Located more than 85 meters below ground and comprising 18 floors, Derinkuyu is the world’s largest underground city ever discovered.

Beneath the cobblestone streets of the city of Derinkuyu lies a network of ancient tunnels and chambers that could have housed 20,000 people, according to IFL Science . The labyrinthine complex beneath Derinkuyu is one of a series of underground cities around the Nevşehir province in central Turkey that are listed as UNESCO World Heritage Sites. A similar complex has been discovered in nearby Cappadocia, 30 kilometers to the north.

The Derinkuyu Underground City was rediscovered in 1963 when a local man kept losing his chickens. After noticing the chickens falling through a crack in his house while he was renovating, the owner found a dark pᴀssageway leading to the complex. Hundreds of houses in the area have similar hidden pᴀssageways leading to the underground metropolis.

Interspersed among the nooks and crannies of the complex are rooms that served as meeting places, stables, kitchens, storage rooms, and small prisons. The structure even had an efficient ventilation system that allowed fresh air to circulate deep within the labyrinthine structure.

Throughout its long history, the underground city of Derinkuyu served primarily as a refuge during war or times of hardship. The city’s age is disputed among scholars, but some estimates place it as old as 2,800 years. It is likely that the underground tunnels were used by people in the Achaemenid Empire, founded by Cyrus the Great in 550 BC. The underground city was later used as a refuge by Arabs during the Arab-Byzantine War between 780 and 1180.