English: In April 2024, NASA scientist Chad Greene flew aboard a Gulfstream III with a team of engineers, monitoring a radar instrument as it probed the Greenland Ice Sheet below. Flying about 150 miles east from Pituffik Space Base in northern Greenland, Greene snapped this photo from the aircraft’s window showing the vast, barren expanse of the ice sheet’s surface. That’s when the radar unexpectedly detected something buried within the ice.

“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.”

Camp Century, also known as the “city under the ice,” is a relic of the Cold War. The U.S. Army Corps of Engineers built the military base in 1959 by cutting a network of tunnels within the near-surface layer of the ice sheet. After it was abandoned in 1967, snow and ice continued to accumulate, and the solid structures associated with the facility now lie at least 30 meters (100 feet) below the surface.

Radar measures distance by sending out radio waves and timing how long it takes for them to reflect back to the sensor. Like an ultrasound for ice sheets, scientists can use radar to map the ice surface, its internal layers, and the bedrock below.

Past airborne surveys that flew over Camp Century have detected signs of the base within the ice. Those flights used conventional ground-penetrating radar, which points straight down and produces a 2D profile of the ice sheet. In that view (map below), Camp Century’s solid structures appear like a blip in the deformed layers of ice.

The April 2024 flights, however, had NASA’s UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar) mounted to the belly of the aircraft. The system looks downward and toward the side, producing maps with more dimensionality.

“In the new data, individual structures in the secret city are visible in a way that they’ve never been seen before,” said Greene, also a cryospheric scientist at JPL. Comparing the new radar map of Camp Century (shown at the top of this page) with historical maps of the base’s planned layout, the parallel structures appear to align with the tunnels built to house an array of facilities.

The added dimension also means interpreting the images can be challenging. For example, the long line that appears “above” the base is the ice bed, which lies at least a mile below the ice sheet’s surface and well below the depth of Camp Century. In this view, the ice bed appears over the base because the radar return is showing part of the ice bed that is far in the distance.

Scientists have used maps acquired with conventional radar to corroborate estimates of Camp Century’s depth—part of an effort to estimate when melting and thinning of the ice sheet could re-expose the camp and any remaining biological, chemical, and radioactive waste that was buried along with it. The scientific utility of the new UAVSAR image of Camp Century remains to be seen; for now, it remains a novel curiosity acquired by chance.

Greene and Gardner had not intended to capture the image of Camp Century. “Our goal was to calibrate, validate, and understand the capabilities and limitations of UAVSAR for mapping the ice sheet’s internal layers and the ice-bed interface,” Greene said. Ultimately, such instruments are expected to help scientists measure the thickness of ice sheets in similar environments in Antarctica and constrain estimates of future sea level rise.

“Without detailed knowledge of ice thickness, it is impossible to know how the ice sheets will respond to rapidly warming oceans and atmosphere, greatly limiting our ability to project rates of sea level rise,” Gardner said. The test flights that captured Camp Century earlier this year will enable the next generation of mapping campaigns in Greenland, Antarctica, and beyond.