Deeper than the Grand Canyon: massive trench found under Antarctica's ice

An enormous valley – deeper than the Grand Canyon and hosting a floor that dips more than two kilometers below sea level in places – lurks beneath the West Antarctic Ice Sheet, researchers have discovered.

The valley, known as the Ellsworth Trough, was discovered by a team of scientists in Britain. They've been mapping a highland region under the ice sheet to better understand where the sheet originated and how far, under a warming climate, it might retreat in the distant future before it stabilizes.

The region, known as the Ellsworth Subglacial Highlands, sits on the continent near the base of the Antarctic Peninsula. The highlands form a Continental Divide-like boundary that sends ice into either the Weddell or Amundsen Seas, on either side of the peninsula.

The newly discovered valley is 25 kilometers (15.5 miles) across at its widest point, stretching for some 325 kilometers.

The valley is so large that it can be seen from space through its effect on the surface contours of the ice sheet above it – one that's several kilometers thick, says Neil Ross, a researcher at Newcastle University in Britain who studies ice streams flowing within the West Antarctic Ice Sheet. He is the lead author of a paper reporting the find in the current issue of the Geological Society of America Bulletin.

In its heyday, the valley formed a fiord that emptied into the Weddell Sea. It now testifies to the remarkable growth of the West Antarctic Ice Sheet from much smaller ice fields that have gathered in the Ellsworth highlands, as well as in the coastal mountains of Marie Byrd Land, east of the Ross Sea and its Ross Ice Shelf, the researchers suggest.

Today, the trough appears well inland. But if the West Antarctic Ice Sheet weren't covering the region, "the ocean would be lapping at the trough mouths" even after the bedrock underneath, relieved of the weight of the ice that hides it, had rebounded, Dr. Ross writes in an e-mail.

The team discovered the giant gash after analyzing data gathered during the project to map the highlands. That project aimed to help resolve a debate over whether the highland landforms under the ice resulted from the advance of the current West Antarctic Ice Sheet or whether they were sculpted by smaller masses of ice. Understanding the origins and history of the West Antarctic Ice Sheet could help inform predictions of what would happen to it as Earth's climate warms.

During three field seasons between 2008 and 2011, the team used ice-penetrating radar flown in aircraft and dragged across the ice sheet's surface. They also incorporated satellite data as well.

As the researchers looked at the landscape below the ice, the network of tributary valleys as well as the Ellsworth Trough itself looked remarkably like the landscape in and around Norwegian fiords.

The topography that the team uncovered "could not have been cut by the present ice sheet," writes Tom Jordan, a researcher with the British Antarctic Survey and a member of the team reporting the results, in an e-mail. "This is because in many cases the ice presently flows in the wrong direction or too slowly to erode the observed feature. Instead the features were cut by a much smaller ice sheet, with confined valley glaciers flowing directly into the sea."

As the team reads the data, the valley probably began to form some 34 million years ago during a moist maritime climate in which ice from ice fields in the highlands began to build and migrate down toward the coast. Over the intervening years of advance, retreat, and advance, the ice gouged it way closer to the coast, even as it was building vertically as part of the growth of the broader West Antarctic Ice Sheet. As this portion of the ice sheet thickened to several kilometers, its enormous weight pushed down on the crust.

Even after it reached the sea, the ice could continue to cut as long as there was enough ice above the surface to keep the base in contact with the sea bottom, Dr. Jordan adds.

While the valley itself is some three kilometers deep at its deepest points, two kilometers of that falls below today's sea level. About one kilometer of that is due to subsidence driven by the weight of the overlying ice, Jordan says.

The continuing discovery of lakes and valleys beneath Antarctica's vast, continental ice sheet can be humbling, Ross suggests.

“To me, this just goes to demonstrate how little we still know about the surface of our own planet," he said in a prepared statement. "The discovery and exploration of hidden, previously-unknown landscapes is still possible and incredibly exciting, even now.”