In 1963, a man in Derinkuyu, Turkey discovered a secret room behind a wall in his house, uncovering an underground city that had been hidden. Archaeologists have since discovered that Derinkuyu extends over 20 levels and 40 meters deep, though it may extend as far as 85 meters. During invasions, thousands of people lived in the underground city to find shelter, including early Christians escaping persecution. The city included living spaces, storage areas, and a ventilation system that could support an estimated 10,000 people.