Antarctica is home to a geological rift system where new crust is being formed, meaning the eastern and western halves of the continent are slowly separating.
The team writes in Nature journal that the canyon is bringing more warm sea water to the ice sheet, hastening melt.
The Ferrigno rift lies close to the Pine Island Glacier where Nasa scientists found a giant crack last year; but the newly discovered feature is not thought to be influencing the "Pig", as it is known.
The rift lies beneath the Ferrigno Ice Stream on a stretch of coast so remote that it has only been visited once previously.
The British Antarctic Survey (BAS) project revisited the area two years ago in the person of Aberdeen University glaciologist Robert Bingham.
The plan was to make ground observations that could link to the satellite data showing unexpectedly pronounced ice loss from the area.
The team towed ice-penetrating radar kit behind a snowmobile, traversing a total of about 2,500km (1,500 miles).
"What we found is that lying beneath the ice there is a large valley, parts of which are approximately a mile deeper than the surrounding landscape," said Dr Bingham. More
