Derinkyu was built in the 8th-7th centuries B.C. and could accommodate 20,000 to 30,000 people. It consisted of 13 levels, more than enough to keep you busy exploring.
The city above
Kaymakli is another underground city similar to Derinkuyu. It's probably older and is more spread out than Derinkuyu, which is built deeper. People in the area still use some of the rooms as stables, cellars, etc.
Entrances dotting the hillside.
I couldn't put this post together without mentioning Petra. Established by the Nabataens around 300 B.C., the city grew rich on the spice trade by charging each caravan for safe conduct through their lands.
Entrance to the city.
View from a distance