DeePsea always makes a backup of a map when nodes are built and during a save operation.
The prior file is always named yourfile.BAK where yourfile is anything you named it.
When Building Nodes
In addition, if nodes are built your level is first saved in ~DEEP.WAD.
Select F5 and select how often DeePsea should make additional backups using the .BKP extension!
You can also manually create a .BKP file at any time by pressing Ctrl+F2.