Derinkuyu Underground City is an ancient multi-level underground city in the Derinkuyu district in Nevşehir Province, Turkey. With its eleven floors extending to a depth of approximately 85 m, it was large enough to shelter tens of thousands of people together with their livestock and food stores. It is the largest excavated underground city in Turkey and is part of a network of several underground complexes found across Cappadocia.
It was opened to visitors in 1969 and to date, only ten percent of the underground city is accessible to tourists.
Additional Underground City info: http://en.wikipedia.org/wiki/Underground_city