In 1963, a man in Derinkuyu, Turkey discovered a secret room hidden behind a wall in his home, which led to the discovery of an extensive underground city. Archaeologists have explored down to 40 meters deep, though it is estimated to extend to 85 meters. The underground city was used by up to 10,000 people to shelter from frequent invasions of the region. It had multiple levels connected by stone doors that could be moved to block passages, with supplies to last long periods underground.