A massive 'canyon' hidden beneath Greenland ice - at least 750 kilometres long and 800 meters deep - has been discovered by scientists.
The previously unknown 'canyon' rivals the Grand Canyon in Arizona, US and is thought to predate the ice sheet that has covered Greenland for the last few million years.
The remarkable chasm in the Earth has the characteristics of a meandering river channel - by comparison, the longest river in the UK, the River Severn, is about 350km long and much less wide and deep.
"With Google Streetview 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," Professor Jonathan Bamber of University of Bristol's School of Geographical Sciences, lead author of the study, said.
The scientists used thousands of kilometres of airborne radar data, collected mainly by NASA and researchers from the UK and Germany over several decades, to piece together the landscape lying beneath the ice sheet that covers most of Greenland and obscures it from view.
At certain frequencies, ice is transparent to radio waves which can travel through the ice and bounce off the bedrock underneath, researchers said.
By analysing all the radar data in a consistent way the team discovered a continuous bedrock canyon that extends from almost the centre of the island and ends at its northern extremity in a deep fjord connecting to the Arctic ocean.
They believe the canyon plays an important role in transporting sub-glacial melt-water produced at the bed from the interior to the edge of the ice sheet and ultimately into the ocean.
Even before the presence of the ice sheet, going back at least four million years, the evidence suggests the canyon provided a pathway for water from the interior to the coast and was a major fluvial system.
"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.
"This area's ice sheet contributes to sea level rise and this work can help us put current changes in context," Professor David Vaughan, ice2sea co-ordinator based at the British Antarctic Survey in Cambridge said. The research was published in journal Science.