This is the Derinkuyu multi-leveled subterranean ancient city in the Derinkuyu district of Nevşehir Province, Turkey (Ancient Cappadocia region). The tunnel complex was discovered by a resident in 1963 when he found a mysterious room behind his home wall. According to a theory, the caves may first have been built by the Phrygians, an ancient Indo-European people native to central Anatolia (Turkey), in the 7th-8th centuries B.C. In the Cappadocian region of central Turkey, ancient underground cities is not a completely rare phenomenon; Derinkuyu, for example, is connected to the underground city of Kaymakli via an 8 kilometer tunnel. It is estimated that, during its prime, Derinkuyu could house up to 20,000 people, and reached as deep as 60 metres.