“Look at this beautiful tree. An oak, Quercus robur. It’s yelling at us, ‘Helloooo!’” says Wim Hof, the doyen of cold endurance stunts, as we head across Hampstead Heath, north London, for a swim in one of the ponds.

Soon he’s catapulting himself into a gaping hole in its trunk, peeping out excitedly like a child. “Here I am, talking to the tree,” he beams. “I see the tree has personality. I go in. We are one. It’s alive. We are alive. Is that crazy? Bloody crazy! Yes I am.”

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Best known for trying to scale Mount Everest in only a pair of shorts and hiking boots, Hof has broken more world records than even he can count. His feats include a near-fatal 57-metre swim under ice during which he went temporarily blind, and a barefoot half-marathon in the Arctic.

He’s also immersed himself in a tub of ice for almost two hours, warmed only by the power of his mind and self-styled breathing techniques. Scientists have begun studying his methods to evaluate if they can offer universal benefits to people.

“Live your body, stimulate your body,” he implores. “Arthritis, Crohn’s disease, psoriasis, depression – all of those diseases are a result of our neglected biochemistry. We need to be stimulated to help fight disease. Cold is a great stimulator.”

Hof and I have come to the pond for what feels like a sort of graduation. A day earlier, I attended a “Wim Hof Method” workshop at London’s Roundhouse to learn about his breathing techniques and how regular cold immersion could boost my immune system and reduce chronic inflammation within my body. Today I’m testing my tolerance to nature’s elements. I’m not the only one.

Cold-water swimming has enjoyed a huge surge in popularity in recent years, with the Outdoor Swimming Society growing from 300 followers in 2006 to 40,000 members this year.

“It’s like a tribe,” says the society’s founder, Kate Rew. “It used to be an eccentric thing, people doing it once or twice as a dare, but now it has an entire identity. It gives you such a feeling of mental calmness and physical robustness that it turns people into missionaries. Everyone who tries it tells their friends and they bring another five people.”

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As I stand on the jetty looking out across the pond – the bright spring sunshine flaring off the water’s surface – I’m concerned about the water temperature. I was told by one enthusiast that between 0 and 5C is considered cold, “but if it gets up to six you’re not ice swimming”. I sheepishly turn towards the lifeguard’s hut to see today’s temperature scrawled on the chalkboard: “7C,” it says.

“Is this cheating?” I ask Hof, who is already topless. “No, no, it’s still cold,” he assures me.

Superhero

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It’s 24 hours earlier, and I am lying on a yoga mat at the nearby Roundhouse under a haze of purple lights, breathing intently.

“Fully in, fully out. Fully in, fully out,” cries Hof from the stage, as more than 700 people hyperventilate in unison. Among them are yoga teachers, life coaches, cold-water swimmers, medical professionals and those, such as Tomasz Radecki from Poland, who have simply been inspired by Hof’s message of hope.

“I first saw Wim on YouTube and he said, ‘anything inside us is within our reach’ and ‘whatever limitations we set ourselves, it’s all bullshit’,” Radecki recalls. “To me it was like being an ordinary person and suddenly realising that I can be a superhero, too.”

Breathing is one of the three main pillars of the Wim Hof Method, alongside cold exposure and commitment. Over a period of 30 minutes, Hof instructs us to fill our lungs rapidly before emptying them passively, in a fashion developed from an ancient Tibetan technique, Tummo. Every so often we hold our breath for as long as we can. The idea is to mimic the natural gasping reflex we experience when we enter cold environments, thereby decreasing the amount of CO 2 in our bodies.

I did a somersault over or through myself. It was a complete loss of control, death, I don’t know Wayne Sequoia

Nick (above) reacts during the breathing exercise at the Wim Hof Method event. Photographs by Jonny Weeks

Soon the effect is so intoxicating that the muscles in my forearms contract, my arms raise involuntarily from my chest and my fingers curl into claws. Around me, people are writhing on their mats, some weeping or howling like wolves.

“Release all trauma. Liberate. Your future. Your mind. Your soul. Who are you? What are you?” urges Hof, before stepping off-stage to attend to Wayne Sequoia, whose limbs are jerking so violently it rouses me from my stupor.

“I left my body,” Sequoia tells me afterwards, his speech slow and slurred. “I had this intense feeling; I did a somersault over or through myself. It was a complete loss of control, death, I don’t know. It felt like a huge healing.”

Another participant, Nick, is similarly delirious. “My wife gave birth to two children not long ago and I can really say I felt the birthing process in my gut,” he says. “I’m a physician and if somebody told me this would happen today I would think they’re crazy. It was completely amazing.”

But not everyone is euphoric. “I just wanted to cry, to sob, but I couldn’t let it out,” says Karren Probyn. “It seemed like there was no end.”

Fight or flight

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True to Hof’s eccentric character, the 60-year-old’s first experience of cold water was spontaneous. Aged 17 and wandering alongside a frozen canal in Amsterdam, he felt a sudden urge to jump in. “Sometimes you don’t know why but you feel attracted to do something,” he explains.

For many years thereafter, Hof was a local oddity known as the “Ice Man”, a guy who would walk his children to school barefoot and do handstands and splits in the snow while the other parents looked at him perplexed. “They stood there smoking, saying, ‘Look at him, he’s crazy’,” he says. Over time, opinion slowly changed.

Wim Hof at Hampstead Heath men’s pond. Photograph by Jonny Weeks

The tragedy of his first wife’s suicide in 1995, as a consequence of her schizophrenia, fomented his belief that cold-water swimming – and his breathing techniques – are a panacea for inflammatory diseases.

“We are estranged from our own deeper physiology because we are no longer in contact with nature,” he explains. “Instead, we are controlling nature with air pollution, heating, technology. But you have to know you have a depth within yourself which needs to be stimulated. If it doesn’t get stimulated it becomes weaker, like a muscle that’s not being used any more.”

Hof claims his method activates the body’s “fight or flight” adrenal response, which in turn reduces chronic inflammation. What’s more, he claims he can regulate his immune response to stress – a feat previously considered impossible because the immune system is autonomic. Scientists have begun to pore over his theories.

Assuming there is a beneficial effect, my guess would be that the effects would not last longer than a day Prof Peter Pickkers

In one logic-defying experiment, he was able to maintain his core body temperature while encased in ice. In another he was injected with an E coli endotoxin that would ordinarily induce sickness, yet he showed no major symptoms. Accusations that he is simply a freak of nature were quashed when he taught a group of volunteers his methods and they, too, showed the same remarkable results.

Professor Peter Pickkers, of Radboud University Medical Centre in the Netherlands, who conducted the E coli tests, told me: “We found that [his method] results in major changes in oxygen and CO 2 levels and an increase in adrenaline to really high levels – higher than observed in people who bungee-jump for the first time.

“Higher adrenaline results in the production of the anti-inflammatory cytokine IL-10 and this inhibits the inflammatory response. We were really surprised that we found these results. The fact that you can influence your immune response willingly with these techniques is of interest, especially for patients with autoimmune diseases.”

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However, he cautions: “We are very reluctant to say it may cure diseases because we have so little data. Assuming there is a beneficial effect, my guess would be that the effects can only be there as long as you practise the techniques – likely not longer than a day.”

Watching Hof on stage, it’s clear that his desire to help others is genuine, even if his evangelical tone and occasional hyperbole could stoke a cultish faith among those seeking “wellness”.

“Anybody is able to doubt me,” Hof says. “I say, ‘Come with your criticism to polish the diamond of the truth.’ People need to wake up to their own power. Everybody has this nature.”

Feel-good factor

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Participants take an ice bath at the Wim Hof Method event. Photographs by Jonny Weeks

Back at the Roundhouse, a line of half-naked people is snaking out into the car park for the day’s big challenge. Everyone is waiting to enter the four inflatable paddling pools, which are being filled with ice.

The first participants gingerly lower themselves in, motivated by a combination of peer pressure and willpower. Two minutes – that’s their target.

For some the experience is visibly excruciating; for others it’s a thrilling sensation. One woman buries her head face-down in the ice and emerges 10 seconds later with a broad smile. The man next to her seems put off by such showboating and quits after a minute.

Catherine Girvan climbs out and begins performing the “horse stance” to warm up. “I was swimming in the pond at Hampstead Heath up until November last year when my husband passed away,” she tells me. “This is my first time [practising cold immersion] since then, and I got a bit of a shock, but I really enjoyed it. It’s all about the endorphins, the serotonin, the feel-good factor. I’m 73 and I’m not on a pill so something’s working.”

Catherine Girvan (above) doing the ‘horse stance’ after her ice bath

Exactly what is “working” remains a mystery, however. A 2017 review of past scientific studies, titled “Cold water immersion: kill or cure”, found a bounty of evidence dating back to the 16th century regarding the potential health implications of cold-water immersion.

Its co-author, Michael Tipton, professor of human and applied physiology at the University of Portsmouth, says: “I believe that cold-water immersion works for a lot of people but we can’t say precisely why.

“There’s an absolute raft of anecdotal evidence but what it can’t tell you is whether the benefits are derived from going into the open air, being with friends, exercising, or being in the cold. There’s not yet been a study that has isolated the cold factor.

“The closest we’ve come was a study of upper respiratory tract infections, where we found that people who go cold- and open-water swimming have fewer infections than their partners who are exposed to the same pathogens. However we also found that they don’t have fewer infections than people who go indoor swimming [in heated water]. So the jury is still out.”

He adds: “It may be that cold-water swimming works for people because of a placebo effect. But so what? If it works for them and if it’s not harmful, and if it’s removing their dependency on drugs, then great.”

After my turn in the ice bath, my body feels unexpectedly warm, so much so I don’t even rush to dry off or get dressed; I’m supercharged with adrenaline for at least an hour.

Birthright

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On the jetty at Hampstead ponds the following morning, Hof is getting changed when Dermot Sullivan, a regular cold-water swimmer, requests a picture.

“Do you know about the new documentary, The Ponds?” he asks. “Some of their stories really shook me up. People have had depression, cancer, they’re going through bereavement, and they come here. One woman said, ‘If you can cope with two-degree water, you can cope with anything.’” On a personal note, he adds: “For me it doesn’t resolve life’s problems, but it’s like momentum.”

Jonny Weeks (above) after swimming at the men’s pond

Hof offers encouragement before racing down the ladder into the water. As I hesitate, I’m reminded of his earlier words at the oak tree: “My mission is to bring happiness, strength and health to all. It’s a birthright,” he says. “But if you want to become strong, happy and healthy, you’d better do this shit.”

• This article is part of a series on possible solutions to some of the world’s most stubborn problems. What else should we cover? Email us at theupside@theguardian.com

• In the UK, Samaritans can be contacted on 116 123 or emailjo@samaritans.org. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org