Underground City, Derinkuyu, Nevşehir Province, Turkey

Features
One of the heavy stone doors. They have a height of 1–1,5 m, 30–50 cm in width and weigh 200–500 kg. The hole in the centre can be used to open or close the millstone, or to see who is outside.
The underground city at Derinkuyu could be closed from the inside with large stone doors. Each floor could be closed off separately.