Early Christians used the underground cities in Cappadocia which include Derinkuyu to hide from Romans. They claim to build and expanding the complex and provide the presiding of Saint Gregory and Saint Basil during the 4th century AD.
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The Derinkuyu underground city is large enough to accommodate 20,000 people at one time. It is well ventilated with more than 1500 ducts that provided the inhabitants with fresh air despite being 60m under the ground. The city also had well-dug wells to provide fresh water and no well was interconnected to keep them from being poisoned from the above by enemies. A large 55-meter (180-foot) ventilation shaft was used as a well also to give water to those below and above.
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Many experts who have explored the tunnels of the Derinkuyu underground city opine that there is a tunnel that extends and connects it to its sister underground city of Kaymakli, which is situated 3 miles away.
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There are many other underground cities in the world which are similar to Derinkuyu and Kaymakli. People of Moosejaw, Saskatchewan in Canada built a good network of tunnels and residences to hide the Chinese immigrants from jail in the 20th century. The Village troglodytique de Barry built an underground residential complex that dated back to 6th century AD.
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