Scientists say they have made an astonishing discovery of a large meteorite crater in Greenland – the first ever discovered there, and the first ever found under an ice sheet.

Published in the journal Science Advances, a team led by Kurt Kjaer from the University of Copenhagen in Denmark described how they found what's believed to be an impact crater hiding under 900 meters (3,000 feet) of ice beneath the Hiawatha Glacier in northwest Greenland.

The crater is thought to be 31 kilometers (19 miles) wide, making it one of the 25 largest impact craters on Earth. The object that caused it was likely an iron asteroid about 1.5 kilometers (0.9 miles) across, the size of a city, weighing 12 billion tonnes (13.2 billion tons). It's thought to have hit Greenland anywhere from 12,000 to 2 million years ago at a speed of about 20 kilometers (12 miles) per second.

“It was hiding in plain sight,” Kjaer told IFLScience. “How often do you go out in the world and make a discovery like this? It’s mind-boggling.”

The crater was first spotted in 2015, when radar was used to map the Greenland ice sheet. Looking at the map, Kjaer and his team noticed there was a large depression towards the northwest of the island.

In May 2016, a German research plane flew back over the area, taking new ice radar images. These started to show that the researchers were right – there was a circular feature hundreds of meters under the ice with a rim around it.

In summer 2016, Kjaer traveled to the site to take samples. He found that meltwater was leaking from the ice, containing material coming from the crater. Studying samples of this water, his team found the “smoking guns” of quartz grains that had been affected by the shock of the impact.

Perhaps most remarkably of all, the team realized a 20-ton chunk of meteorite that had been sitting in the University of Copenhagen’s Centre for GeoGenetics – collected from Greenland in 1963 – was most likely a chunk of this impactor. And other pieces found in this area may have come from the impact, too.

Some major questions about the impact remain, however, including its age. The team have dated it to the Pleistocene era (2.6 million to 11,700 years ago), saying it must have impacted after ice began to cover Greenland 3 million years ago and after the nearby bedrock took shape 2 million years ago, but before the last ice age ended at the end of the Pleistocene.

The other question is what effect this impact had on the planet. Large impacts like these can have dramatic effects on Earth’s climate, and this may have been no exception, burying itself 7 kilometers (4 miles) into the ice. This event could have melted and evaporated huge amounts of ice, releasing considerable amounts of water into the North Atlantic.

Kjaer noted that this impact could even have had a role to play in the Younger Dryas – a period 12,900 to 11,700 years ago when temperatures in the Northern Hemisphere mysteriously dropped to glacial levels.

In their paper the team also said there could be more discoveries to make in this area, with “one of the most promising regions,” being southwest of this crater. “We are seeing a hidden landscape that is starting to emerge,” said Kjaer. “And there are surely more discoveries to be made.”

One of the next key steps will be working out how old the crater is. This could be done by looking for debris ejected from the impact nearby. And if we can find that out, we might unveil what effect this large asteroid had on Earth.