Only a few seconds in the icy depths would be enough to kill most mere mortals.
But yesterday, protected by nothing more than a pair of Speedo trunks and his extraordinary central heating, Lewis Pugh took the plunge and became the first man to swim at the North Pole.
The 36-year-old Londoner spent almost 19 minutes at minus 1.8C as he front crawled for a full kilometre - more than half a mile in the coldest water a human has ever swum.
He has swum the whole of the Thames, been first to swim the length of the world's longest fjord and first to swim round the most northerly point of Europe.
He is also the first man to swim at the South Pole - where the waters are a comparatively warm zero celsius.
Although the North Pole temperature is below zero, the salt is enough to prevent the water freezing at that temperature.
Pugh carries out his swims to Channel Swimming Club rules - which outlaw wetsuits, flippers or snorkels.
He chooses not to cover himself in goose grease as he does not want to be too slippery for his team to fish him out of the water in case he develops hypothermia.
