Long-distance swimmer Lynne Cox: Johnny Watterson talks to a woman who has braved icy seas, 50-foot whirlpools and killer whales, in just swimsuit, hat and goggles

Geeks in the laboratory will tell you that since the Big Bang, pain has been a useful sensation. Pain tells you when you are damaging yourself. Pain tells your body to stop. Pain says "let go". The geeks will say that pain is a notional thing.

They'll tell you the idea of pain being an end point on a single carriage highway is to take a flat-world view of its personality and its relationships. For some people pain can be slipped into the back pocket or banned from the front room for hours on end. Pain can be a terrifying enemy one moment and a comforting big brother the next. It can kill you or it can open doors, allow you to feel more alive than you have ever felt in your life.

Somewhere between the US and the Soviet Union, Lynne Cox swam through the Bering Sea for her life. She experienced pain. It was August 1987 and she was in the middle of the glacial water mass that separates Alaska from Siberia.

Initially, the sensation had been one of swimming through frozen tapioca, millions of small particles of near frozen and frozen beads of water pouring over her shoulders, stinging and burning at her skin. Her face felt like it was "shot full of Novocaine" and was "separating from my skull". Her hands were numb. Disembodied, she could not tell whether they were functioning as paddles any more. They were no longer hers, the colour of ash, the arms of "a dead person".

The water was 38 degrees F, fractionally above freezing point. She had jumped off a rock on the shore of Little Diomede island into the Arctic Ocean. All she was wearing as she set out for the snow-capped volcanic island of Big Diomede two and a half miles away was a swimsuit, hat and goggles. The international dateline runs between the two islands and marks the border between the continents. The sea between had not been open to boats since the beginning of the Cold War in 1948 and had never been swum.

In essence, the attempt was in preparation for her dream of swimming to Antarctica. As she beat small icebergs aside and scrambled over the bigger ones towards the snow beach on the Soviet side of the border she finally hauled herself across the ice sheet and into a tent after two hours and six minutes in the water. Inside she was met by an islander offering a bouquet of wild flowers. A doctor stood beside her with a cardiac defibrillator on the table.

"In managing pain, you also manage your focus," she says. "Years ago I came here to London to do a series of experiments with Dr William Keatinge, an expert on hypothermia. One of the tests was to immerse my hand in 32F water for half an hour. They were looking at the blood flow through the hands. It was the most painful thing I have ever done in my life.

"They then asked me to do it again and I thought that as I'd done this before my mind is going to accept it and it won't be as hard to do. But it was as painful, if not more so, than the first time. Half way through the doctor asked 'would you like me to hold your hand?' I said 'yes'. That cut the pain in half. I changed the focus so that it was not just the one hand immersed in freezing water. Something or someone was there with me and it made, not a small change, a huge change."

At the end of that year Mikhail Gorbachev flew to Washington to sign the INF treaty, which would help reduce nuclear missile production. At the official dinner hosted by US president Ronald Reagan, Gorbachev referred to the thawing of relations between the two countries.

"It took one brave American by the name of Lynne Cox just two hours to swim from one of our countries to the other. She proved by her courage how closely to each other our peoples live," he said.

A perplexed Reagan hastily had the national security adviser call the US State Department to find out whom Gorbachev was referring to.

Cox is an assembly of things. Disarmingly gentle, eccentrically courageous and unrestrained by conventional wisdom, her outrageous endurance feats have also drawn on a spiritual dimension. Swimming the oceans has been a liberation.

Like the free divers who plunge several hundred feet under the sea on one gulp of breath and then stifle and fight the body's demand for oxygen for six or seven minutes while their lungs are compressed to the size of a fist, so, too, has Cox explored the earth's canvas in a different way.

At 16-years-of-age in 1973 she re-drew the men's and women's record in swimming the English Channel in nine hours and 57 minutes. It was her second trip to Dover in two years as she had held the world record at 15 until it was broken the same year by a man. Two years later she became the first woman to swim between the North and South islands of New Zealand across the notorious Cook Strait. Five hours into that swim she was further from the finish of the 12-mile stretch than when she'd started. Twelve hours, two and a half minutes later she finished. A first again.

In 1978 she swam the length of the shark-infested waters off South Africa's Cape of Good Hope, where the Indian Ocean meets the Atlantic. During the swim a 12-foot bronze whaler shark was speared by one of the security divers in the process of making an open-mouthed attack.

The lakes, too, came in for conquering. Lake Baikal in Siberia, the biggest, the deepest. Lake Titicaca in Bolivia, the highest. She swam the Strait of Magellan in South America, where 50-foot whirlpools jockeyed to slurp her and the accompanying skiffs underneath, and in Egypt front-crawled into a dead dog during a distance race in the Nile. The subsequent dysentery and hypothermia almost killed her.

In the Bering Strait they wondered if orcas could sniff out human meat or if the 1,000lb leopard seals liked the taste of humans as much as penguins - whole.

"I tried to wear a suit that didn't look like food," she says. It wasn't just Cox's physiology that was being put to the test.

"They found I am able to close down the blood flow to the peripheral areas of the body very quickly and keep it closed down," she says. "That helps to move the blood from the outside (cold) area and into the core (warm) and it doesn't circulate out much, so that protects the vital organs. Also the blood does not circulate to the peripheral areas and bring the cooled blood back to the core.

"They also found I work at a very high rate. I can normally work at 70-80 per cent of my max and sustain that for a long time. I have a large muscle mass that generates heat and a good distribution of body fat. Most women do but I have more than average. That helps keep me warm and buoyant.

"But it's the mindset and the spirit that drives you forward, a combination of all of it. Don't think you can just believe you can do something without training. And if you don't think you can do it, it's not going to happen even if you train."

The whys are philosophical. Logical, concrete reasons don't exist among the icebergs and killer whales. Where it is logical, even reasonable to get out when the early stages of hypothermia set in, Cox's instincts were always to push harder.

"I know there is dark in the world. I like to focus on light," she says with unabashed simplicity. "It is all in your perspective. The story is about having dreams and realising your life is your life and you need to do the things you want. If you can fathom swimming to Antarctica in a swimsuit, maybe you can fathom doing what it is you want."

To dive head first into the water one mile from the southern-most ice mass would be to stop the heart beating. Even with Cox's meticulous preparation and history of successes, she was anxious as the wind blew at 30 knots off the surrounding glaciers. She wondered if the 32-degree water, the temperature at which fresh water freezes, would freeze her cells and cause permanent damage. Unknown to her, the nerve endings in her hands and feet from a preparatory swim had already been freeze-damaged. Her initial warning system, pain, had been compromised.

She slipped in and sank under water. The cold made her hyperventilate, sharp, short, uncontrollable breaths. She didn't know it but her body was trying to compensate, trying to warm the smaller volumes of cold air before they entered the body cavity and lungs. Her skin burned. Like touching wet fingers to frozen metal, she fought for seven minutes, almost unable to breathe. A wave hit her and she opened her mouth. It didn't hurt her teeth. It tasted sweet from the glacial melt. She looked down and the water was clear crystal blue. Endless. She was swimming in air.

"Where there is no land lines to follow, you create your own course," says Cox. "That's exciting. It can be sometimes depressing and exhausting but somehow it matches life. Everything is changing, the wind, the currents, the tide. It's spectacular.

"You feel that. You are immersed in a very special environment. There is a sense of your inner body being able to power through it, the feeling that this is a different place in the world.

"I had a sense of swimming the Channel when I was nine years old. I found out early I was good at swimming distances, I could go further. In the pool I learned pace and discipline but in the ocean I think there was always a sense of freedom."

Cox is of Irish descent. Her mother's great, great grandparents both died on a famine ship and the children were adopted by a French-Canadian family. Raised in Boston she lives in southern California. The Antarctic swim of just over a mile was completed in 25 minutes. At the end she was so cold she could not feel anything and slurred her speech. Her core temperature had dropped two and a half degrees and she shook uncontrollably for more than half an hour. But she felt strong. She felt warm inside. There was no pain, only a warm, outstretched hand holding on to hers.

Swimming to Antarctica (Tales of a Long Distance Swimmer, by Lynne Cox), Weidenfeld&Nicholson £14.99.