Minecraft wiki entry for Chunk_format, associated text: Each section is a 16×16×16-block area, with up to 16 sections in a chunk : from 0 at the bottom, to 15 on top. Empty sections are not saved. Each section has a "Y" byte for its Y-index (0 to 15), a "Palette" list linking IDs to block states, and a "BlockStates" long array storing the IDs per block location, compressed by fitting multiple IDs inside each entry (see NBT_structure above for details on the compression). There is an additional section at the top and bottom of the world used to store light, so that light travels properly over and under the world limits.