A vast hole in the Earth on the same scale as the Grand Canyon lies buried under ice in Greenland, scientists have learned.
The hidden mega-canyon is at least 466 miles (740km) long and up to 2,624ft (800m) deep in places.
The feature, resembling a meandering river channel, is believed to pre-date the ice sheet that has covered Greenland for millions of years.
Professor Jonathan Bamber, from the school of geographical studies at University of Bristol, said:
"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."
The canyon was uncovered by airborne radar which can penetrate ice and bounce off the land beneath.
British adventurer Mark Wood has warned of the dangers of embarking on Arctic challenges, claiming people "fall in love with the romance of a polar expedition" without being aware of the true dangers.
The 47-year-old, from Coventry, attempted to become the first person in history to ski solo to both the South and North Poles, but had to halt the expedition after funding cuts.
Speaking to ITV News from his base camp at Mount Everest, he said: "The Arctic is one of the most unforgiving environments on the planet.
"The dangers are hidden; cold, isolation and the ever-changing landscape.
"People fall in love with the romance of a polar expedition but when you're on the ice it's a very different matter - it sucks the strength out of you."
The Queen’s Nursing Institute has paid tribute to Philip Goodeve-Docker, who hoped to raise money for the charity with his tragic Greenland expedition.
Mr Goodeve-Docker also undertook the challenge in memory of his late grandfather, who was a treasurer for the QNI, which is dedicated to improving the nursing care of people in their own homes.
The British man who died during a charity trek across the Greenland ice cap was trapped in a severe snow storm for around 30 hours before emergency teams could reach him and his two friends.
Philip Goodeve-Docker, an events manager, along with expedition leader Roan Hackney and Andy Norman, a former IT professional, got into difficulty when the sudden storm hit. Mr Goodeve-Docker died before the team could be airlifted to safety.
Temperatures in Greenland dropped to as low as minus 10.5C (13F) on Saturday, while gusts of 95 miles (152km) an hour were reported on the east coast. Both Mr Hackney and Mr Norman remain in a "critical condition" in hospital, according to Mr Goodeve-Docker's brother, Mark.
The charity page set up by Philip Goodeve-Docker ahead of his trek across Greenland has attracted in excess of £6,000 as strangers continue to honour the adventurer.
Mr Goodeve-Docker took on the challenge in a bid to raise £4,000 for The Queen's Nursing Institute but died on Sunday after he and his two teammates Roan Hackney and Andy Norman got into difficulty.
Philip Goodeve-Docker and his team mate Andy Norman were both "rookies to the Arctic" according to Mr Norman.
Writing on his website before the trip, the 33-year-old former IT professional said he was "setting foot for the first time in the Arctic Circle" and described Mr Goodeve-Docker as "completely new to the Arctic and indeed this adventure lark" but added that his friend had been "training furiously".
Mr Norman and Mr Goodeve-Docker undertook the unsupported crossing with experienced expedition leader Roan Hackney.
The trio was supported and monitored during the trip from the UK, managed by a world leader in managing Greenland expeditions.
Mr Norman and Mr Hackney remain in hospital.
Well-wishers continued to pledge donations to Philip Goodeve-Docker's nominated charity today in tribute to the adventurer.
Mr. Goodeve-Docker died on Sunday after he and his friends Andy Norman and Roan Hackney attempted to cross the world's second largest icecap unsupported.
He undertook the challenge in a bid to raise more than £5,000 for the The Queen's Nursing Institute.
Writing on his Just Giving page this morning, one contributor wrote: "Would have paid any money to have you back my friend still can't believe you are gone. rest in peace hope to see you again one day."
Another wrote: "In memory, you made a difference and lived large!!"
A Foreign Office spokeswoman said it is providing assistance to relatives of Philip Goodeve-Docker and to the families of the two rescued men from his party.
Relatives of Philip Goodeve-Docker have paid tribute to him in a Facebook post:
The 30-year-old charity promoter from Ealing, London, was said to be "on the expedition of his lifetime" with two friends in Greenland, when the group got into difficulty on Sunday.
Philip Goodeve-Docker undertook a charity trek across the Arctic in tribute to his late grandfather, according to his Just Giving page.
The 30-year-old charity promoter described the 550-600km trek as "one of the great polar challenges."