A major feature of the Earth has escaped notice — until now.

Scientists reported Thursday that they've discovered a vast canyon, twice as long as the Grand Canyon. It carves a deep scar from the center of the world's largest island out to the coast. And, oh one more detail: It's buried beneath as much as two miles of ice.

Yes, we're talking about icy Greenland.

You may not think of it when you fly over, but beneath all that ice is a hidden terrain of bedrock. Jonathan Bamber from the University of Bristol has been trying to understand what that bedrock looks like. He pulled together a vast amount of data, collected by research airplanes that have been zigzagging over Greenland for the past 40 years.

Those planes have been using radar that can peer through the ice to see what lies below.

"We started looking at the data, and once we put it all together, we realized there was this strange feature in the middle of Greenland," he says. "And we started looking into it in a bit more detail."

The picture that emerged was that of a canyon rivaling the one carved through the red rock in the American Southwest.

"It's almost twice the length of the Grand Canyon, about half its depth and about twice its width in most places," he says. And it looks like it was carved by rivers, back before Greenland was covered with ice, four million years ago.

"Nowhere does it look like a typical U-shaped valley," which is what you would expect if it were carved by glaciers, he says. "At its northern limit, it looks pretty much like a river valley. It's got relatively steep slopes, and it's quite deep."

"We think this was a major river system in Greenland before the ice sheet was there," he says. "And it's just survived the cover by the ice sheet."

Bamber and his colleagues have published their description of this remarkable feature in the journal Science. And it's drawing oohs and aahs from people like Michael Studinger at NASA's Goddard Space Flight Center.

"It really shows how little we know about what's below the major continental ice sheets, like the Greenland ice sheet and the Antarctic ice sheet," Studinger says.

He runs a NASA program called IceBridge, which flies airplanes over these ice sheets to peer down into — and through — the glaciers.

"We can't do it from space yet, and doing it from airplanes and from the ground takes a long time, actually," Studinger says.

That's why this feature has remained hidden for so long. Another reason: Scientists studying these ice sheets have mainly focused their attention on the coastlines. That's where the ice sheets are most unstable — and most likely to melt or collapse, and by so doing raise the global sea level.

Eric Rignot, a geophysicist at the University of California, Irvine, and NASA's Jet Propulsion Laboratory, says he knew about the mouth of this canyon because it lies under the huge Petermann glacier, where it meets the sea.

"To be honest, I never imagined that some of these canyons could extend that far beneath the ice sheet," Rignot says. "So when I saw the figure of this paper, I suddenly went, wow! That's quite a feature."

The canyon is probably serving as a drain, which carries meltwater under the glacier and out to sea. Bamber says that running water could explain some odd grooves that scientists have discovered on the underside of the Petermann glacier. It will take some time for scientists to understand how this canyon fits into the story of Greenland and its glaciers.

"You know, it's not every year, it's not every decade, it's not every five decades that you discover something quite as substantial and extensive as a feature like this, so it was a big surprise for us," Bamber says.

Does he think he's discovered the last large canyon on our planet?

"Well, who knows," he says with a laugh. "We were pretty stunned when we found it, and I think there can't be too many!"

If there is another Grand Canyon yet to be discovered, he says it would probably be buried under the ice of Antarctica.

Copyright 2021 NPR. To see more, visit https://www.npr.org.