Derinkuyu could house up to 20,000 people.

Derinkuyu cave, Cappadocia, Turkey
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For hundreds of years, when invaders came to the Cappadocia region of what is now Turkey, inhabitants would retreat to a vast underground city, taking with them their belongings — including large livestock — seal up the entry with large stones, and live there until the threat passed.

The underground haven of Derinkuyu was built in the Byzantine era in 780-1180 AD, and it could likely accommodate up to 20,000 people and their animals, the Independent reported. But there are other reports that claim the city could have been built as early as the 8th century B.C.

Discovered in the late 1960s, this unusual site is open to visitors who can explore nearly half of all of its vast underground passages. Its discovery is straight from a newspaper headline: A local man stumbled upon the city when he knocked down a wall in his basement. The 18-story city includes kitchens, stables, churches, and tombs, all carved into the soft volcanic rock that covers much of the Nevsehir region of Turkey, according to the Independent.

Turkey underground cave
Credit: Photo by Ozge Elif Kizil/Anadolu Agency/Getty Images

The caves remained in use up until the 1920s, when persecuted Christians took refuge following a population exchange with Greece.

Tourists who have visited this wonder have written hundreds of rave reviews on Trip Advisor, giving it an average of 4.5 stars out of 5. But claustrophobes beware: Many reviewers noted that those unaccustomed to small spaces might find the experience uncomfortable, as many of the passageways are quite narrow.

Derinkuyu Underground City, Turkey
Credit: Images

“Such a wonderful experience to experience how the ancient people temporary lived in this underground city,” wrote one reviewer, with many others describing it as a “must see.”

Derinkuyu is not the only underground city in this area of Turkey. Construction workers clearing for a real estate development discovered an even more ancient underground city in 2015 that may be up to 5,000 years-old. Little is known about this new city, as it is still being excavated, but it contains waterways and churches, according to an ABC News report.