diff --git a/src/entity/Entity.php b/src/entity/Entity.php
index e483b4348..1631667c5 100644
--- a/src/entity/Entity.php
+++ b/src/entity/Entity.php
@@ -635,7 +635,7 @@ abstract class Entity{
 
 		$this->checkBlockIntersections();
 
-		if($this->location->y <= -16 and $this->isAlive()){
+		if($this->location->y <= World::Y_MIN - 16 and $this->isAlive()){
 			$ev = new EntityDamageEvent($this, EntityDamageEvent::CAUSE_VOID, 10);
 			$this->attack($ev);
 			$hasUpdate = true;
diff --git a/src/network/mcpe/serializer/ChunkSerializer.php b/src/network/mcpe/serializer/ChunkSerializer.php
index 6fe392b67..af5336465 100644
--- a/src/network/mcpe/serializer/ChunkSerializer.php
+++ b/src/network/mcpe/serializer/ChunkSerializer.php
@@ -46,8 +46,8 @@ final class ChunkSerializer{
 	 * Chunks are sent in a stack, so every chunk below the top non-empty one must be sent.
 	 */
 	public static function getSubChunkCount(Chunk $chunk) : int{
-		for($count = count($chunk->getSubChunks()); $count > 0; --$count){
-			if($chunk->getSubChunk($count - 1)->isEmptyFast()){
+		for($y = Chunk::MAX_SUBCHUNK_INDEX, $count = count($chunk->getSubChunks()); $y >= Chunk::MIN_SUBCHUNK_INDEX; --$y, --$count){
+			if($chunk->getSubChunk($y)->isEmptyFast()){
 				continue;
 			}
 			return $count;
@@ -59,7 +59,7 @@ final class ChunkSerializer{
 	public static function serializeFullChunk(Chunk $chunk, RuntimeBlockMapping $blockMapper, PacketSerializerContext $encoderContext, ?string $tiles = null) : string{
 		$stream = PacketSerializer::encoder($encoderContext);
 		$subChunkCount = self::getSubChunkCount($chunk);
-		for($y = 0; $y < $subChunkCount; ++$y){
+		for($y = Chunk::MIN_SUBCHUNK_INDEX, $writtenCount = 0; $writtenCount < $subChunkCount; ++$y, ++$writtenCount){
 			self::serializeSubChunk($chunk->getSubChunk($y), $blockMapper, $stream, false);
 		}
 		$stream->put($chunk->getBiomeIdArray());
diff --git a/src/world/World.php b/src/world/World.php
index a06f2ad5e..dcd27805c 100644
--- a/src/world/World.php
+++ b/src/world/World.php
@@ -331,7 +331,7 @@ class World implements ChunkManager{
 	private const BLOCKHASH_Y_OFFSET = self::BLOCKHASH_Y_PADDING - self::Y_MIN;
 	private const BLOCKHASH_Y_MASK = (1 << self::BLOCKHASH_Y_BITS) - 1;
 	private const BLOCKHASH_XZ_MASK = (1 << self::MORTON3D_BIT_SIZE) - 1;
-	private const BLOCKHASH_XZ_EXTRA_BITS = 6;
+	private const BLOCKHASH_XZ_EXTRA_BITS = (self::MORTON3D_BIT_SIZE - self::BLOCKHASH_Y_BITS) >> 1;
 	private const BLOCKHASH_XZ_EXTRA_MASK = (1 << self::BLOCKHASH_XZ_EXTRA_BITS) - 1;
 	private const BLOCKHASH_XZ_SIGN_SHIFT = 64 - self::MORTON3D_BIT_SIZE - self::BLOCKHASH_XZ_EXTRA_BITS;
 	private const BLOCKHASH_X_SHIFT = self::BLOCKHASH_Y_BITS;
diff --git a/src/world/format/Chunk.php b/src/world/format/Chunk.php
index f1b1320ae..8bb5eed9f 100644
--- a/src/world/format/Chunk.php
+++ b/src/world/format/Chunk.php
@@ -35,7 +35,9 @@ class Chunk{
 	public const DIRTY_FLAG_BLOCKS = 1 << 0;
 	public const DIRTY_FLAG_BIOMES = 1 << 3;
 
-	public const MAX_SUBCHUNKS = 16;
+	public const MIN_SUBCHUNK_INDEX = 0;
+	public const MAX_SUBCHUNK_INDEX = 15;
+	public const MAX_SUBCHUNKS = self::MAX_SUBCHUNK_INDEX - self::MIN_SUBCHUNK_INDEX + 1;
 
 	public const EDGE_LENGTH = SubChunk::EDGE_LENGTH;
 	public const COORD_BIT_SIZE = SubChunk::COORD_BIT_SIZE;
@@ -71,10 +73,10 @@ class Chunk{
 		$this->subChunks = new \SplFixedArray(Chunk::MAX_SUBCHUNKS);
 
 		foreach($this->subChunks as $y => $null){
-			$this->subChunks[$y] = $subChunks[$y] ?? new SubChunk(BlockLegacyIds::AIR << Block::INTERNAL_METADATA_BITS, []);
+			$this->subChunks[$y] = $subChunks[$y + self::MIN_SUBCHUNK_INDEX] ?? new SubChunk(BlockLegacyIds::AIR << Block::INTERNAL_METADATA_BITS, []);
 		}
 
-		$val = ($this->subChunks->getSize() * SubChunk::EDGE_LENGTH);
+		$val = (self::MAX_SUBCHUNK_INDEX + 1) * SubChunk::EDGE_LENGTH;
 		$this->heightMap = HeightArray::fill($val); //TODO: what about lazily initializing this?
 		$this->biomeIds = $biomeIds;
 
@@ -118,7 +120,7 @@ class Chunk{
 	 * @return int|null 0-255, or null if there are no blocks in the column
 	 */
 	public function getHighestBlockAt(int $x, int $z) : ?int{
-		for($y = $this->subChunks->count() - 1; $y >= 0; --$y){
+		for($y = self::MAX_SUBCHUNK_INDEX; $y >= self::MIN_SUBCHUNK_INDEX; --$y){
 			$height = $this->getSubChunk($y)->getHighestBlockAt($x, $z);
 			if($height !== null){
 				return $height | ($y << SubChunk::COORD_BIT_SIZE);
@@ -280,21 +282,21 @@ class Chunk{
 	}
 
 	public function getSubChunk(int $y) : SubChunk{
-		if($y < 0 || $y >= $this->subChunks->getSize()){
+		if($y < self::MIN_SUBCHUNK_INDEX || $y > self::MAX_SUBCHUNK_INDEX){
 			throw new \InvalidArgumentException("Invalid subchunk Y coordinate $y");
 		}
-		return $this->subChunks[$y];
+		return $this->subChunks[$y - self::MIN_SUBCHUNK_INDEX];
 	}
 
 	/**
 	 * Sets a subchunk in the chunk index
 	 */
 	public function setSubChunk(int $y, ?SubChunk $subChunk) : void{
-		if($y < 0 or $y >= $this->subChunks->getSize()){
+		if($y < self::MIN_SUBCHUNK_INDEX or $y > self::MAX_SUBCHUNK_INDEX){
 			throw new \InvalidArgumentException("Invalid subchunk Y coordinate $y");
 		}
 
-		$this->subChunks[$y] = $subChunk ?? new SubChunk(BlockLegacyIds::AIR << Block::INTERNAL_METADATA_BITS, []);
+		$this->subChunks[$y - self::MIN_SUBCHUNK_INDEX] = $subChunk ?? new SubChunk(BlockLegacyIds::AIR << Block::INTERNAL_METADATA_BITS, []);
 		$this->setTerrainDirtyFlag(self::DIRTY_FLAG_BLOCKS, true);
 	}
 
@@ -303,7 +305,11 @@ class Chunk{
 	 * @phpstan-return array<int, SubChunk>
 	 */
 	public function getSubChunks() : array{
-		return $this->subChunks->toArray();
+		$result = [];
+		foreach($this->subChunks as $yOffset => $subChunk){
+			$result[$yOffset + self::MIN_SUBCHUNK_INDEX] = $subChunk;
+		}
+		return $result;
 	}
 
 	/**
diff --git a/src/world/format/io/FastChunkSerializer.php b/src/world/format/io/FastChunkSerializer.php
index bb8bec687..c8ab53a73 100644
--- a/src/world/format/io/FastChunkSerializer.php
+++ b/src/world/format/io/FastChunkSerializer.php
@@ -23,6 +23,7 @@ declare(strict_types=1);
 
 namespace pocketmine\world\format\io;
 
+use pocketmine\utils\Binary;
 use pocketmine\utils\BinaryStream;
 use pocketmine\world\format\BiomeArray;
 use pocketmine\world\format\Chunk;
@@ -96,7 +97,7 @@ final class FastChunkSerializer{
 
 		$count = $stream->getByte();
 		for($subCount = 0; $subCount < $count; ++$subCount){
-			$y = $stream->getByte();
+			$y = Binary::signByte($stream->getByte());
 			$airBlockId = $stream->getInt();
 
 			/** @var PalettedBlockArray[] $layers */
diff --git a/src/world/format/io/leveldb/LevelDB.php b/src/world/format/io/leveldb/LevelDB.php
index a96e329b3..3fe2e359c 100644
--- a/src/world/format/io/leveldb/LevelDB.php
+++ b/src/world/format/io/leveldb/LevelDB.php
@@ -266,7 +266,7 @@ class LevelDB extends BaseWorldProvider implements WritableWorldProvider{
 			case 3: //MCPE 1.0
 				$convertedLegacyExtraData = $this->deserializeLegacyExtraData($index, $chunkVersion);
 
-				for($y = 0; $y < Chunk::MAX_SUBCHUNKS; ++$y){
+				for($y = Chunk::MIN_SUBCHUNK_INDEX; $y <= Chunk::MAX_SUBCHUNK_INDEX; ++$y){
 					if(($data = $this->db->get($index . self::TAG_SUBCHUNK_PREFIX . chr($y))) === false){
 						continue;
 					}
diff --git a/src/world/light/SkyLightUpdate.php b/src/world/light/SkyLightUpdate.php
index 42d12be17..2c4214568 100644
--- a/src/world/light/SkyLightUpdate.php
+++ b/src/world/light/SkyLightUpdate.php
@@ -115,11 +115,10 @@ class SkyLightUpdate extends LightUpdate{
 		//have to avoid filling full light for any subchunk that contains a heightmap Y coordinate
 		$highestHeightMapPlusOne = max($chunk->getHeightMapArray()) + 1;
 		$lowestClearSubChunk = ($highestHeightMapPlusOne >> SubChunk::COORD_BIT_SIZE) + (($highestHeightMapPlusOne & SubChunk::COORD_MASK) !== 0 ? 1 : 0);
-		$chunkHeight = count($chunk->getSubChunks());
-		for($y = 0; $y < $lowestClearSubChunk && $y < $chunkHeight; $y++){
+		for($y = Chunk::MIN_SUBCHUNK_INDEX; $y < $lowestClearSubChunk && $y <= Chunk::MAX_SUBCHUNK_INDEX; $y++){
 			$chunk->getSubChunk($y)->setBlockSkyLightArray(LightArray::fill(0));
 		}
-		for($y = $lowestClearSubChunk, $yMax = $chunkHeight; $y < $yMax; $y++){
+		for($y = $lowestClearSubChunk; $y <= Chunk::MAX_SUBCHUNK_INDEX; $y++){
 			$chunk->getSubChunk($y)->setBlockSkyLightArray(LightArray::fill(15));
 		}
 
@@ -130,7 +129,7 @@ class SkyLightUpdate extends LightUpdate{
 		for($x = 0; $x < Chunk::EDGE_LENGTH; ++$x){
 			for($z = 0; $z < Chunk::EDGE_LENGTH; ++$z){
 				$currentHeight = $chunk->getHeightMap($x, $z);
-				$maxAdjacentHeight = 0;
+				$maxAdjacentHeight = World::Y_MIN;
 				if($x !== 0){
 					$maxAdjacentHeight = max($maxAdjacentHeight, $chunk->getHeightMap($x - 1, $z));
 				}
@@ -174,21 +173,21 @@ class SkyLightUpdate extends LightUpdate{
 	 * @phpstan-param \SplFixedArray<bool> $directSkyLightBlockers
 	 */
 	private static function recalculateHeightMap(Chunk $chunk, \SplFixedArray $directSkyLightBlockers) : HeightArray{
-		$maxSubChunkY = count($chunk->getSubChunks()) - 1;
-		for(; $maxSubChunkY >= 0; $maxSubChunkY--){
+		$maxSubChunkY = Chunk::MAX_SUBCHUNK_INDEX;
+		for(; $maxSubChunkY >= Chunk::MIN_SUBCHUNK_INDEX; $maxSubChunkY--){
 			if(!$chunk->getSubChunk($maxSubChunkY)->isEmptyFast()){
 				break;
 			}
 		}
 		$result = HeightArray::fill(World::Y_MIN);
-		if($maxSubChunkY === -1){ //whole column is definitely empty
+		if($maxSubChunkY < Chunk::MIN_SUBCHUNK_INDEX){ //whole column is definitely empty
 			return $result;
 		}
 
 		for($z = 0; $z < Chunk::EDGE_LENGTH; ++$z){
 			for($x = 0; $x < Chunk::EDGE_LENGTH; ++$x){
 				$y = null;
-				for($subChunkY = $maxSubChunkY; $subChunkY >= 0; $subChunkY--){
+				for($subChunkY = $maxSubChunkY; $subChunkY >= Chunk::MIN_SUBCHUNK_INDEX; $subChunkY--){
 					$subHighestBlockY = $chunk->getSubChunk($subChunkY)->getHighestBlockAt($x, $z);
 					if($subHighestBlockY !== null){
 						$y = ($subChunkY * SubChunk::EDGE_LENGTH) + $subHighestBlockY;
diff --git a/src/world/utils/SubChunkExplorer.php b/src/world/utils/SubChunkExplorer.php
index 9d7faeb96..69bdb44e1 100644
--- a/src/world/utils/SubChunkExplorer.php
+++ b/src/world/utils/SubChunkExplorer.php
@@ -68,7 +68,7 @@ class SubChunkExplorer{
 		if($this->currentSubChunk === null or $this->currentY !== $newChunkY){
 			$this->currentY = $newChunkY;
 
-			if($this->currentY < 0 or $this->currentY >= $this->currentChunk->getHeight()){
+			if($this->currentY < Chunk::MIN_SUBCHUNK_INDEX or $this->currentY > Chunk::MAX_SUBCHUNK_INDEX){
 				$this->currentSubChunk = null;
 				return SubChunkExplorerStatus::INVALID;
 			}