Minecraft wiki entry for Server_level.dat, associated text: The first 65536 sequential bytes of the array make up the top-most 256×256 "sliver" of the level, with the north-most row of bytes, from left to right, being at locations 0 ... 255, the row below that at, from left to right, locations 256 ... 511, and so on. The default maps are 64 "slivers" deep.