Dawn is breaking over Hampstead Heath and the temperature is barely above freezing. Out of the gloom emerges Mary Rimington, deputy principal of City and Islington college in north London, to take her early-morning dip. For the past 18 years she has braved rain, wind and snow, sometimes breaking the ice when necessary, to take the plunge.
The history lecturer is one of hundreds of hardy souls whose daily year-round ritual is to swim in one of the Heath's three swimming ponds.
"I find it both physically and spiritually renewing," she said. "It is sheer joy to swim with the kingfishers flying around before cycling to work and facing the traumas of city life. "People think I am mad to do this, but it's not madness. It is life-enhancing."
In winter her dip lasts for no more than five minutes. "It's a quick 100 strokes and out," she added. "Any longer when the water temperature is just above freezing could be dangerous. But in the summer I like to do several circuits of the pond."
Just how dangerous it can be was brought home to her when, in October 2003, she helped to rescue and resuscitate one of the regular swimmers, who suffered a heart attack after diving in. "She had missed a couple of weeks swimming, which can be crucial in cold water," she said. "Your body has to stay acclimatised to the cold."
Her addiction, as she describes it, began during one Indian summer in the mid-Eighties.
"I started swimming there in the summer when it was hot, and one year the summer carried on into October and I saw no point in stopping," she said.
"I don't feel well if I don't swim early in the morning. I don't get colds and flu and I haven't had a day off work in that time."
She says she judges the severity of the winter by the number of days the pond is frozen over. Swimmers are no longer permitted to break the ice, for safety reasons. While no days have been lost to ice this year, there is one threat to her ritual.
The Corporation of London, which maintains the three swimming ponds on the Heath, says that providing free swimming there is "an unsustainable drain" on resources.
She is on the Kenwood Ladies Pond Committee, fighting proposals to close the ladies, men's and mixed ponds or to introduce charging. Kenwood Ladies Pond officially opened in 1925 and stayed open even during the Second World War.
If you know of FE people who pursue unusual activities, please tell us.