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.

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