Jolyon Jenkins meets the people who want to swim a mile through freezing cold water. This isn't like a quick dip on New Year's Day - it takes about 40 minutes to swim a mile. As the swimmers battle the second law of thermodynamics, only the fit, or fat, will make it.

Some swimmers acclimatise by sitting in icy paddling pools in their gardens and sleeping without bedclothes. Others pile on the calories to build bulk. But apart from the danger of hypothermia, the risks are legion: cold shock as you enter the water, loss of brain function or motor control leading to drowning, and non-freezing cold injury that can leave sufferers with permanent pain in their extremities. Acclimatisation can even increase the risks, by lowering the temperature at which you start to shiver and generate heat. No wonder that not everyone thinks ice swimming is a good idea.

So why do the ice swimmers want to do it? Jolyon travels to the ice swimming world championships in Bavaria to find out.

Presenter/producer: Jolyon Jenkins. Show less