|Image from Wikipedia.
The Catacombs of Paris are another example of a real-world labyrinth (I first came across them here). Many levels deep, and (supposedly) millions of dead interred. Maps derived from real structures, in my opinion, have a sense of verisimilitude that is hard to get from purely invented maps (though maybe that is at least partially a placebo effect). See also this post at Aaron Steele’s blog on St. Paul’s Catacombs.
Also mentioned on Dragonsfoot. Map here (very atmospheric, but not really ready for RPG prime-time). (Thank you, user Grim, for that link.) Perhaps usable as a player handout, if one were to create a separate referee map (though the French might take players out of the game if this were used in a fantasy setting).