Camp Century, a subterranean military base in Greenland, was spotted under the icy expanse during a flight earlier this year.

As it turns out, this abandoned "secret city" was the site of a secret Cold War project known as Project Iceworm which called for the construction of 2,500 miles (4,023 km) of tunnels that could be used to nuclear intermediate range ballistic missiles (IRBMs) at the Soviet Union.

While flying roughly 150 miles east of northern Greenland’s Pituffik Space Base, the NASA researchers also snapped photos of the polar landscape from the plane’s windows showing a barren, icy expanse. It wasn’t until analyzing the radar images, however, that the team realized what they had flown over without even realizing it.