Location of the 19-mile-wide (31 kilometers) impact crater in northwestern Greenland.

The location of the 19-mile-wide (31 kilometers) impact crater in northwestern Greenland.

An unusually large asteroid crater measuring 19 miles wide has been discovered under a continental ice sheet in Greenland. Roughly the size of Paris, it’s now among the 25 biggest asteroid craters on Earth.

An iron-rich asteroid measuring nearly a kilometer wide (0.6 miles) struck Greenland’s ice-covered surface at some point between 3 million and 12,000 years ago, according to a new study published today in Science Advances.

The impact would’ve flung horrific amounts of water vapor and debris into the atmosphere, while sending torrents of meltwater into the North Atlantic—events that likely triggered global cooling (a phenomenon sometimes referred to as a nuclear or volcanic winter). Over time, however, the gaping hole was obscured by a 1,000-meter-tall (3,200-foot) layer of ice, where it remained hidden for thousands of years.

Remarkably, the crater was discovered quite by chance—and it’s now the first large crater to be discovered beneath a continental ice sheet.

In 2015 I was looking at a new map of the bedrock below the Greenland Ice Sheet and discovered a large circular feature under the Hiawatha glacier in northwest Greenland,” Nicolaj K. Larsen, a co-author of the study and a geoscientist at Aarhus University, told Gizmodo. “In other words, it was a coincidence that the crater was discovered.”

Larsen, along with his colleague Kurt Kjaer from the Natural History Museum of Denmark, immediately recognized that they had stumbled upon something special, but it soon became apparent that the depression would be hard to confirm as a remnant of an ancient asteroid strike.

The first step was to analyze aerial surveys taken of Greenland from 1997 to 2014 by researchers from the University of Kansas. But the data resolution of these surveys “was not sufficient,” according to Larsen, so a team was sent to Greenland to collect superior, higher-resolution ice-radar data of the Hiawatha glacier and the bedrock beneath. This was accomplished in 2016 using wideband ground-penetrating radar (or in this case, ice-penetrating radar) developed at the University of Kansas.

You can see the rounded structure at the edge of the ice sheet, especially when flying high enough,” John Paden, a co-author of the study and an engineer at the University of Kansas, said in a statement.

“For the most part the crater isn’t visible out the airplane window. It’s funny that until now nobody thought, ‘Hey, what’s that semicircular feature there?’ From the airplane it is subtle and hard to see unless you already know it’s there. Using satellite imagery taken at a low sun angle that accentuates hills and valleys in the ice sheet’s terrain.