Derinkuyu Underground City

Dipping some 200 feet under the streets of Derinkuyu, Turkey, exists another city, big enough to support a population — at around 20,000 — far greater than its aboveground contemporary. No one knows for sure who wielded 7th and 8th century tools to create this multi-level metropolis (was it the Phrygians? The Persians?) But these mystery people left behind a shelter with all the amenities that you might find above ground, for use in times of conflict or natural disasters. Discovered in the 1960s, the underground city of Derinkuyu houses remnants of shops, cellars, areas for livestock and food storage — even access to fresh water. [Source]