View Images Summer meltwater has drained through a snow-covered channel in Greenland. Scientists say that the Greenland Ice Sheet sits atop a canyon twice as long as the Grand Canyon. 
 
 Photograph by James Balog, National Geographic 
 
 Imagine if you could pick up the Greenland Ice Sheet and see what lies beneath. Surely 1.7 million square kilometers of slowly thawing ice must rest on a massive pool of melted water, right? 
 
 Not necessarily, according to a study released today in the journal Science. Unlike the ice sheet covering Antarctica that sits atop numerous lakes, the Greenland Ice Sheet blankets a giant subglacial canyon nearly twice as long as the Grand Canyon located in Arizona. (Read: "Changing Greenland" in National Geographic magazine.) 
 
 Scientists suggest the canyon—which runs as deep as half a mile (800 meters) and as long as 466 miles (750 kilometers)—is paleofluvial, meaning it originated as a system of rivers in Greenland's hard bedrock surface. According to this research, the canyon is part of an organized valley system that carries meltwater away from the inland ice sheet and funnels it toward coastal fjords connecting to the Arctic Ocean. (Related: "Ancient Global Warming Raised Sea Levels Nearly 70 Feet.") 
 
 Jonathan Bamber, lead author of the study and a physical geography professor at the University of Bristol in the United Kingdom, says he believes the canyon pre-dates the ice sheet. Before the land was completely glaciated at least four million years ago, melt from partial ice cover likely flowed through the bedrock canyon. 
 
 Since then, the features of the land have survived glaciation without significant erosion. 
 
 Beyond the Surface 
 
 While flying over the ice sheet, scientists over the past three decades have measured the depths of the canyon using a radar system that operates at frequencies where ice is transparent to radio waves—from around 50 megahertz to 500 megahertz. A pulse of energy is sent down to penetrate through the ice, bounce off the bedrock, and travel back to the radar system. (Also read: "'Shocking' Greenland Ice Melt: Global Warming or Just Heat Wave?") 
 
 Bamber and his team compiled this decades-worth of airborne radar data, mainly from NASA, to build a more comprehensive picture of what sits under the Greenland Ice Sheet. 
 
 The data have made him curious about other parts of the world. The Antarctic Ice Sheet, which is ten times as large as Greenland's, sits on a complex topography that includes bedrock and mountain ranges. After the recent canyon discovery in Greenland, Bamber says it is hard not to wonder about the areas of Antarctica still left to be explored. 
 
 Greenland has been glaciated for about 3.5 million years, and will continue to remain so for thousands more. But because of constantly increasingly global temperatures, it is difficult to determine when exactly the glacial land ice will completely erode. Understanding what happens to meltwater will be crucial for research on the future of glacial melt and rising sea levels. (Watch: "Greenland Ice Sheet.") 
 
 "If we carry on warming up the planet, the ice is going to melt faster than we want it to," Bamber said. "It really depends on what we do to the planet, doesn't it?" ||||| Greenland's 'grand canyon' was part of a large river system before an ice sheet covered it millions of years ago, a new study says. Now it appears to be a vital part of the island's plumbing. 
 
 A canyon similar in scale, if not in grandeur, to Arizona's Grand Canyon lies beneath Greenland's ice sheet, according to a new study. 
 
 Running from deep within the island's interior north to Greenland's northwest coast, the canyon measures at least 470 miles long, six miles across at its widest, and as many as 2,600 feet deep – reaching its widest and deepest points near the coast. The Grand Canyon, by comparison, is 277 miles long, 18 miles wide, and 6,000 feet deep. 
 
 The portrait points to how little scientists know about what lies beneath the world's great ice sheets. It also could help researchers understand how the ice sheet and melt water are working together to feed outlet glaciers along a coast where glaciers have been thinning at an increasing pace within the past decade. 
 
 Mapping Greenland's hidden landscape is important "so we can understand how the ice sheet is presently routing mass out toward its edges and how the ice sheet and water underneath the ice will interact," says Mark Fahnsetock, a geophysicist at the University of Alaska at Fairbanks. 
 
 In addition, the discovery could help solve a longstanding riddle: Why does Antarctica's ice sheet have lakes underneath it, but Greenland's ice sheet apparently does not? For the region the canyon traverses, the answer may be that the island has more efficient plumbing, says Dr. Fahnsetock, who was not involved in the study. 
 
 The first hints that the island's ice sheet, nearly two miles thick, was hiding such a feature emerged in the later 1990s. But the evidence was limited. No one anticipated a canyon of such magnitude. 
 
 The canyon lacks features that would signal it was formed by glaciers, suggesting that its predates the initial appearance of glaciers there about 38 million years ago, according to the team led by Jonathan Bamber, a glaciologist at the University of Bristol in Britain. His team built a profile of the canyon using 40 years of ice-penetrating radar data, which show the canyon meandering in ways that suggest it was carved by a large river system. 
 
 The team concludes that water would have flowed through the canyon constantly until Greenland's island-wide ice sheet formed 3.5 million years ago. Once the ice sheet formed, its pressure would have pushed water at the base out of the canyon in some locations. But the canyon would have remained a major route for under-ice water as it flowed toward the coast and under what today is known as the Petermann Glacier. 
 
 That could help explain why the ice shelf at the end of the Petermann Glacier has been prone to high rates of melting. The ice shelf is grounded in a fjord, and scientists have thought that warm sea water was entering the fjord and melting the shelf from underneath. 
 
 Now, it turns out that the shelf also could be under assault form melt water moving north through the canyon in what the research team calls "well-organized" flows. Indeed, scientists have uncovered deep, wide channels carved upward into the underside of the shelf, suggesting that the underside is exposed to relatively focused, rapidly flowing melt water. Researchers also have discovered a hydrothermal heat source near the northern end of the canyon, which would increase the amount of melt water at the base of the ice as it moves over the hot spot. 
 
 A formal presentation of the results is set to appear in Friday's issue of the journal Science. 
 
 For Eric Rignot, who has mapped Greenland's coastal areas, including the northern end of the canyon, the extent of the canyon is an eye-opener. 
 
 "It didn't occur to me that the canyon could go that far inland," says Dr. Rignot, a glaciologist at the University of California, Irvine and a senior scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We did not even bother looking at that." 
 
 Researchers are keeping a keen eye on the region. 
 
 The northern end of the canyon represents a conduit for ice moving down from the ice sheet, but it also represents a pathway for ocean water to migrate inland to continue melting the glacier from underneath if the Petermann ice shelf breaks up and the glacier retreats. 
 
 "The ocean water will basically follow the ice along that canyon," he says. 
 
 If nothing else, the study "shows how little we still know about the bedrock below large continental ice sheets," said Michael Studinger, project scientist for a NASA ice-monitoring program whose data played a significant role in uncovering the canyon, in a prepared statement.
– Scientists have—totally by accident—come across a hidden canyon that dwarfs the Grand one, the BBC reports. The researchers were using radar to map out Greenland's bedrock when they stumbled upon the 2,625-foot deep feature, which, at 500 miles, is longer than the Grand Canyon's 277 miles, Design & Trend notes. It's full of ice now—the stuff is up to two miles thick—but a river forged the canyon some four million years ago, predating the ice. Now, a little meltwater drains out of the northern end of the unseen canyon, which runs from central Greenland to its north coast. "With satellite images instantly available on a mobile phone we could assume that the Earth has been fully mapped, but there's clearly a lot left to discover," says a scientist. The canyon could also be bad news: Water moving under Greenland's ice could contribute to faster melting, the Christian Science Monitor notes. So are there more of these in the world? The lead researcher tells National Geographic he's now wondering what lies under the Antarctic Ice Sheet—which is 10 times the size of Greenland's.