Ninety metres underwater is not a sensible place for a human being. At a point that is almost as deep as Big Ben is high, the water pressure on your body is 10 times greater than air pressure at sea level. Your lungs are squeezed to the size of fists and your body is no longer naturally buoyant. Rather than floating, it sinks like a stone. To swim back to the surface, even with the aid of fins, will take you two minutes.

Now imagine swimming to that depth, and back again, on a single lungful of air. Because that’s what the British diver Sara Campbell did last month in Dahab, Egypt, when she set a world record of 90 metres (295ft) in the discipline of “constant weight” freediving.

She did it with the help of a mermaid-style fin, and the dive took 3min 46sec. It was one of three new world records she set in a single weekend. Last week she won gold in the constant weight category at the world championships in Egypt.

Freediving – popularised in the 1988 Luc Besson film, The Big Blue – is demanding and dangerous. But perhaps the most extraordinary thing about Campbell is that she has been freediving competitively for less than a year. And since she took up the sport in spring 2006 she has spent seven months out of the water because of illness.

Campbell is a little surprised herself by what she’s achieved. “One of my earliest memories is swimming underwater and pretending to be a fish,” she recalls. “But I never imagined it would come to this.”

A dream of becoming a yoga teacher brought Campbell to Dahab two years ago. She had been working in public relations and as a part-time aerobics instructor in London, and was looking for a fresh start in sunnier climes. Campbell, 35, stumbled on the sport when a couple of freedivers came to a yoga class. They noticed she was very good at breath-holding and suggested she try freediving.

Almost instantly, she realised she was a natural. “After about six weeks, I was already reaching depths of over 40 metres [130ft] in the discipline of ‘constant weight no fins’,” she says. That involves the diver reaching his or her depth and surfacing again without any help from equipment. “I met a woman who had been diving with the British team for two years. Her best depth was only 37 metres [121ft]. She suggested I apply for a place in the team.”

Two days after she filled out her application forms to join the British team, Campbell caught hepatitis A, and was forced out of the water for seven months. But one week after she returned, and in her first competitive dive, in April this year, she set a new British women’s freediving record in constant weight no fins.

How does she do it? “Like every diver, I get help from a phenomenon known as the mammalian dive reflex,” Campbell says. “It’s a physical response in all humans to immersion in water, and its discovery has been one of the most fascinating aspects of freediving.”

Our heartrate slows when we hit the water and the blood vessels in our arms and legs constrict to allow more blood to concentrate in our vital organs. We become more oxygen efficient and can hold our breath underwater for longer than we can in the air.

Campbell is able to hold her breath for an extraordinary 4min 30sec. There’s something unusual about her physiology, and a team of Italian doctors is about to conduct tests on her in an attempt to discover what that is.

Just as important is her attitude. She loves competing and is unfazed by the risks – underlined this year when former world record holder Loïc Leferme died off the south coast of France. She says Leferme’s disciplines, “no limits” and “variable weights”, are different from her own – the freediver is dragged to extraordinary depths by weights (the no limits record is 214 metres – 702ft). In no limits they rise to the surface with the help of an air bag, which runs up a line, and fatal accidents can occur when these jam. The divers are too deep to be able to swim back to the surface.

The most worrying problems for Campbell’s discipline are the blackouts that freedivers experience as they rise to the surface. With their brains suffering from a lack of oxygen, and the water pressure rapidly decreasing, it’s easy for divers to drift in and out of consciousness. “It hasn’t happened to me yet,” she says, “but I’ve watched the film of my 90-metre dive, and I can see myself doing things at the end of which I have no memory whatsoever.”

It’s because of this that divers train with “buddies”, who descend to 80ft to await their companion’s return, and bring them to the surface if necessary (nearly all blackouts occur during the last 65ft of ascent). During competitions, there’s also a counterbalance, which will drag the diver to the surface in a few seconds.

As a result, Campbell insists her version of the sport is safer than scuba diving. If she’s honest, she finds it a bit of a doddle. “Each dive feels easy. Apart from the [300ft] dive, every one has felt well within my limits.”

So is there a chance that one day, at the bottom of a dive, she’ll leave her guide rope behind and swim away into the infinite blue – as Jean-Marc Barr did at the end of Besson’s movie? “No chance,” she says. “There’s too much I want to do with my life. My goal is to be the first woman to reach 100 metres [328ft] in the constant weight discipline and after that, who knows? Maybe it will be time to start a family.”

Makes you wonder if she might raise a pod of dolphins.

Sara Campbell is competing in the AIDA individual world championships. See www.sharmfreedivingworldchampionship.com for details

How to get started

- There are eight categories of freediving competition – three swimming-pool disciplines, two involving the use of weights, and three self-propelled. The weight-assisted dives are most dangerous and take divers to the greatest depths – the no limits record is 214 metres (702ft), set by the Austrian Herbert Nitsch in June

- There are numerous British freediving clubs, mainly in and around London. Try the Tribe Freediving Club, which meets at a private pool in Streatham, south London, and can offer training to beginners (www.deeperblue.net/ courses/tribe)

- For those who fancy training in warmer climbs, Vertical Blue (www.verticalblue.net) offers courses on Long Island in the Bahamas. A basic three-day beginner’s course costs $290 (£140), although you’ll have to get there first. The course director is William Trubridge, a record-breaking freediver

- Basic freediving equipment is similar to that used for scuba diving. You’ll need a wetsuit and some fins – either two separate ones or a mono-fin, essentially two fins joined together. You’ll also need a specialist freediving face mask – scuba masks are too bulky

- The Manual Of Freediving: Underwater On a Single Breath, by Umberto Pelizzari and Stefano Tovaglieri, is the definitive illustrated guide for aspiring freedivers. It is available at www.amazon.co.uk

- For more details about freediving, visit www.britishfreediving.org