Earth still holds plenty of secrets, even though some of them are hiding in plain sight.

Take, for instance, a recently confirmed impact crater hidden beneath Greenland's Hiawatha Glacier. Measuring about 19 miles (31 kilometers) in diameter and 1,000 feet (305 meters) deep, the crater is one of the 25 largest impact craters on the planet. Scientists weren't even sure it was here until July 2015 when a new map made using radar data compiled by NASA's Operation IceBridge mission revealed a circular depression along the northwestern edge of the glacier.

"Previous radar measurements of Hiawatha Glacier were part of a long-term NASA effort to map Greenland's changing ice cover," Joe MacGregor, a NASA glaciologist at Goddard Space Flight Center, said in a NASA statement. "What we really needed to test our hypothesis was a dense and focused radar survey there. The survey exceeded all expectations and imaged the depression in stunning detail: a distinctly circular rim, central uplift, disturbed and undisturbed ice layering, and basal debris — it's all there."

Indeed, the crater is "exceptionally well-preserved," according to Kurt Kjær, a professor at the Center for GeoGenetics at the Natural History Museum of Denmark. This is surprising since the glacial ice is a potent agent of erosion that should have removed all traces of the impact crater. Kjær was the lead author on the paper regarding the impact crater published in Science Advances.

The crater is probably fairly young, at least by geological standards. Scientists estimate it formed less than 3 million years ago and as recently as 12,000 to 115,000 years ago when an iron meteorite more than half a mile wide slammed into the Earth. Ice may have already formed around Greenland, and this meteorite would have vaporized billions of tons of ice in the process.

A massive impact crater has been hiding under Greenland's ice sheet
Located under the Hiawatha Glacier, the impact crater is one of the largest on Earth.