A huge canyon has been found beneath the ice sheet that covers most of Greenland.
Scientists used radar to make the discovery which was carved by a great river more than four million years ago.
The BBC reports it is 800km long and up to 800m deep and packed with ice. The cannyon was carved by a great river more than four million years ago, before the ice arrived.
It was discovered by researchers from Britain and Germany studying data from NASA and using radar to map Greenland's bedrock. Ice is transparent to radio waves at certain frequencies.
The British Antarctic Survey said it was remarkable to find so huge a geographical feature previously unseen.
Professor David Vaughan said the canyon would have been partly uncovered at the time of the last interglacial age 100,000 years ago
The valley is longer than the Grand Canyon in Arizona. Glaciologists think it plays an important role in transporting sub-glacial meltwater towards the ocean.
The Greenland ice sheet is up to 3km thick.