An unearthly sort of dungeon crawl, with weird magic running all about. A rather labyrinthine dungeon, as even the most justified and world-built dungeon tends to be.
So are they going to call it a dungeon or a labyrinth? Hmmm -- decisions, decisions.
Meanwhile outlining their discussions about the monsters, and lack of ecosystem, and how the lights are needed for the party but not the monsters, and how the doors shut for the party but not the monsters. . . a real sort of underworld dungeon.