An anonymous Turkish man broke a wall in his basement in 1963, which led him to discover a complex network of tunnels, halls, and chambers. This vast abandoned subterranean city, which was up to 18 stories and 280 feet (76 m) deep– was enough to house 20,000 people. Located in Derinkuyu, Turkey, no one knows exactly why it was built, who was it built for, or when and how it was abandoned. Read more about the possible reasons why here!
image credit: Yasir999, CC BY-SA 4.0