A team of NASA scientists made an astonishing discovery while using a high-tech, airborne radar system to probe Greenland’s massive ice sheet — an abandoned U.S. military base hidden 100 feet below the surface.

“We were looking for the bed of the ice and out pops Camp Century,” said Alex Gardner, a cryospheric scientist at NASA’s Jet Propulsion Laboratory, who helped lead the project. “We didn’t know what it was at first.”

Known as the “city under the ice,” Camp Century is a Cold War relic that was built in 1959 by the Army Corps of Engineers and became buried under snow and ice after it was abandoned in 1967, according to NASA.