When it first appeared in their radar images, NASA scientist Chad Greene and his team of engineers weren’t sure what they were seeing.

Flying above northern Greenland in a Gulfstream III in April of this year, Greene and his crew were monitoring radar information collected from the ice sheet below when, about 150 miles east of Pituffik Space Base—formerly Thule Air Base and still the northernmost installation operated by the U.S. Armed Forces—they spotted something unexpected.

The aircraft’s radar system had detected some kind of structure buried beneath the ice.

“We didn’t know what it was at first,” recalled cryospheric scientist Alex Gardner with NASA’s Jet Propulsion Laboratory (JPL). In the radar imagery, what appeared to be a massive structure had been revealed deep beneath the frozen landscape.

“We were looking for the bed of the ice,” Gardner said, “and out pops Camp Century.” //

A remote U.S. military base once used as a top-secret testing site for the deployment of nuclear missiles from the Arctic, Camp Century was constructed by the U.S. Army Corps of Engineers within the Greenland Ice Sheet in 1959. Remaining in use for less than a decade, the base was decommissioned after just eight years and abandoned beneath Greenland’s frozen landscape.

Also known as the “city under the ice,” this forgotten Cold War relic consists of a network of tunnels hewn into the near-surface portions of the ice sheet. Today, the remnants of the secretive base lay hidden beneath close to 100 feet of snow and ice that have continued to accumulate since it was decommissioned. //

Although the radar imagery obtained in April by Greene and Gardner could prove useful in terms of ongoing monitoring of such threats as melting continues, the researchers said the images of this forgotten vestige of the Cold War they obtained occurred entirely by chance.

“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.

“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 added.