Scientists think they've found one of the world's biggest impact craters, but confirmation is tricky given its location—under a glacier in Greenland. In the journal Science Advances, researchers make the case that a meteorite perhaps a mile wide slammed into Greenland somewhere between 12,000 and 3 million years ago, reports the Guardian. The resulting crater, first spotted by a plane using ice-penetrating radar, is 19.3 miles wide and would rank among the 25 biggest on Earth. The city of Paris could fit inside it, notes Space.com. After spotting the indentation in the radar images, the researchers set about getting samples to confirm their hunch. Because the crater lies under the giant Hiawatha glacier, they had to settle for examining sediment meltwater—and that sediment included "shocked quartz grains," pointing toward a meteor strike.
"You have to go back 40 million years to find a crater of the same size, so this is a rare, rare occurrence in Earth's history,” says Kurt Kjaer of the Natural History Museum of Denmark, one of the researchers in the discovery. However, an expert at the Lunar and Planetary Institute in Houston tells Space.com that he is "intrigued" but not wholly sold yet on a meteor strike. The researchers would have to drill through a half-mile of ice to settle the debate, and it's not clear when, or whether, that might happen. Assuming the crater is from a meteor strike, the big question is exactly when that strike occurred. If it's on the recent end of the scale, around 12,800 years ago, Astronomy.com notes that this could be the missing crater that would explain a mysterious 1,000-year cooling period known as the Younger Dryas. (A rock used as a doorstop turned out to be worth a pretty penny.)