NASA scientists have discovered the long-buried remains of Camp Century, a secret U.S. military base from the Cold War, hidden 100 feet below Greenland’s ice sheet.

Originally presented as a research station, the base was a covert front for Project Iceworm, a plan to deploy nuclear missiles under the Arctic.

“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 (JPL), who helped lead the project. “We didn’t know what it was at first.”

Built in secret between June of 1959 and October of 1960 by the U.S. Army Corps of Engineers, Camp Century—also known as “the city under the ice”—was comprised of 21 underground tunnels spanning 9,800 feet, according to Interesting Engineering.

Popular Mechanics report: In radar images of the site, Greene said, many of the base’s individual structures are clearly discernible. To study the base, NASA used Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR), a technology similar to LiDAR that is commonly used in searching for hidden structures like Maya ruins. The difference is that where LiDAR uses laser light, UAVSAR uses radio waves.

The U.S. and Denmark signed the Defense of Greenland agreement in 1951 “to negotiate arrangements under which armed forces of the parties to the North Atlantic Treaty Organization may make use of facilities in Greenland in defense of Greenland and the rest of the North Atlantic Treaty area,” according to the National Museum of Nuclear Science and History. (Greenland was, at the time of the agreement, a county of Denmark). This allowed the U.S. to build bases in Greenland.

Even without the -70 degree temperatures and 125-mile-per-hour winds that were possible on the ice shelf, the construction of Camp Century sounds like a total nightmare. The camp was built from 6,000 tons of material, transported via heavy bobsleds that topped out at two miles per hour. Materials were shipped to Thule, another U.S. base (this one above ice)—on the sleds, that trek was a 70-hour trip.

Army engineers first dug trenches—the longest of which was a 1,000-foot-long passageway called Main Street—deep in the snow and ice before wooden buildings and steel roofs built out Camp Century.

The crown jewel of the base was one of the first PM-2 medium-power nuclear reactors that, in the freezing conditions, had to be treated with the utmost care in order to power the site.

While operating at the base, scientists made major geological breakthroughs. They were some of the first to study ice cores, and soil from Greenland itself revealed an ancient history of verdant forestry and diverse wildlife.

But that research was just a cover-up.

Camp Century itself was not a secret. Its establishment was known, and the Army even made a promotional video for the project. The scientific research angle, as significant as the discoveries were, was merely a front for a major U.S. nuclear weapon strategy of which the Danish government wasn’t even aware.

Known as “Project Iceworm,” the plan was for Camp Century to house ballistic missiles under the Greenland ice. An additional 52,000 square miles of tunnels were planned—enough to fit 600 missiles. It would require 60 launch centers to be built, and would be manned by 11,000 soldiers living full-time in the city under the ice.

If that sounds impossible on a number of levels to you, you’re not alone. Project Iceworm never came to be because of a litany of obstacles, almost all of them boiling down to some version of “well, this simply isn’t feasible.” By 1967, Camp Century was decommissioned and abandoned—a frigid fossil of the U.S.’s Cold War efforts. The potential nuclear weapon plan was publicized by the Danish Institute of International Affairs in 1997.

So, now the remains sit alone, buried by even more ice and snow in the 57 years since the closure of the base and giving its ‘Camp Century’ name even more significance. Ultimately, it’s just a harmless footnote in U.S. military history. Right?

Well, not exactly. As is U.S. tradition during international ventures, there were some residual drawbacks. Remember that nuclear reactor that was lugged piece by piece below the ice, only to operate for 33 months? Well, during that time, it produced over 47,000 gallons of nuclear waste, according to the National Museum of Nuclear Science and History. When the base shut down, they removed the reactor, but not the waste, which is still lying under the ice—ice that is in serious peril with a warming climate. Right now, it’s just sitting there, frozen in time. But a study by experts has projected that the base could begin losing ice by 2090.

“They thought it would never be exposed,” William Colgan, a climate and glacier scientist at Toronto’s York University and the leader of the study, told The Guardian in 2016. “Back then, in the ‘60s, the term global warming had not even been coined. But the climate is changing, and the question now is whether what’s down there is going to stay down there.”