Not many extreme sports demand as much from the bodies of competitors as freediving, but that hasn't stopped the world's elite taking it to a further level than ever before.
The sport, in which athletes descend under water on a single breath and without the aid of oxygen, has just taken a dive deeper in Norway. There, divers have foregone warm seas and plummeted below lakes of barely melting ice.
In the freezing waters of Lake Lutvann, near Oslo, the French world record-holder Guillaume Nery reached a depth of 52m at the weekend. The level of fitness and stamina required for that dive is beyond the realm of fantasy for most international athletes. By the time freedivers are 10m below the surface, their lung capacity is half what it is usually; 30m down it is reduced to a quarter of its original size. At 100m down or further, a freediver's lungs are effectively the size of two oranges, while their pulse is reduced to eight or nine beats a minute.
Nery and his rivals have to be able to hold their breath for up to 10 minutes, pushing them into a state known as apnoea. The compression of their lungs means that, with very little oxygen being transferred into the bloodstream, their brains could be starved of oxygen altogether, leading to a blackout. Nery's world record depth is 113m, which he set in warm waters off his home city of Nice last July.
And the extreme pressure levels under water can cripple a freediver's legs or rupture ear drums. Top competitors pinch their noses to force air into their ear canals.
All of which might raise the question of why on earth they would want to push their bodies to even further limits, by tackling ice instead of warm water.
"We knew there were different dangers with diving under ice", said Steinar Schjager, who organised the event at Lake Lutvann. "That's part of why we wanted to try it. The extreme cold can reduce pulse even further, and because it's pitch black even 20m below the surface, rescue attempts after a blackout are very difficult."
Nery defeated Christian Ernist of Sweden and fellow Frenchman Christian Maldame in the inaugural under-ice competition.
Like most extreme sports, freediving is a niche activity with a small and loyal fan base. But Nery's profile, in France and beyond, is doing much to help the sport reach new audiences. Yesterday the 26-year-old spoke about his exhilaration at discovering the dark world under Norway's ice. "It's very different to under water diving", he said. "When you dive, the light fades almost immediately. You can feel a very intense chill through your body. All your reference points fade and it is a very strange and sort of interesting feeling. You suddenly become very conscious of a sense of discovery, as if you are in another dimension".
Nery began diving 12 years ago, when he was just 14. Before any major dive or record attempt he spends two to three months training, running up to 10 miles a day and swimming many more. Before his dive in Lake Lutvann he spent weeks cross-country skiing.
His girlfriend, Juliet Gautier, 29, is the French women's record holder in freediving, after reaching 68m last year. Her intimacy with sport helps allay the fears that have plagued Nery's mother, a maths teacher in Nice, and father, a hospital administrator.
"My mother is getting better than she was but it's still very horrible for her," said Nery. "When I was at school she used to hate it. Then when I had my first blackout she asked me to stop. I don't think she'll ever be happy, no matter how many records I break."