Derinkuyu, in Turkey, is an ancient underground city dating back 2500 years:

[tblue37]

Derinkuyu, in Turkey, is an ancient underground city dating back 2500 years and extending to a depth of about 60 metres and able to support 20,000 people, and their cattle. Subterranean cities were though to protect inhabitants from invasion, allowing thousands to live in secrecy

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[tblue37]

1. 2nd tweet: more info & another picture--

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The Derinkuyu dungeon is a complex branching system of rooms, halls, tunnels and wells, diverging downwards (covered with bars), upwards and to the sides. The city was built in such a way that it was impossible to capture it. Every precaution was taken.

[Zorro]

2. Kaymakli is another underground city not too far away

There are rumors that Derinkuyu and Kaymakli were connected, although I think no one has found that connecting passage.

I visited Derinkuyu many years ago; one of the locals had turned a few rooms into a discotheque, and no one seemed to mind.

[tblue37]

3. Thanks! I looked for pics and found a bunch. So cool!

[Richard D]

4. I heard . .

. . . that there is at least one other underground city, Possibly more.