Briton Tanya Streeter has broken the world freediving record by descending 400ft (122 meters) and resurfacing without the help of air tanks or buoyancy aids.
Streeter was 'overwhelmingly relieved' to complete the dive
Tanya Streeter said it had been a "hard dive" and she was relieved.
"Everything went pretty well considering it's a really long way to swim down
and back up while holding your breath and expending a lot of energy.
"I'm just happy I lived up to my expectations."
She said she had been trying to prove that freediving need not be dangerous, even though the extreme sport involves pushing the body beyond its normal limits - and can result in death or serious injury.
Freediving's reputation was harmed by the death last year of a French female competitor who drowned in waters off the Dominican Republic.
Streeter, 30, who was educated at Roedean and Brighton University and now lives in Austin, Texas, had 15 safety divers monitoring her bid on Monday, in the Turks and Caicos Islands in the Caribbean.
Each safety diver was equipped with an independent lift bag that could be quickly attached to Streeter's wet suit to send her to the surface if something went wrong.
A rescue boat with an emergency physician and life-support equipment was standing by, with a shore ambulance and hyperbaric chamber also on alert.
As she descends, the carbon dioxide concentrated in Streeter's body makes her feel light-headed and her legs become partially paralysed.
"The pressure is building as I descend. I don't have a mask on. I keep my
eyes open or closed, depending on how I feel," Tanya said.
Her lungs are crushed to the size of small fists and her heart slows down to 15 beats a minute.
When she reached the bottom she took hold of the cable and began a series of
long, gliding pulls toward the surface and kicked with her fins when her arms
"I'm always very calm underwater, I can hold my breath for six minutes,"
Streeter, whose family comes from West Sussex, shattered both the men's (393.7 feet) and women's (311.7 feet) world records in freediving's variable-ballast category.
She also holds world records in five out of the seven disciplines in freediving, beating the men's record in three of them.