The Poles tried to conquer the deadly K2 on the top of the world last winter, and hot air balloonists have gambled their lives in riding the unpredictable jet stream at the limits of the atmosphere, but there are other mad, extreme sport enthusiasts going not up but down - into the depths of the ocean.

Can you imagine standing on an anchor-like "sled" and descending 400ft (122 metres) into the ocean, without the aid of any SCUBA diving gear?

This is exactly what international Brit Tanya Streeter, 30, did on 21 July 2003 in Grace Bay off the Turks and Caicos Islands in the Caribbean Sea, achieving a world record in the "variable ballast" category of freediving. She smashed the previous women's record of 311.7ft held by Deborah Andollo of Cuba. She also destroyed the men's record of 393.7ft, set by Patric Musimu of Belgium.

Tanya stood in a weighted metal sled and slid down a stretched guide line into the darker depths below. A brake mechanism prevented her from descending too rapidly. A special device pinched her nose shut to stop the high pressure forcing cold, salty water up her nostrils and a wetsuit covered her whole body, hiding her long, blond hair. With her forearms crossed in front of her around the sled column and closed eyes, she looked like some poor corpse-like soul being banished to the ocean depths as some kind of punishment, summoning all her concentration to resist the crushing pressure. Brings back memories of Ed Harris falling alone into the darkness in the film "The Abyss". Only this time she didn't have the special rat-tested, oxygen-rich breathing fluid filling her lungs.

15 safety divers, one of them her husband Paul, were stationed every 50ft down the guide line - in case anything went horribly wrong. Last year a French female competitor drowned in the waters off the Dominican Republic, harming freediving's reputation. Each diver was equipped with an independent lift bag that could be quickly attached to the extreme sport victim and take her to the surface if the pressure won.

Above the waves a rescue boat was on stand by with an emergency physician and life-support equipment, and a shore ambulance and hyperbaric chamber were also on alert.

As she descended the concentrated carbon dioxide in her body made her feel faint and her legs became partially paralysed. Her lungs were crushed to the size of small oranges - and starved of oxygen, her heart beat slowed down to 15 beats a minute.

When the sled reached the bottom of the measured descent, a safety diver checked she was still conscious and then she began the long journey back to the surface - pulling on the cable guide line and kicking with her unusually long fins, adapted for maximum propulsion.

When Tanya broke the surface she gasped for air and gave a thumbs-up. She had held her breath for 3 minutes 38 seconds. She was attempting to prove that freediving need not be dangerous, even though this extreme sport involves pushing the body beyond its normal limits - which can lead to death or serious injury.

Tanya's family come from West Sussex in Southern England, but she was born in the Cayman Islands and educated in Britain at Roedean and Brighton University. She now lives in Austin, Texas. She said breaking the records would help her to support environmental organisations like the Whale and Dolphin Conservation Society and Coral Reef Alliance.

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