Those are all fictional examples. But in 1963, that barrier was breached for real. Taking a sledgehammer to a wall in his basement, a man in the Turkish town of Derinkuyu got more home improvement than he bargained for. Behind the wall, he found a tunnel. And that led to more tunnels, eventually connecting a multitude of halls and chambers. It was a huge underground complex, abandoned by its inhabitants and undiscovered until that fateful swing of the hammer.
    
    The anonymous Turk—no report mentions his name—had found a vast subterranean city, up to 18 stories and 280 feet (76 meters) deep and large enough to house 20,000 people. Who built it, and why? When was it abandoned, and by whom? History and geology provide some answers.