Through a basement door in southeastern Turkey lies a sprawling underground city -- perhaps the country's largest -- which one historian believes dates back to the ninth century before Jesus Christ.

Archaeologists stumbled upon the city-under-a-city "almost by chance" after an excavation of house cellars in Midyat, near the Syrian border, led to the discovery of a vast labyrinth of caves in 2020.

Workers have already cleared more than 50 subterranean rooms, all connected by 120 metres (131 yards) of tunnel carved out of the rock.

But that is only a fraction of the site's estimated 900,000-square-metre area, which would make it the largest underground city in Turkey's southern Anatolia region.

"Maybe even in the world," said Midyat conservation director Mervan Yavuz who oversaw the excavation.

"To protect themselves from the climate, enemies, predators and diseases, people took refuge in these caves which they turned into an actual city," Yavuz added.

The art historian traces the city's ancient beginnings to the reign of King Ashurnasirpal........

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