I once had a dungeon modelled after that underwater city from Star Wars episode 1 (yuck!). The idea was that the PCs were acting diplomats for a local city state (of which one player was the elected mayor, it's complicated), and the underwater bubble city was constructed by extra-planar gnomes and then sent through a portal to the prime material some few hundred years ago. When the players arrived large parts of it had been left uninhabited by the dwindling population. Those parts made for great dungeons, with corridors linking magical bubble rooms with hostile aquatic monsters and old artefacts.