NetHack has persistent levels, up until just before you enter the Elemental Planes. When you leave a level, NetHack writes the level to a temporary file on disk (in the playground). These temporarily files also provide the basis to recover a crashed game. When you save your game, NetHack stores the levels in your save file. Persistent levels were an innovation in NetHack's predecessor, Hack. Actually, Jay Fenlason included the ability to write a dungeon level to disk and read it later in the original design of Hack.