Chunk

A chunk is a 16x128x16 volume of blocks. Usually these are compressed with the deflate algorithm.

1. NBT
2. Block ordering

NBT

Inside of an uncompressed chunk file are only a few NBT Tags

Field	Type	Description
Entities	List	All entities in this chunk
TileEntities	List	All tile entities in this chunk, such as Mob Spawners, Chests or Signs
LastUpdate	Long	The last tick value for when the chunk was updated
TerrainPopulated	Byte	If the chunk is populated with folliage
xPos	Integer	The Chunk-Space X Coordiante of the Chunk
zPos	Intger	The Chunk-Space Z Coordinate of the Chunk
BlockLight	Byte Array	The Block light values of the chunk
Blocks	Byte Array	The Block values of the chunk
Data	Byte Array	The Metadata/damage values of the chunk
HeightMap	Byte Array	A top-down heightmap of the chunk
SkyLight	Byte Array	The sky light values of the chunk

Block ordering

Blocks are stored as vertical columns (Y-Axis). To iterate over a stored Byte Array from a Chunk the ordering is as follows.

for (int x = 0; x < 16; x++) {
    for (int z = 0; z < 16; z++) {
        for (int y = 0; y < 128; y++) {
            // Y is in Z which is in X
            block[y + (z*128) + (x*128*16)];
        }
    }
}