While walking in the West Virginia woods in 1978, Dr. Robert Conte saw something that made him stop in his tracks: an aluminum door between green cement walls. With an ominous feeling, he quickly turned around and walked straight back to The Greenbrier, where he had just been hired as the hotel’s historian.

Dr. Conte had heard the rumors—everyone working at the luxury resort had—about an underground bunker built by the government during the Cold War that was said to exist somewhere on the property. “It didn’t take long around here to figure out that you didn’t want to bring the subject of the bunker up—it wasn’t a good career move,” says Conte.

Exposed in a Washington Post story that broke in 1992, the long-standing secret of the 120,000-square-foot “Government Relocation Facility” built for members of the U.S. Congress was out. Hidden in plain sight, part of it was even in continual use as a convention center and theater for Greenbrier guests. Gaudy wallpaper hid iron doors that could withstand a 30-ton blast. “I walk in and out of that space all of the time,” says Dr. Conte. “Now it’s obvious it was there.” Today, more than 33,000 people visit annually.

Underground exploration is part adventure, part history, and part plain curiosity. “Often, it’s like discovering something that’s slipped through time to the present day. It brings history to life,” says Steve Duncan, an urban explorer whose work takes him underground to the abandoned subway tunnels, empty tombs, and unused aqueducts in New York City, and who was part of the Discovery Channel’s short-lived television series “Urban Explorers.” Over the past decade, Duncan says he’s seen a real growth in the public’s desire to investigate what’s below the surface. “Part of what makes it exciting is that there’s always more to dig up.”