In 1963, a man in Derinkuyu, Turkey discovered a mysterious room behind a wall in his house that led to the accidental discovery of an extensive underground city. Archaeologists have since uncovered 20 levels that could house up to 10,000 people and reach depths of 85 meters. The underground city was used by inhabitants as shelter and protection from frequent invasions, and included living quarters, stables, kitchens, warehouses, and schools connected by tunnels.