Unlike the moon or Mercury, where impact craters dominate the landscape, the pock marks caused by meteorite hits are much harder to find on Earth. That’s because our atmosphere limits the size of space rocks that actually smash into us, and erosion and rainfall often erase traces of ancient impacts. But some of the depressions survive the eons, and researchers have just found one of the largest ever discovered trapped beneath the ice of Greenland’s Hiawatha glacier.

Signs of the crater were first detected by NASA’s Operation Icebridge, an airborne mission that uses radar to track changes in ice on Greenland’s ice sheet. Researchers from the University of Copenhagen who examined the publically available data noticed an anomaly underneath the ice of Hiawatha that appeared to be a 19-mile-wide, 1,000-foot-deep crater, which, if confirmed, would be one of the top 25 largest craters known on Earth and the first to be found under the ice. (And it would be big enough to "swallow Washington, D.C., writes Paul Voosen at Science.)

The team then spent three years confirming the NASA data. Satellite images seemed to show a circular depression in the surface of the ice. The team also sent a German research plane equipped with new type of high-powered ice radar to map the crater in stunning detail, getting images of the 1,000-foot crater rim and the upwellings in the middle that accompany a meteorite strike. The team also put boots on the ground, collecting samples of sediment from channels washing out of the crater, which included bits of shocked quartz that can only be formed during a high-energy impact. They conclude that there is indeed a crater locked beneath the ice, the team reports in a study published in the journal Science Advances.

The next big questions ask exactly when the meteor hit and what kind of effect it had on the planet.

“The crater is exceptionally well-preserved, and that is surprising, because glacier ice is an incredibly efficient erosive agent that would have quickly removed traces of the impact,” says lead author Kurt H. Kjær from the Center for GeoGenetics at the Natural History Museum of Denmark in a press release. “But that means the crater must be rather young from a geological perspective. So far, it has not been possible to date the crater directly, but its condition strongly suggests that it formed after ice began to cover Greenland, so younger than 3 million years old and possibly as recently as 12,000 years ago—toward the end of the last ice age.”

Science's Voosen reports that the impact would have been a pretty big global event. It’s believed that to create the crater, the iron meteor that struck Greenland would have to be half a mile to a mile across and would have had the force of a 700 megaton warhead. Such an impact would have been felt hundreds of miles away, would have warmed up that area of Greenland and may have rained rocky debris down on North America and Europe.

Some researchers believe it could have had an even more significant impact. About 12,800 years ago toward the end of the last ice age, the world was steadily warming up. Then, abruptly, the paleoclimate record shows that temperatures plummeted back to ice age norms for about 1,000 years, a cooling period called the Younger Dryas that has no definite explanation. According to one theory, a comet impact in Greenland would have melted ice and diluted the ocean current that transports warm water through the Atlantic, causing a re-freeze. Some have even suggested such an event could have led to massive forest fires in Europe and North America, leading to the end of megafauna like the mastodon and the human communities that hunted them, which also disappear from the record around this time.

“It’s a very speculative idea, but if this does turn out to be [the link], it would have had an outsize impact on human history,” Joseph MacGregor, a glaciologist with NASA tells Brian Clark Howard at National Geographic.

But that’s only one possibility. In fact, Ludovic Ferriere of the Natural History Museum in Vienna tells Howard that he’s not convinced the site is definitely an impact crater and not some sort of natural depression. To be convinced he’d like to see more sediment tested and—the ultimate proof—drilling through the 0.6 mile-thick glacier to collect samples from the crater itself. Let's just hope it is a crater and the buzzing of the drill doesn't hit the nest of something more sinister lurking below the ice.