It's a snowy Saturday morning. A group of seven men sporting woolly caps and backpacks gather at the top of Parliament Hill on London's Hampstead Heath.
They've been meeting every weekend morning like this for the past two years – never less than two, sometimes as many as 15, and their numbers are growing.
Ranging in age from 44 to 57, they head off shortly after 9am, jogging down the southern side of the hill.
This is no ordinary running group. In their professional lives they work for banks, hedge funds and private-equity firms. The 25-minute run is just a preamble. Soon they strip to swimming shorts, gloves and swimming caps, not wetsuits – and dive into a pond, which on this day is at barely 2 degrees. It can dip well below that.
"That's Cold in Anybody's Language," the chalkboard temperature warning sign outside the lifeguard hut affirms.
While there is a long history in Europe of freezing-water bathing – the Finns are known for avantouinti, or plunging into ice holes after a sauna – cold-water swimming has taken off in London.
Highgate Ponds are unique in Britain as the only life-guarded open-water swimming facilities accessible to the public every day of the year.
This particular group of enthusiasts is jokingly named the EGLST, an acronym for the East Germans Ladies Swim Team. It's a riff on the testosterone-laden doping scandal that surrounded that team's gold medal sweep of the 1976 Olympics.
Alongside the financiers, there are architects, photographers, IT professionals and lawyers including a Queen's Counsel, or senior barrister.
Most are committed to swimming at least twice a week. Others attempt to sneak in an early morning dip before work. Snow and ice – all the better. The only criterion for membership is getting in the water.
Alcohol seems to be the common theme in terms of their recruitment. Many an introduction occurred over a wine-soaked Saturday night dinner where, according to James Knowles, a managing director at Knight Vinke Asset Management LLC, one of the team's earliest members, a regular will sell "the poor sucker on how he should spend his Sunday morning".
Paul Mylrea, director of communications at the BBC and a regular swimmer for more than 18 months, says "it's a good reason to watch what you're drinking at parties".
Some come just once, unwilling to give it another try after the shock. Others, intrigued by the challenge of taking themselves out of their comfort zones, return.
Upon entering the water, there are different styles, but for all it's a shock to the body, said to be like a heart attack or an "inability to breathe". While some keep quiet, others yell. Andrew Willett, a managing director at Lloyds Bank, prefers to "growl."
Hedge fund executive Knowles describes the changes as "the body's natural signal forewarning death". If you can get past the initial 90 seconds, "one feels a sense of euphoria as the body seeks to compensate, and then calmness", he says.
David Alberts, the chief creative officer at global advertising company MoFilm, agrees: "Unlike running where my mind is still active, with cold-water swimming, my thinking is restricted to what is going on in the water, how long I could stay in, how far I could swim do I have to get out?"
On exiting the water, most reference a numbing of the limbs and a tingling of the skin. It's a good idea to put on shoes fast before the shakes set in and laces become impossible.
Geoff Burgess, a corporate partner at international lawyers Debevoise & Plimpton LLP, has recently recruited his 12-year-old son Asa to the scene. He describes a feeling of "cold blood entering internal organs".
Tom Kearney, a former commodities executive, relishes the daily dose of "pure, unadulterated exhilaration."
On vacations, "team" members will seek out the waters no matter where they are. They have tackled the piranha-filled Black river in Brazil, Norway in October, the Scottish Highlands and Helsinki in November, and Snowdonia in December.
"In France this year, I found myself leaping into rivers, much to the amusement of the locals," says Rick Wells, owner of the Fernandez & Wells chain of coffee bars and cafes.
Within the short time that they've known each other, nicknames have been assigned, including the Gimp (who wears a cap that covers most of the face and secures with a strap), and the newest recruit, the Ice Man (who needs continual reminders to exit the water).
Alberts explains it this way: "It's a group of guys acting like kids, nobody talks about work. There's a bit of competitive banter but it's just a laugh with a little bit of danger."
The group swims alongside the Lifebuoys, who sponsor the annual Christmas Day Race at the Highgate Ponds, and whose average age "hovers around 60," according to club president Chris Ruocco. There's also a group of women who regularly swim at the nearby Ladies' Pond. The Serpentine has its own club, as does the Tooting Bec Lido.
It might be argued that this is just another fad in the Type-A professionals' portfolio: They've done the marathons, segued into biking, moved on to the triathlons. Still, the social aspect of their pursuit suggests that this extreme sport is more than a macho pursuit.
As Kearney puts it: "If you start the day with a swim, no one can take that away from you."