Scientists have discovered a rift the size of the Grand Canyon hidden under the Antarctic ice sheet, which they say is contributing to ice melt and a consequent rise in the sea level.

The rift, some 1.5 kilometres (one mile) deep, 10 kilometres wide and 100 kilometres long, was found by researchers using radar to measure the subglacial topography, glaciologist Robert Bingham told AFP.

"If you imagine the Grand Canyon but filled with ice and then even that whole feature is then also uniformly beneath another kilometre of ice," is how he described the feature whose magnitude he said was a "surprise" to the team.

Scientist believe a shrinking West Antarctic Ice Sheet is responsible for about 10 percent of climate-change-induced sea level rise, which if unchecked threatens to flood many coastal cities within a few generations.

The sheet, a huge mass of ice up to four kilometres thick that covers the land surface and stretches into the sea, is melting faster than any other part of Antarctica.

But scientists' sketchy knowledge of the sub-surface topography has made it difficult to predict the exact rate and extent of ice sheet loss, said the study published in Nature.

The newly discovered valley, formed long before the region was covered by ice, is believed to be part of a wider West Antarctic rift system, "which we've known exists but we don't know where it goes," said Bingham.

"We are now getting a better idea that parts of this rift system actually go ... further west than we previously knew about."

The type of rift found under the Ferrigno Ice Stream is caused when a continental plate starts to tear apart -- like the large lakes that fill rift systems in parts of East Africa today.

"It is the shape of the rift that contributes to the fact that the region is vulnerable to ice melt," Bingham explained of the Antarctic discovery.

"Because the rift is there it means that the ice is both deeper and slopes inland as you move away from the sea and both of those conditions make this a vulnerable topography to ice thinning effects" by allowing warm sea water to flow inland along a trough created by the rift to attack the ice on the coastline.

Bingham said the find showed that not only modern climate factors but also geology is contributing to ice loss.

"I think where it changes our view just ever so slightly is that this issue is traditionally conceptualised as a modern effect of global warming, and what we see is that that modern effect is actually superimposed on a very ancient geological evolution.

"It helps us to appreciate that the whole process is something that occurs over many cycles of time."

Scientists had only visited the region once before, over 50 years ago, in 1961.

This time, experts from the University of Aberdeen and the British Antarctic Survey conducted three months of fieldwork in 2010.

"We targeted the area because we knew from satellite measurements that there was ice thinning taking place," said Bingham.

"When we did the survey and we found this rift, that actually was a surprise that it was much, much deeper and preconditioned to this thinning than we expected."

The only way to find such a valley covered by an ice sheet is by using radar on the spot, he added.