diff --git a/src/Util.hpp b/src/Util.hpp
index 1f50958..163813d 100644
--- a/src/Util.hpp
+++ b/src/Util.hpp
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <cstdint>
+#include <limits>
 #include <stdexcept>
 #include <utility>
 
@@ -29,13 +30,15 @@ static inline float clamp(float v, float min, float max) {
 // A block coordinate relative to a chunk/section (X/Y/Z)
 typedef uint8_t block_idx_t;
 // A section index in a chunk (Y)
-typedef uint8_t section_idx_t;
+typedef int8_t section_idx_t;
 // A chunk coordinate relative to a region (X/Z)
 typedef uint8_t chunk_idx_t;
 
 // A block coordinate relative to a region (X/Z)
 typedef uint16_t region_block_idx_t;
 // A block coordinate (Y)
-typedef uint16_t y_idx_t;
+typedef int16_t y_idx_t;
+
+const y_idx_t y_idx_min = std::numeric_limits<y_idx_t>::min();
 
 }
diff --git a/src/World/Chunk.cpp b/src/World/Chunk.cpp
index 8b4b9eb..a447cda 100644
--- a/src/World/Chunk.cpp
+++ b/src/World/Chunk.cpp
@@ -18,27 +18,35 @@ namespace MinedMap {
 namespace World {
 
 Chunk::Chunk(const ChunkData *data) {
-	std::shared_ptr<const NBT::CompoundTag> level =
-		assertValue(data->getRoot()->get<NBT::CompoundTag>("Level"));
+	std::shared_ptr<const NBT::ListTag> sectionsTag;
 
-	std::shared_ptr<const NBT::ListTag> sectionsTag = level->get<NBT::ListTag>("Sections");
-	if (!sectionsTag || sectionsTag->empty())
-		return;
+	auto level = data->getRoot()->get<NBT::CompoundTag>("Level");
+	if (level) {
+		sectionsTag = level->get<NBT::ListTag>("Sections");
+		if (!sectionsTag || sectionsTag->empty())
+			return;
 
-	auto biomesIntArray = level->get<NBT::IntArrayTag>("Biomes");
-	auto biomesByteArray = level->get<NBT::ByteArrayTag>("Biomes");
+		auto biomesIntArray = level->get<NBT::IntArrayTag>("Biomes");
+		auto biomesByteArray = level->get<NBT::ByteArrayTag>("Biomes");
 
-	if (biomesIntArray && biomesIntArray->getLength() == BSIZE*BSIZE*BMAXY) {
-		biomeInts = std::move(biomesIntArray);
-		biomeFormat = INT_ARRAY;
-	} else if (biomesIntArray && biomesIntArray->getLength() == SIZE*SIZE) {
-		biomeInts = std::move(biomesIntArray);
-		biomeFormat = INT_ARRAY_PRE1_15;
-	} else if (biomesByteArray && biomesByteArray->getLength() == SIZE*SIZE) {
-		biomeBytes = std::move(biomesByteArray);
-		biomeFormat = BYTE_ARRAY;
+		if (biomesIntArray && biomesIntArray->getLength() == BSIZE*BSIZE*BMAXY) {
+			biomeInts = std::move(biomesIntArray);
+			biomeFormat = INT_ARRAY;
+		} else if (biomesIntArray && biomesIntArray->getLength() == SIZE*SIZE) {
+			biomeInts = std::move(biomesIntArray);
+			biomeFormat = INT_ARRAY_PRE1_15;
+		} else if (biomesByteArray && biomesByteArray->getLength() == SIZE*SIZE) {
+			biomeBytes = std::move(biomesByteArray);
+			biomeFormat = BYTE_ARRAY;
+		} else {
+			throw std::invalid_argument("corrupt biome data");
+		}
 	} else {
-		throw std::invalid_argument("corrupt biome data");
+		sectionsTag = data->getRoot()->get<NBT::ListTag>("sections");
+		if (!sectionsTag || sectionsTag->empty())
+			return;
+
+		biomeFormat = SECTION;
 	}
 
 	auto dataVersionTag = data->getRoot()->get<NBT::IntTag>("DataVersion");
@@ -48,25 +56,34 @@ Chunk::Chunk(const ChunkData *data) {
 		auto s = std::dynamic_pointer_cast<const NBT::CompoundTag>(sTag);
 		std::unique_ptr<Section> section = Section::makeSection(s, dataVersion);
 		section_idx_t Y = section->getY();
+		if (sections.empty())
+			sectionOffset = Y;
+
+		Y -= sectionOffset;
+		assertValue(Y >= 0 && size_t(Y) >= sections.size());
 		sections.resize(Y);
 		sections.push_back(std::move(section));
 	}
 }
 
 uint8_t Chunk::getBiome(block_idx_t x, y_idx_t y, block_idx_t z) const {
-	if (x > SIZE || y > MAXY || z > SIZE)
-		throw std::invalid_argument("corrupt chunk data");
+	if (x >= SIZE || z >= SIZE)
+		throw std::invalid_argument("getBiome: invalid block coordinate");
 
 	switch (biomeFormat) {
 	case INT_ARRAY:
+		if (y < 0 || y >= MAXY)
+			break;
 		return biomeInts->getValue((y>>BSHIFT)*BSIZE*BSIZE + (z>>BSHIFT)*BSIZE + (x>>BSHIFT));
 	case INT_ARRAY_PRE1_15:
 		return biomeInts->getValue(z*SIZE + x);
 	case BYTE_ARRAY:
 		return biomeBytes->getValue(z*SIZE + x);
 	default:
-		return 0xff;
+		break;
 	}
+
+	return 0xff;
 }
 
 Block Chunk::getBlock(block_idx_t x, Chunk::Height height, block_idx_t z) const {
@@ -74,16 +91,19 @@ Block Chunk::getBlock(block_idx_t x, Chunk::Height height, block_idx_t z) const
 
 	block.depth = height.depth;
 
-	section_idx_t Y = height.y >> HSHIFT;
+	if (height.y == y_idx_min)
+		return block;
+
+	section_idx_t Y = (height.y >> HSHIFT) - sectionOffset;
 	block_idx_t y = height.y & HMASK;
 
-	if (Y < sections.size() && sections[Y])
+	if (Y >= 0 && size_t(Y) < sections.size() && sections[Y])
 		block.type = sections[Y]->getBlockStateAt(x, y, z);
 
-	section_idx_t Yt = (height.y + 1) >> HSHIFT;
+	section_idx_t Yt = ((height.y + 1) >> HSHIFT) - sectionOffset;
 	block_idx_t yt = (height.y + 1) & HMASK;
 
-	if (Yt < sections.size() && sections[Yt])
+	if (Yt >= 0 && size_t(Yt) < sections.size() && sections[Yt])
 		block.blockLight = sections[Yt]->getBlockLightAt(x, yt, z);
 
 	return block;
@@ -93,17 +113,17 @@ bool Chunk::getHeight(
 	Chunk::Height *height, const Section *section,
 	block_idx_t x, block_idx_t y, block_idx_t z, int flags
 ) const {
-	if (height->depth > 0)
+	if (height->depth > y_idx_min)
 		return false;
 
-	if (!(flags & WITH_DEPTH) && height->y > 0)
+	if (!(flags & WITH_DEPTH) && height->y > y_idx_min)
 		return false;
 
 	const Resource::BlockType *type = section->getBlockStateAt(x, y, z);
 	if (!type || !(type->flags & BLOCK_OPAQUE))
 		return false;
 
-	if (height->y == 0)
+	if (height->y == y_idx_min)
 		height->y = (section->getY() << HSHIFT) + y;
 
 	if (!(flags & WITH_DEPTH))
@@ -119,9 +139,14 @@ bool Chunk::getHeight(
 
 Chunk::Heightmap Chunk::getTopLayer(int flags) const {
 	uint32_t done = 0;
-	Heightmap ret = {};
+	Heightmap ret;
 
-	for (int16_t Y = sections.size() - 1; Y >= 0; Y--) {
+	for (block_idx_t z = 0; z < SIZE; z++) {
+		for (block_idx_t x = 0; x < SIZE; x++)
+			ret.v[x][z] = Height { .y = y_idx_min, .depth = y_idx_min };
+	}
+
+	for (section_idx_t Y = sections.size() - 1; Y >= 0; Y--) {
 		if (done == SIZE*SIZE)
 			break;
 
diff --git a/src/World/Chunk.hpp b/src/World/Chunk.hpp
index 1a59efe..34a5184 100644
--- a/src/World/Chunk.hpp
+++ b/src/World/Chunk.hpp
@@ -26,7 +26,7 @@ class Chunk {
 public:
 	// Number of blocks in a chunk in x/z dimensions
 	static const uint32_t SIZE = Section::SIZE;
-	// Maximum Y value
+	// Maximum Y value for pre-1.18 chunks
 	static const y_idx_t MAXY = 256;
 
 	// Shift to get from height to section index
@@ -55,6 +55,7 @@ public:
 	};
 
 private:
+	section_idx_t sectionOffset;
 	std::vector<std::unique_ptr<Section>> sections;
 
 	enum BiomeFormat {
@@ -62,6 +63,7 @@ private:
 		BYTE_ARRAY,
 		INT_ARRAY_PRE1_15,
 		INT_ARRAY,
+		SECTION,
 	} biomeFormat = UNKNOWN;
 
 	std::shared_ptr<const NBT::ByteArrayTag> biomeBytes;
@@ -73,9 +75,9 @@ private:
 	) const;
 
 	const Resource::BlockType * getBlockStateAt(block_idx_t x, y_idx_t y, block_idx_t z) const {
-		section_idx_t Y = y >> HSHIFT;
+		section_idx_t Y = (y >> HSHIFT) - sectionOffset;
 
-		if (Y >= sections.size() || !sections[Y])
+		if (Y < 0 || size_t(Y) >= sections.size() || !sections[Y])
 			return nullptr;
 
 		return sections[Y]->getBlockStateAt(x, y & HMASK, z);
diff --git a/src/World/Section.cpp b/src/World/Section.cpp
index 948fee3..bc10f96 100644
--- a/src/World/Section.cpp
+++ b/src/World/Section.cpp
@@ -16,7 +16,7 @@ namespace MinedMap {
 namespace World {
 
 Section::Section(const std::shared_ptr<const NBT::CompoundTag> &section) {
-	Y = assertValue(section->get<NBT::ByteTag>("Y"))->getValue();
+	Y = int8_t(assertValue(section->get<NBT::ByteTag>("Y"))->getValue());
 	blockLight = section->get<NBT::ByteArrayTag>("BlockLight");
 }
 
@@ -25,18 +25,32 @@ const Resource::BlockType * Section::getBlockStateAt(block_idx_t, block_idx_t, b
 }
 
 std::unique_ptr<Section> Section::makeSection(const std::shared_ptr<const NBT::CompoundTag> &section, uint32_t dataVersion) {
-	std::shared_ptr<const NBT::LongArrayTag> blockStates = section->get<NBT::LongArrayTag>("BlockStates");
-	if (blockStates) {
-		const std::shared_ptr<const NBT::ListTag> palette = assertValue(section->get<NBT::ListTag>("Palette"));
+	{
+		const std::shared_ptr<const NBT::CompoundTag> blockStates = section->get<NBT::CompoundTag>("block_states");
+		if (blockStates) {
+			const std::shared_ptr<const NBT::ListTag> palette = assertValue(blockStates->get<NBT::ListTag>("palette"));
+			std::shared_ptr<const NBT::LongArrayTag> data = blockStates->get<NBT::LongArrayTag>("data");
 
-		return std::unique_ptr<Section>(new PaletteSection(section, std::move(blockStates), palette, dataVersion));
+			return std::unique_ptr<Section>(new PaletteSection(section, std::move(data), palette, dataVersion));
+		}
 	}
 
-	std::shared_ptr<const NBT::ByteArrayTag> blocks = section->get<NBT::ByteArrayTag>("Blocks");
-	if (blocks) {
-		std::shared_ptr<const NBT::ByteArrayTag> data = assertValue(section->get<NBT::ByteArrayTag>("Data"));
+	{
+		std::shared_ptr<const NBT::LongArrayTag> blockStates = section->get<NBT::LongArrayTag>("BlockStates");
+		if (blockStates) {
+			const std::shared_ptr<const NBT::ListTag> palette = assertValue(section->get<NBT::ListTag>("Palette"));
 
-		return std::unique_ptr<Section>(new LegacySection(section, std::move(blocks), std::move(data)));
+			return std::unique_ptr<Section>(new PaletteSection(section, std::move(blockStates), palette, dataVersion));
+		}
+	}
+
+	{
+		std::shared_ptr<const NBT::ByteArrayTag> blocks = section->get<NBT::ByteArrayTag>("Blocks");
+		if (blocks) {
+			std::shared_ptr<const NBT::ByteArrayTag> data = assertValue(section->get<NBT::ByteArrayTag>("Data"));
+
+			return std::unique_ptr<Section>(new LegacySection(section, std::move(blocks), std::move(data)));
+		}
 	}
 
 	return std::unique_ptr<Section>(new Section(section));
@@ -81,7 +95,7 @@ PaletteSection::PaletteSection(
 		expectedLength = (4096 + blocksPerWord - 1) / blocksPerWord;
 	}
 
-	if (blockStates->getLength() != expectedLength)
+	if (blockStates && blockStates->getLength() != expectedLength)
 		throw std::invalid_argument("corrupt section data");
 
 	for (const auto &entry : *paletteData) {
@@ -97,6 +111,9 @@ PaletteSection::PaletteSection(
 }
 
 const Resource::BlockType * PaletteSection::getBlockStateAt(block_idx_t x, block_idx_t y, block_idx_t z) const {
+	if (!blockStates)
+		return palette.at(0);
+
 	size_t index = getIndex(x, y, z);
 	size_t bitIndex;
 
diff --git a/src/regiondump.cpp b/src/regiondump.cpp
index a091536..62fe359 100644
--- a/src/regiondump.cpp
+++ b/src/regiondump.cpp
@@ -24,7 +24,7 @@ int main(int argc, char *argv[]) {
 	}
 
 	World::Region::visitChunks(argv[1], [&] (chunk_idx_t X, chunk_idx_t Z, const World::ChunkData *chunk) {
-		std::cout << "Chunk(" << X << ", " << Z << "): "
+		std::cout << "Chunk(" << unsigned(X) << ", " << unsigned(Z) << "): "
 			<< *chunk->getRoot() << std::endl;
 	});