A ravine the size of the Grand Canyon has been discovered beneath the Greenland ice sheet.
The mega-canyon is at least 750km long and 800 metres deep and is believed to predate the ice sheet that has covered Greenland for millions of years.
Researchers at the University of Bristol said the canyon has the characteristics of a winding river channel and is on a similar scale to parts of the Grand Canyon in Arizona. The mega-canyon is half the length of the longest river in the UK, the Severn, which is 350km long.
The team found the canyon using thousands of kilometres of radar data collected by Nasa and researchers from the UK and Germany. Their findings, spanning several decades, mapped the landscape lying beneath the ice sheet covering most of Greenland.
Ice is transparent to radio waves so radar imaging can pass through to bounce back off the rock beneath.
Researchers found a continuous bedrock canyon extending from the centre of the island to its northern extremity in a deep inlet connecting to the Arctic Ocean.
David Vaughn, from the funding programme ice2sea, based at the British Antarctic Survey in Cambridge, said: "A discovery of this nature shows that the Earth has not yet given up all its secrets. A 750km canyon preserved under the ice for millions of years is a breathtaking find in itself, but this research is also important in furthering our understanding of Greenland's past."
The researchers believe the canyon transports sub-glacial meltwater from the interior bed to the edge of the ice sheet and into the ocean. Before the ice sheet, there is evidence to show the mega-canyon acted as a pathway for water from the middle of Greenland to the coast and was a "major fluvial system".
"This area's ice sheet contributes to sea level rise and this work can help us put current changes in context," Vaughn said.
Michael Studinger, of Nasa's Goddard Space Flight Centre, said: "It is quite remarkable that a 750km-long channel the size of parts of the Grand Canyon is discovered in the 21st century below the Greenland ice sheet. It shows how little we still know about the bedrock below large continental ice sheets."
Lead author Jonathan Bamber said: "With Google Street View available for many cities around the world and digital maps for everything from population density to happiness one might assume that the landscape of the Earth has been fully explored and mapped. Our research shows there's still a lot left to discover."