Dungeon Ideas from Rpg.net

* Libraries
* Lunatic asylums, hospitals and sanitoria
* Prisons
* Fortresses, military installations, docks, hangars
* Places to keep very, very valuable things - Like museums
* Opera houses
* Gigantic war-machines
* A siege-world/land where vast fortifications, earthworks and trenches cover the area where an Iliad-style generational siege once took place, now abandoned and infested with fell creatures.
* City sprawl submerged in flood and fire; current city sits on top ( in places ) two to three diff levels of ancient buildings. 2,000 year old aquaducts still run, cutting and weaving through a plan centuries old. Would be very easy to envision flooded submrged aquaducts pushing out debris and dirt in such an environ, clearing out blocks of ancient ruined city through abandoned sewers. A perfect home for cultists and what ever evils they summoned, deep beneath the city above?
* Crashed/Sunked Ship
* A previous civilization's holdfast/retreats, built in anticipation of an apocalyptic invasion of monsters/demons
* The catacombs of Paris, before they were turned into giant ossuaries, were limestone quarries - the stone that was used to build parts of Paris was quarried far beneath its foundations.