Derinkuyu: The Ancient underground cave city that can house 20,000 people

[Still Waters]

Beneath the cobbled roads of Derinkuyu in Turkey, there lies an ancient warren of tunnels and chambers that could have once accommodated as many as 20,000 people. Burrowing more than 85 meters (280 feet) beneath the Earth's surface and encompassing 18 levels, the Derinkuyu underground city is the world’s largest subterranean city –  or, at least, the largest that's ever been discovered.

Visitors to the town are told that Derinkuyu's underground history was only "rediscovered" in 1963 by a local man who kept losing his chickens. After noticing his poultry slip into a small crevasse during a renovation of their house, they unearthed a dark passageway that led to the complex. It was eventually revealed that hundreds of private houses in the town contained similar hidden passages leading to the subterrestrial settlement.

https://www.iflscience.com/derinkuyu-the-ancient-underground-cave-city-that-can-house-20000-people-69287