American endurance swimmer Diana Nyad has begun her attempt to become the first person to swim the shark-filled Florida Straits without the aid of a safety cage.
Ms Nyad, 61, is trying to swim 103 miles from Havana to the Florida Keys in 60 hours, a feat she says has been a lifelong dream.
She said she hoped she would inspire people to live vigorously during their golden years.
Tanned and freckled from long hours training in the open seas of the Caribbean, Ms Nyad said: "The joke is 60 is the new 40, and it's true. We are a younger generation than the 60 that went before us."
Five boats are sailing alongside her with a 45-strong support crew on board, from navigators, nutritionists and doctors to shark wranglers and a film crew that has been documenting her story.
Ms Nyad first attempted the crossing as a 28-year-old in 1978, when she swam inside a steel shark cage for about 42 hours before sea currents hammered her off course.
This time she will be relying on special equipment that surrounds her with an electric current imperceptible to humans but strong enough to keep most sharks at bay.
Whitetip sharks are not deterred by the field, so divers will be standing by to gently discourage any of those who get curious - without harming them.
For the record to be considered valid, Ms Nyad will have to make the swim without a wetsuit.
Her crew will navigate, monitor her health and provide nourishment, but she is not allowed to touch the boat, nor can they touch her, until she emerges fully on to dry land.