An illustration of an underground city like Derinkuyu. Note the church on the bottom level. Cities like this were used during times of Christian persecution, so religious items would be placed on the lowest levels for protection. The elaborate subterranean city was connected via stairways and passages, and even connected to other underground cities through tunnels that stretched for miles. It's thought to have been initially built during the seventh and eighth centuries BCE, and was in continual, frequent use through the 12th century. Based on the church found on the fifth and lowest level, it seems the population was Christian, and probably used the city during wartime. The city was also used as a refuge from the Mongolian invasion in the 1300s and up through the 20th century for Christian people fleeing persecution. It was finally abandoned for good in 1923.