A huge mega-canyon, nearly double the length of America's Grand Canyon, has been discovered beneath a mile of ice in Greenland.

The canyon looks like a winding river channel and is at least 750km (466 miles) long, making it longer than the famous tourist spot (which is a comparatively paltry 446km, or 277 miles) and is as deep in places at 800m (2,600ft).

"One might assume that the landscape of the Earth has been fully explored and mapped," said Jonathan Bamber, professor of physical geography at the University of Bristol and lead author of the study. "Our research shows there's still a lot left to discover."

Boffins used thousands of kilometres of airborne radar data collected by NASA and researchers from the UK and Germany over several decades to map the landscape lying under Greenland's ice sheet. A large portion of the data came from the space agency's Operation Icebridge, a six-year mission that is the largest airborne survey of the Earth's polar ice ever flown.