NASA performed a scientific mission earlier this year, surveying Greenland's massive ice sheet to test the limits and capabilities of its Uninhabited Aerial Vehicle Synthetic Aperture Radar technology. The agency didn't expect to "see" anything interesting down there, yet they rediscovered a long-abandoned base built by the US during the Cold War.

Source: https://www.techspot.com/news/105763-nasa-got-new-view-secret-cold-war-base.html