diff --git a/core/src/Dimension.cpp b/core/src/Dimension.cpp
new file mode 100644
index 0000000..5f88a0e
--- /dev/null
+++ b/core/src/Dimension.cpp
@@ -0,0 +1,152 @@
+#include "vf/Dimension.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstdlib>
+
+namespace vf {
+
+namespace {
+
+constexpr long long ticksPerDay = 24000;
+
+// A destination portal site is only worth taking if it sits within this many
+// blocks of the scaled arrival height; otherwise a platform is cheaper than a
+// long fall or a climb to the roof.
+constexpr int safeSpotVerticalBand = 32;
+
+uint32_t deriveSeed(uint32_t baseSeed, uint32_t salt) {
+    uint64_t h = baseSeed;
+    h ^= salt + 0x9E3779B97F4A7C15ull + (h << 6) + (h >> 2);
+    h *= 1099511628211ull;
+    return static_cast<uint32_t>(h ^ (h >> 32));
+}
+
+} // namespace
+
+DimensionConfig DimensionConfig::forDimension(DimensionId id) {
+    DimensionConfig cfg;
+    cfg.id = id;
+    switch (id) {
+        case DimensionId::Nether:
+            cfg.floorY = 0;
+            cfg.ceilingY = 128;
+            cfg.hasSky = false;
+            cfg.hasCeiling = true;
+            cfg.hasDayCycle = false;
+            cfg.waterEvaporates = true;
+            cfg.bedsExplode = true;
+            cfg.coordinateScale = 8.0f;
+            cfg.ambientLight = 0.10f;
+            cfg.seedSalt = 0x4e544852u;
+            break;
+        case DimensionId::End:
+            cfg.floorY = 0;
+            cfg.ceilingY = 128;
+            cfg.hasSky = false;
+            cfg.hasCeiling = false;
+            cfg.hasDayCycle = false;
+            cfg.waterEvaporates = false;
+            cfg.bedsExplode = true;
+            cfg.coordinateScale = 1.0f;
+            cfg.ambientLight = 0.04f;
+            cfg.seedSalt = 0x454e4421u;
+            break;
+        default:
+            cfg.floorY = 0;
+            cfg.ceilingY = 128;
+            cfg.hasSky = true;
+            cfg.hasCeiling = false;
+            cfg.hasDayCycle = true;
+            cfg.waterEvaporates = false;
+            cfg.bedsExplode = false;
+            cfg.coordinateScale = 1.0f;
+            cfg.ambientLight = 0.0f;
+            cfg.seedSalt = 0u;
+            break;
+    }
+    return cfg;
+}
+
+const char* DimensionConfig::nameOf(DimensionId id) {
+    switch (id) {
+        case DimensionId::Nether: return "nether";
+        case DimensionId::End: return "end";
+        default: return "overworld";
+    }
+}
+
+int DimensionConfig::clampY(int y) const {
+    int low = lowestPlaceableY();
+    int high = highestPlaceableY();
+    if (y < low) return low;
+    if (y > high) return high;
+    return y;
+}
+
+Dimension::Dimension(const DimensionConfig& dimensionConfig, uint32_t worldSeed)
+    : cfg(dimensionConfig), derivedSeed(deriveSeed(worldSeed, dimensionConfig.seedSalt)) {
+    switch (cfg.id) {
+        case DimensionId::Nether: {
+            NetherGenerator::Params netherParams;
+            netherParams.seed = derivedSeed;
+            netherParams.floorY = cfg.floorY;
+            netherParams.ceilingY = cfg.ceilingY;
+            nether = std::make_unique<NetherGenerator>(netherParams);
+            break;
+        }
+        case DimensionId::End: {
+            EndGenerator::Params endParams;
+            endParams.seed = derivedSeed;
+            endParams.floorY = cfg.floorY;
+            endParams.ceilingY = cfg.ceilingY;
+            end = std::make_unique<EndGenerator>(endParams);
+            break;
+        }
+        default: {
+            WorldGenParams overworldParams;
+            overworldParams.seed = derivedSeed;
+            overworldParams.maxY = cfg.ceilingY;
+            overworld = std::make_unique<WorldGen>(overworldParams);
+            break;
+        }
+    }
+}
+
+bool Dimension::columnGenerated(int columnX, int columnZ) const {
+    return columns.find(Vec3i{columnX, 0, columnZ}) != columns.end();
+}
+
+void Dimension::generateColumn(int columnX, int columnZ) {
+    if (!columns.insert(Vec3i{columnX, 0, columnZ}).second) return;
+
+    if (nether) nether->generateColumn(blocks, columnX, columnZ);
+    else if (end) end->generateColumn(blocks, columnX, columnZ);
+    else overworld->generateColumn(blocks, columnX, columnZ);
+}
+
+int Dimension::generateArea(int columnX, int columnZ, int columnRadius) {
+    int made = 0;
+    for (int dx = -columnRadius; dx <= columnRadius; ++dx) {
+        for (int dz = -columnRadius; dz <= columnRadius; ++dz) {
+            if (columnGenerated(columnX + dx, columnZ + dz)) continue;
+            generateColumn(columnX + dx, columnZ + dz);
+            ++made;
+        }
+    }
+    return made;
+}
+
+float Dimension::dayFraction() const {
+    if (!cfg.hasDayCycle) return 0.0f;
+    long long phase = elapsed % ticksPerDay;
+    if (phase < 0) phase += ticksPerDay;
+    return static_cast<float>(phase) / static_cast<float>(ticksPerDay);
+}
+
+int Dimension::highestSolidY(int gx, int gz) const {
+    for (int y = cfg.ceilingY - 1; y >= cfg.floorY; --y)
+        if (blocks.solidAt(gx, y, gz)) return y;
+    return cfg.floorY - 1;
+}
+