An ancient multi-level underground city in the Derinkuyu district in Nevşehir Province, Turkey, extending to a depth of approximately 85 metres (279 ft). It is large enough to have sheltered as many as 20,000 people together with their livestock and food stores. It is the largest excavated underground city in Turkey and is one of several underground complexes found throughout Cappadocia.

It was initially built in about 800 BC, and expanded through the years. It is believe to have 18 subterranean levels.

The video is fascinating!

Derinkuyu underground city