US illusionist David Blaine held a press conference shortly before starting his underwater challenge in New York on Monday.
He announced an unexpected twist to the challenge, telling reporters he would be handcuffed into 150lb of metal chains for the dramatic finale.
Blaine entered the tank wearing trousers, rubber shoes and a special diving mask. He has a special wetsuit to use if needed.
Members of the public filed past the sphere to wave at Blaine and take photographs shortly after he began the stunt.
The US performer's actions have captured the attention of the world's media.
Blaine is encouraging fans to come up close and communicate with him through the sphere.
Blaine will end his seven-day challenge with an attempt to hold his breath under water longer than the current record of eight minutes, 58 seconds.
He will receive air and nutrition via tubes throughout the week-long challenge.