The danger of hypothermia hasn't put off Kiwi ice swimmers, who are pushing their bodies to the limit to find out how long they can last in freezing-cold water.
Marathon swimmer Susan Sherwen has just completed her most challenging swim to date — a mere 2km but in a lake that was only 3C.
"It's called an extreme ice mile," she told Seven Sharp.
"It took me 40 minutes. That last 10 minutes was really, really hard."
Early last month, some of the country's most hardened swimmers plunged into Otago's Blue Lake, near St Bathans, for the national ice swimming championships.
Competitors were only allowed to wear a cap, pair of goggles, and standard togs, with water required to be below 5C.
Despite having swum the Cook and Foveaux Strait before, Sherwen said the "extreme ice mile" she undertook at the championships was the hardest.
It comes without saying that the extreme sport is not for the faint of heart. Ice swimming poses serious risks to the body, such as hypothermia.
That's why the swimmers were accompanied by a paramedic who monitored and intervened if necessary. Debbie Keightley kept an eye on Sherwen.
"The main danger for ice swimming is hypothermia," Keightley said.
"We don't like them to be in the water for anything more than 45 minutes.
"And any sign of anything not normal, we will pull them out."
Keightley said she was amazed by Sherwen's ability to withstand the cold for so long.
She said most people would not be able to survive in ice water for that long.
"Susan is just an amazing swimmer. She's incredibly fit, and she trains in these conditions. This is not for anyone that doesn't train in these conditions."
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