The underground city that could hide 20,000 people

In 1963, a man in the town of Derinkuyu in Cappadocia was renovating his house when he knocked through a wall and found a tunnel behind it. The tunnel led to another, and then another. He had stumbled into one of the largest underground cities ever found.

What's down there?

Its levels hold stables, kitchens, wine and oil presses, wells, storage rooms, a chapel and even a school. Over 50 ventilation shafts carry fresh air to the deepest floors. Each level could be sealed with a rolling stone door up to a metre across that could only be moved from the inside.

Who built it, and why?

Cappadocia's rock is volcanic tuff, which is soft enough to carve with hand tools but hardens when exposed to air. Digging probably began around the 8th century BC, possibly by the Phrygians, although historians still argue about that. It was greatly expanded in the Byzantine era, when local people hid underground from raids for weeks at a time.

Derinkuyu opened to visitors in 1969. Only some of its levels are open to the public today, and dozens more underground settlements have been found across the region.