Thirty six underground cities have been discovered in Cappadocia so far. Some are huge complexes going down eight levels and are large enough for several thousand inhabitants. The original caves were probably cut for shelter and storage or for refuge in times of danger and enlarged over the generations.
The Christians developed them further and there are several churches deep underground. They are about 85 metres deep, or until water was reached, and have efficient ventilation systems - even eight floors down the air was still fresh.
Millstone doors were used to block tunnels while the inhabitants escaped. The round doors were wedged shut and could only be opened from one side.