I work in an opera house with catacombs - well, pretty impressive cellars, anyway! It's great, our cellars go on for miles, mostly round in circles under the main auditorium, there are air raid shelters and all sorts of stuff down there. Every so often I go exploring, but I've yet to find an underground lake... we do have an opera-singing ghost though! It's a nightmare trying to take people on tours down there though, I'm tempted to take a ball of string with me, otherwise the odds of getting lost are fairly high...