Minecraft classic level format
Minecraft classic level format is a data format used in the Minecraft Classic game (not the current Minecraft game). This and other Minecraft formats are extensively documented in the Minecraft Gamepedia wiki.
It consists of Java objects serialized with Java Object Serialization, prepended by a format-specific header, and compressed with gzip. Single-player levels are saved in files with a .mine extension, while the Classic Creative server saves them in a file called server_level.dat.