Illusionist David Blaine has begun his latest stunt - spending seven days underwater in his "human aquarium" challenge in New York.
The 33-year-old lowered himself into a 2.5m (8ft) acrylic sphere filled with water at the Lincoln Center for the Performing Arts.
A mask and air line will keep Blaine alive, while food will be provided through a tube.
"My only fear is the unknown," Blaine said before descending into the sphere.
Hundreds of people turned out on Monday to see Blaine - who spent 44 days in a suspended acrylic box over the River Thames in 2003 - begin his latest stunt.
After seven days he will attempt to break the world record for holding breath under water, which currently stands at eight minutes, 58 seconds.
He revealed an unexpected twist to the challenge, telling reporters he would be handcuffed into 150lb (68kg) of metal chains for the dramatic finale.
"So basically besides holding my breath for about nine minutes hopefully I will have to escape from all these chains, and if not I will drown and the world will see something pretty insane," he said.
Dr Murat Gunel, who specialises in surgical treatment of strokes, said a lack of adequate oxygen in Blaine's brain during the breath-holding challenge, especially after seven days under water, carried a risk of irreversible brain injury.
"David might never be the same again," he said.
"Even if he is fine immediately after, there is a risk for developing seizures in the long run.
"We all advised him against this stunt."
Blaine entered the "bubble" wearing trousers, rubber shoes and a special diving mask.
During the seven-day challenge the water temperature will be maintained at around 36C and he will be able to control his own core body temperature by wearing specially designed wetsuits.
He has also undergone intensive training for the challenge, shedding 50lbs (23kg) in body weight since January to improve the efficiency with which his body uses oxygen.
"I feel pretty good. Though I've been sleeping very little and breath-holding non-stop," Blaine said.
"My system is completely empty and I did that so there's no waste. That way in the sphere I don't have to use the bathroom."