Underground City -- Derinkuyu (Cappadocia, Turkey)

"It is believed that these underground cities were enlarged during early Christians times to provide refuge from invaders and persecutors. One of the largest of about forty such cities, called Derinkuyu, was found accidentally in 1963...Derinkuyu is believed to have held up to twenty thousand people and could be as many as eighteen to twenty stories deep. Much of this underground city has not been explored due to cave-ins. " -http://www.rdricketts.com/uncity.html