An unused NYC subway station at City Hall (see images of secret cities in the movies referenced in previous post):
From The Straight Dope:
The Mole People: Life in the Tunnels Beneath New York City details Toth's early-90s encounters with several dozen of what she estimated at the time to be 5,000 homeless people living beneath the streets of New York, mostly in subway and railroad tunnels. Particularly large populations inhabit (or inhabited, anyway) the multilevel labyrinths beneath Grand Central and Penn stations. In a few cases, sizable communities have coalesced, some allegedly numbering 200 people or more, complete with "mayors," elaborate social structures, even electricity. Toth describes one enclave deep under Grand Central with showers using hot water from a leaky steam pipe, cooking and laundry facilities, and an exercise room. The community has a teacher, a nurse, and scampering children. "Runners" return frequently to the surface to scavenge food and such, but others--the real "mole people" — routinely go for a week or more without seeing the light of day.
The remains of the underground cities of Cappadocia (Turkey):
The underground resistance in the film “Metropolis”:
The lair of the Phantom of the Opera in the Paris sewers:
