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Here's news of a huge natural feature of our planet that's only just been discovered. It's 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. So, why has it escaped notice until now? It's buried beneath as much as two miles of ice.

NPR's Richard Harris has the story.

RICHARD HARRIS, BYLINE: Yes, we're talking about icy Greenland. You may not think if 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.

JONATHAN BAMBER: 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. And we started to look into it in a bit more detail.

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

BAMBER: It's almost twice the length of the Grand Canyon, about half its depth, and got similar width in most places.

HARRIS: And if you could remove the ice and sort of fly down to this canyon, would it look more like a U-shaped glacial canyon or would it look like a more sharply defined canyon carved by more traditional river systems?

BAMBER: Nowhere does it look like a typical U-shaped valley. At its northern limit, it looks pretty much like a river valley. And it's got relatively steep slopes and it's quite deep.

HARRIS: That means the canyon was carved more than 4 million years ago, some time before Greenland was covered with glaciers.

BAMBER: We think that this was a major river system in Greenland before the ice sheet was there. And it's just survived the cover by the ice sheet.

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

MICHAEL STUDINGER: It really kind of shows how little we know about what's below the major continental ice sheets, like the Greenland ice sheet and the Antarctic ice sheet.

HARRIS: Studinger runs a NASA program that flies airplanes over these ice sheets in order to peer down into, and through, the glaciers.

STUDINGER: So we can't do it from space yet, and doing it from airplanes and from the ground takes a long time, actually.

HARRIS: 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 at U.C. Irvine says he knew about the mouth of this canyon because it lies under the huge Petermann glacier, where it meets the sea.

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

HARRIS: The canyon is probably serving as a drain, which carries melt water under the glacier and out to sea. Jonathan 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 newly discovered feature fits into the story of Greenland and its glaciers.

BAMBER: 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.

HARRIS: Do you think there is another feature like this on the planet or have you discovered the last big unfound canyon?

BAMBER: Well, yeah, who knows? We were pretty stunned when we found it and I think there can't be too many.

HARRIS: If there is another Grand Canyon yet to be discovered, he says it would probably be buried under the ice of Antarctica. Richard Harris, NPR News.

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