());
+        assertNull(plan.getCoarse());
+        assertEquals(1, plan.getStrideChunks());
+    }
+
     @Test
     public void regionPlanMatchesOnlyTargetRegionCells() {
         IrisRegion regionA = region("reg_a");
@@ -326,6 +343,24 @@ public class HintedLocatorTest {
         assertFalse(plan.isPossible());
     }
 
+    @Test
+    public void regionPlanIsUnprunedForStackLayerRegion() {
+        DimensionStackContext stackContext = mock(DimensionStackContext.class);
+        DimensionTerrainContext terrainContext = mock(DimensionTerrainContext.class);
+        IrisDimension stackedDimension = mock(IrisDimension.class);
+        IrisRegion stackedRegion = region("stacked_region");
+        when(engine.getDimensionStackContext()).thenReturn(stackContext);
+        when(stackContext.getLayersTopToBottom()).thenReturn(List.of(terrainContext));
+        when(terrainContext.getDimension()).thenReturn(stackedDimension);
+        when(stackedDimension.getAllRegions(terrainContext)).thenReturn(new KList<>(stackedRegion));
+
+        HintedLocator.SearchPlan plan = HintedLocator.regionPlan(engine, "stacked_region");
+
+        assertTrue(plan.isPossible());
+        assertNull(plan.getCoarse());
+        assertEquals(1, plan.getStrideChunks());
+    }
+
     @Test
     public void objectPlanPrunesToHostBiomes() {
         IrisRegion regionA = region("reg_a");

core/src/test/java/art/arcane/iris/engine/modifier/IrisFloatingChildBiomeModifierStackTest.java

@@ -0,0 +1,22 @@
+package art.arcane.iris.engine.modifier;
+
+import art.arcane.iris.engine.DimensionStackLayout;
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisFloatingChildBiomeModifierStackTest {
+    @Test
+    public void floatingBiomeMarkersSkipStackProtectedCells() {
+        DimensionStackLayout layout = mock(DimensionStackLayout.class);
+        when(layout.isHostFeatureProtectedY(60)).thenReturn(true);
+        when(layout.isHostFeatureProtectedY(30)).thenReturn(false);
+
+        assertFalse(IrisFloatingChildBiomeModifier.shouldWriteHostBiomeMarker(layout, 60));
+        assertTrue(IrisFloatingChildBiomeModifier.shouldWriteHostBiomeMarker(layout, 30));
+        assertTrue(IrisFloatingChildBiomeModifier.shouldWriteHostBiomeMarker(null, 60));
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisBiomeLayerSlopeSourceTest.java

@@ -0,0 +1,68 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.IrisComplex;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.util.project.noise.CNG;
+import art.arcane.iris.util.project.stream.ProceduralStream;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.same;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public final class IrisBiomeLayerSlopeSourceTest {
+    @Test
+    public void explicitSlopeStreamControlsSurfacePaletteSelection() {
+        assertExplicitSlopeStreamControlsSurfacePaletteSelection(false);
+    }
+
+    @Test
+    public void explicitSlopeStreamControlsLockedSurfacePaletteSelection() {
+        assertExplicitSlopeStreamControlsSurfacePaletteSelection(true);
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void assertExplicitSlopeStreamControlsSurfacePaletteSelection(boolean lockLayers) {
+        IrisData data = mock(IrisData.class);
+        IrisComplex hostComplex = mock(IrisComplex.class);
+        ProceduralStream<Double> hostSlope = mock(ProceduralStream.class);
+        ProceduralStream<Double> sourceSlope = mock(ProceduralStream.class);
+        IrisBiomePaletteLayer paletteLayer = mock(IrisBiomePaletteLayer.class);
+        CNG heightGenerator = mock(CNG.class);
+        PlatformBlockState surfaceBlock = mock(PlatformBlockState.class);
+        RNG rng = new RNG(19L);
+
+        IrisBiome biome = new IrisBiome();
+        biome.setLockLayers(lockLayers);
+        KList<IrisBiomePaletteLayer> paletteLayers = new KList<>();
+        paletteLayers.add(paletteLayer);
+        biome.setLayers(paletteLayers);
+
+        when(hostComplex.getSlopeStream()).thenReturn(hostSlope);
+        when(hostSlope.getDouble(12D, -8D)).thenReturn(0D);
+        when(sourceSlope.getDouble(12D, -8D)).thenReturn(5D);
+        when(paletteLayer.getZoom()).thenReturn(1D);
+        when(paletteLayer.getMinHeight()).thenReturn(1);
+        when(paletteLayer.getMaxHeight()).thenReturn(1);
+        when(paletteLayer.getSlopeCondition()).thenReturn(new IrisSlopeClip(5D, 5D));
+        when(paletteLayer.getHeightGenerator(any(RNG.class), same(data))).thenReturn(heightGenerator);
+        when(heightGenerator.fit(1, 1, 12D, -8D)).thenReturn(1);
+        when(paletteLayer.get(rng, 0, 12D, 0D, -8D, data)).thenReturn(surfaceBlock);
+
+        KList<PlatformBlockState> hostLayers = biome.generateLayers(
+                new IrisDimension(), 12D, -8D, rng, 1, 32, data, hostComplex);
+        KList<PlatformBlockState> sourceLayers = biome.generateLayersWithSlope(
+                new IrisDimension(), 12D, -8D, rng, 1, 32, data, sourceSlope);
+
+        assertTrue(hostLayers.isEmpty());
+        assertEquals(1, sourceLayers.size());
+        assertSame(surfaceBlock, sourceLayers.get(0));
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisDimensionReachableBiomesTest.java

@@ -16,6 +16,43 @@ import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 public class IrisDimensionReachableBiomesTest {
+    @Test
+    @SuppressWarnings("unchecked")
+    public void includesOutOfPoolFocusBiomeAndRegion() {
+        IrisDimension dimension = new IrisDimension()
+                .setRegions(new KList<>("declared"))
+                .setFocus("focused-biome")
+                .setFocusRegion("focused-region");
+        IrisRegion declared = new IrisRegion().setLandBiomes(new KList<>("declared-biome"));
+        IrisRegion focused = new IrisRegion().setLandBiomes(new KList<>("focused-region-biome"));
+        declared.setLoadKey("declared");
+        focused.setLoadKey("focused-region");
+        IrisBiome declaredBiome = biome("declared-biome");
+        IrisBiome focusedBiome = biome("focused-biome");
+        IrisBiome focusedRegionBiome = biome("focused-region-biome");
+
+        IrisData data = mock(IrisData.class);
+        ResourceLoader<IrisRegion> regionLoader = mock(ResourceLoader.class);
+        ResourceLoader<IrisBiome> biomeLoader = mock(ResourceLoader.class);
+        when(data.getRegionLoader()).thenReturn(regionLoader);
+        when(data.getBiomeLoader()).thenReturn(biomeLoader);
+        when(regionLoader.load("declared")).thenReturn(declared);
+        when(regionLoader.load("focused-region")).thenReturn(focused);
+        when(biomeLoader.load("declared-biome")).thenReturn(declaredBiome);
+        when(biomeLoader.load("focused-biome")).thenReturn(focusedBiome);
+        when(biomeLoader.load("focused-region-biome")).thenReturn(focusedRegionBiome);
+
+        Set<String> regionKeys = dimension.getAllRegions(() -> data).stream()
+                .map(IrisRegion::getLoadKey)
+                .collect(Collectors.toSet());
+        Set<String> biomeKeys = dimension.getReachableBiomes(() -> data).stream()
+                .map(IrisBiome::getLoadKey)
+                .collect(Collectors.toSet());
+
+        assertEquals(Set.of("declared", "focused-region"), regionKeys);
+        assertEquals(Set.of("declared-biome", "focused-biome", "focused-region-biome"), biomeKeys);
+    }
+
     @Test
     @SuppressWarnings("unchecked")
     public void includesExactRecursiveGenerationClosure() {

core/src/test/java/art/arcane/iris/engine/object/IrisDimensionTypeKeyTest.java

@@ -20,4 +20,20 @@ public class IrisDimensionTypeKeyTest {
 
         assertEquals("worlds_my_pack", dimension.getDimensionTypeKey());
     }
+
+    @Test
+    public void customBiomeKeyPreservesFlatDimensionNamespace() {
+        IrisDimension dimension = new IrisDimension();
+        dimension.setLoadKey("Overworld");
+
+        assertEquals("overworld:aurora", dimension.getCustomBiomeKey("Aurora"));
+    }
+
+    @Test
+    public void customBiomeKeyMapsRecursiveDimensionPath() {
+        IrisDimension dimension = new IrisDimension();
+        dimension.setLoadKey("Layers/Sky");
+
+        assertEquals("layers:sky/aurora", dimension.getCustomBiomeKey("Aurora"));
+    }
 }

core/src/test/java/art/arcane/iris/engine/platform/EngineBukkitOpsBiomeTest.java

@@ -0,0 +1,30 @@
+package art.arcane.iris.engine.platform;
+
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisWorld;
+import org.bukkit.Location;
+import org.junit.Test;
+
+import static org.junit.Assert.assertSame;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class EngineBukkitOpsBiomeTest {
+    @Test
+    public void mantleBiomeLookupConvertsWorldYToInternalY() {
+        Engine engine = mock(Engine.class);
+        IrisWorld world = IrisWorld.builder().minHeight(-64).maxHeight(320).build();
+        IrisBiome biome = mock(IrisBiome.class);
+        Location location = mock(Location.class);
+        when(engine.getWorld()).thenReturn(world);
+        when(location.getBlockX()).thenReturn(12);
+        when(location.getBlockY()).thenReturn(-25);
+        when(location.getBlockZ()).thenReturn(-7);
+        when(engine.getBiomeOrMantle(12, 39, -7)).thenReturn(biome);
+
+        assertSame(biome, EngineBukkitOps.getBiomeOrMantle(engine, location));
+        verify(engine).getBiomeOrMantle(12, 39, -7);
+    }
+}