Aydıntepe underground City

Discovered by coincidence during excavation works for a building construction in 1988,[3] -to some sources in 1998–[4] the underground city at Aydıntepe features chambers, halls and hallways carved out of volcanic tufa rock formation without using building materials.[2] It is situated 2–5 m (6.6–16.4 ft) under the surface. The chambers and the halls are connected with 1 m (3.3 ft)-wide and 2 m (6.6 ft)-high hallways stretching over about 1 km (0.62 mi) in length. There are eight conical shafts over the hallways, which are believed to be for observation or ventilation purposes.[2][5][6] Some original entrances and structures are now inaccessible because of collapse or later construction.[7]

A round rock piece of 1.5 m (4.9 ft) diameter stands as a door at the entrance to close the underground city from the outer world when needed. The tombs above the underground city and the wall figures inside it point out to a history of three millennia including the Late Roman or early Byzantine era.[3][8] It was used by Christians as well as by Muslims. Various people sheltered in the city at different times, hiding from religious persecution or war.[8] Inside the underground city, there are cellars, storage rooms, water sources, and a pool.[2] The city was well-fortified while still in use; it once housed a closing device and guard chamber.[7]

The site was archaeologically researched, and was put under protection by the Cultural and Natural Heritage Preservation Board in 2008. About 850 m (2,790 ft) long part of the underground city covering an area of 1,200 m² (13,000 sq ft) is open to the public for visit.[2][3]

• Aydıntepe underground city at YouTube