The environmentalist and motivational speaker, Lewis Gordon Pugh, wore only swimming trunks, goggles and a swimming hat to swim 1km (0.62 miles) across Pumori Lake underneath Mount Everest.
The icy lake lies 17,000ft (5,300m) up the famous mountain near the Khumbu Glacier and its waters are around just 2C.
Lewis took 22 minutes and 51 seconds to breast stroke across the lake and had to find a "delicate balance" between going too fast and too slowly.
He says that if he swam too quickly he could have lost energy and drowned but if he had been too slow he would have suffered hypothermia.
His epic dip has become the first ever long distance swim to be completed under the summit of Everest, however swimming in freezing water is nothing new to Lewis, he has prevously swam in Antarctica and across the North Pole to draw attention to melting sea ice.
But he says his latest challenge was his toughest swim yet as he had to battle altitude sickness as well as freezing conditions.
He said: "It's one of the hardest swims I've ever undertaken. When I swam in Antarctica and across the North Pole I swam with speed and aggression but on Mount Everest you can't use the same tactics.
''Because of the altitude you need to swim very slowly and deliberately. I felt I was going to drown. I was gasping for air and if I had swum any faster I would have gone under.
"I was deeply concerned that I wouldn't make 1km and I'm delighted that I've finally achieved it''
Lewis says he took part in the swim to bring the "world's attention" to the melting of glaciers and its effect on the region.
He said: ''The glaciers in the Himalayas are not just ice. They are a lifeline - they provide water to approximately two billion people.''
Copyright Speakers Corner 2017