In Turkey, there are ancient underground cities, the largest of which discovered to date is 18 stories deep, extends 280 feet beneath the surface, and could have held 30,000 people. BLDGBLOG writes:

Manhattan will be gone, Los Angeles gone, Cape Canaveral flooded and covered with seaweed, London dissolving into post-Britannic muck, the Great Wall of China merely an undetectable line of minerals blowing across an abandoned landscape – but there, beneath the porous surface of Turkey, carved directly into tuff, there will still be underground cities.

There is an MIT Press book about this: Caves of God: The Monastic Environment of Byzantine Cappadocia, by Spiro Kostof (sample chapter PDF available with some gorgeous black and white illustrations that could make wonderful player handouts). Definitely something to check out from the library. I hope it has maps.