Photo: TobyJ · CC BY-SA 4.0 · via Wikimedia Commons
Mud Flood · Derinkuyu, Turkey
Derinkuyu underground city
The open question
Who carves a city 85 metres down, and why go underground at all?
On the record
A multi-level underground city about 85 m deep, with air shafts laid out to move air. It was forgotten until 1963, when a resident knocked through a basement wall.

