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 so far, though it is estimated to extend to 85 meters total. The underground city was used by up to 10,000 people at a time to take shelter from frequent invasions of the region. It included living quarters, stables, dining rooms, a church, kitchens, warehouses, and more, with an underground river and ventilation system.