server_level.dat is the name of the file used by the Minecraft Classic server for loading and saving the in-game map. The file can be backed up to save content which helps to protect constructions against griefers or to use the file for map editing.
When uncompressed, the format of the file is as follows:
|0||4||Magic ID||A magic ID is a constant number used to identify the Minecraft file format. The current value is 0x271bb788.|
|4||1||Version Number||The version number represents the current format used to save the level. The current value is 2.|
|5||Variable||Serialized Java com.mojang.minecraft.level.Level Class||More information about the serialization format used by Java is available in the manual, however, the easiest way to edit the file is to use the classes already provided with the official .|