Lewis Pugh is no stranger to tough swims - he's swum in Antarctica and across the North Pole, highlighting the impacts of global warming on the ice caps. But swimming at altitude - two thirds of the way up Mount Everest, 5,300m above sea level - proved his hardest challenge yet.
After three days of test swims and battles with altitude sickness, Lewis successfully completed his 'Swim for Peace' across the Pumori glacial lake next to the Khumbu glacier. In icy-cold 2° waters - wearing just a pair of Speedo swimming trunks, cap and goggles - he finished the 1km swim in 22 minutes 51 seconds.
Afterwards Lewis explained: I had to find a delicate balance between going too fast, in which case I might drown due to hyperventilation - and going too slowly, in which case I might die of hypothermia.
"Swimming even 20 metres at full speed in the test swim, I felt I was going to drown. I was gasping for air, and if I'd swum any faster I would have gone under.
"I learned I had to respect this unique terrain and swim as slowly as possible. I had to swim breaststroke so I could breathe more efficiently."
Climate change, water and peace
The reason behind the swim was never far from his mind. "All along the Khumbu Glacier I've seen pools of melted ice," says Lewis. "Millions of people rely on this water supply, and preserving it is vital to peace in the region.
"I have seen glaciers in the Arctic, the Alps, central Africa, Antarctica and the Himalayas - and it's the same story everywhere. Most glaciers are melting away.
"I would urge David Cameron, Nick Clegg and the new UK government, as well as leaders worldwide, to put climate change at the very top of their agendas.
"When I arrive back in London I hope to be able to share my experiences with the government and show them the footage of what we've seen."