Freediving: breaching the surface of the body's capabilities

Freediving is a bit of an odd extreme sport. On the one hand, it's completely natural: humans have been diving with their breath held for thousands of years, be it for food, treasure or pleasure. And ask any swim teacher and they'll tell you that, as soon as a kid is comfortable in water, they'll start exploring how long they can hold their breath and how far they can swim. On the other hand, those kids all come up when they get the urge to breathe. Air is such an essential part of life that the thought of going without it voluntarily is enough to give most people the shivers. You can go without food for weeks, without water for days, but air? A minute, maybe two?

The official world-record breath-hold is more than 11 minutes. Yes, 11. Stéphane Mifsud of France can lay face-down in a shallow pool for nearly a dozen minutes, get up and be perfectly OK. (Branko Petrović lodged a Serbian national record of 12min 11sec, but the conditions weren't ratified by world freediving governing body Aida.) There are people out there swimming more than 250m underwater – five lengths of an Olympic-sized pool. They wear a big monofin, but even without those they can manage more than four lengths. And there's a Russian grandmother out there who regularly beats these guys. She's also the only woman who has dived below 100m on breath-hold in competition. The men are approaching 130m. How is that possible?

Well, like any sport, it's mostly training. Sure, natural ability helps: Branko has 15-litre lungs and weighs about as much as a pack of spaghetti, so all that air doesn't have to feed a whole lot of body. But in essence, what you're training is an ability to withstand CO2 – carbon dioxide. This is another aspect in which freediving is an odd one out. Most sports focus on VO2 max: your ability to transport and burn oxygen effectively. Freedivers want to make their oxygen last as long as possible. In terms of cars, it would be the difference between racing and making one tank of fuel go farthest.

Most people think that the urge to breathe comes from a lack of oxygen – but actually that feeling is your levels of CO2 rising. So, as you hold your breath, you still have lots of oxygen in your lungs, in your blood and your tissues, but no way to release the CO2 that keeps building up. At a certain point the balance tips so that it becomes uncomfortable. People think at that point they need to breathe in, get oxygen, or they're going to choke and die, but actually what their body wants to do is breathe out – to get rid of the carbon dioxide. They still have plenty of oxygen, and all that happens when they keep holding their breath is that it will become more and more uncomfortable and their diaphragm – the main muscle responsible for breathing – will start to involuntarily contract to try and make the body exhale.

If this sounds unpleasant, that's because it is. Tolerating high levels of CO2 is uncomfortable and annoying – not quite painful, but definitely not fun. But it's part of the challenge: if you want to see how low your oxygen can go, you have to learn how to deal with high carbon dioxides. As with any type of training, you have to get out of your comfort zone if you want to improve. You'll have to find a way to surrender to the discomfort – because you shouldn't fight it. Here's where freediving shows its third odd characteristic: whereas most extreme sports are about adrenaline and hearts racing, will power and toughness, freediving is about learning to relax, completely.

A relaxed body consumes much less oxygen than a tense body. Hold your breath and your body will tell you exactly where you are tense: that's where it'll start to annoy first. And if you were to try and go deep, a tense body will not be able to equalise the pressure that comes with the depth. Fight the sea and you will fail. All you can do is relax. Relaxation is not some mystical state of mind, by the way – first and foremost it is a surrender; your body giving in to gravity. When you're holding your breath on the surface, that means you can let the water carry you – you don't have to do anything to hold yourself in some position, like when you're seated or standing. When you're swimming, it means you adjust to the pace of the water; you don't try to dominate it but work with it instead, in conjunction with its properties. And when you dive deep, it means that at a certain depth you can stop swimming and just fall, for your lungs are so compressed that you are negatively buoyant.

But these are just numbers and theories – none of them explain why you would do something that seems so unnatural and dangerous. In fact, it is natural – human capacity for breath-hold diving is on par with some marine mammals, and not really dangerous, with not a single deadly accident in 20 years of freediving competitions. The 'Why do it?' question is rarely asked of other sports (and most sports, when practised on a professional level, are extreme). Something in the human mind finds holding your breath very scary. So I suppose, on this logic, the question might warrant an answer.

The world depth freediving championships begin today in Kalamata, Greece, and about 140 athletes from all over the world will come to this little industrial city by the sea to compete. They'll have trained for months, adhered to a very boring diet, not drunk (much), laid off the coffee and other stimulants. They won't get paid – as a matter of fact, they have to pay to attend the event. Most of them have no chance of winning or breaking a world record. They'll try for a national record but even when they get that, it won't bring them fame or glory. And yet they come, more of them each year. Why? Well, think about it this way: each of us is roughly two-thirds water. You've probably experienced the freedom that being in water gives you – that sensation of flying. Now combine that with this little nugget of odd human-body adaptation: as soon as your face immerses into water, your heart beat drops. The deeper you go the more profound this effect is: the water is actually helping you relax. Now think of all the flying dreams you've ever had, and imagine diving past the point where you have to swim – where you just sink and surrender to the ocean and her pressure. My guess is that this is why we freedive: to fly into that endless blue that gives you the mother of all hugs.

• The 2013 world depth freediving championships are in Kalamata, Greece, until 22 September.

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