Minecraft wiki entry for Server_level.dat, associated text: After un-gzipping the datafile (the player may simply decompress the file itself using a tool, which can decompress gzipped files), sequentially the datafile consists of the number 656127880 as 32-bit integer (0x27 0x1B 0xB7 0x88 in HEX), followed by the number 2 as a byte (0x02 in HEX), then followed by serialized Level Java classfile instance. The level's block values (material type, such as stone) are stored inside of a byte array inside of this class.