The Derinkuyu underground city is an ancient multi-level underground city in the Derinkuyu district in Nevsehir Province, Turkey. Derinkuyu was fully formed in the Byzantine era, when it was heavily used as protection from Muslim Arabs during the Arab-Byzantine wars. It can be closed from the inside with large stone doors. Each floor could be closed off separately. The city could accommodate up to 20,000 people and had all the usual amenities found in other underground complexes across Cappadocia, such as wine and oil presses, stables, cellars, storage rooms, refectories, and chapels. These tunnels were rediscovered in 1963, after a resident of the area found a mysterious room behind a wall in his home. Further digging revealed access to the tunnel network. Continue reading for a video, more pictures and additional information.