Attribution: Nevit Dilmen (talk), with CC BY-SA 3.0 license, original file:Derinkuyu_Underground_City_9831_Nevit_Enhancer.jpg , Wikipedia
Ventilation well in Derinkuyu Underground City in Cappadocia, Turkey. Christians fled the enemies and hid in this underground cities. All digging starts with a ventilation well and afterwards city expands horizontally.