Veteran US long distance swimmer Diana Nyad is making her third attempt to swim the 166 kilometers (103 miles) from Cuba to Florida.
The 62-year-old launched herself into Hemingway Marina in Havana for a swim expected to take a grueling 60 hours.
She's being accompanied across the Florida Straits by a fleet of small boats and a support team of about 30 people.
Nyad first attempted the Cuba-Florida swim in 1978 but quit after almost 42 hours in open sea because of current and weather conditions.
She tried it again in August this year but abandoned that attempt after 29 hours because of a lengthy asthma attack, shoulder pain and heavy seas.
This time she's going prepared with medication at the ready to tackle shoulder problems and asthma, and plans to eat more during her periodic breaks for liquids and food.
She's also protected from sharks by a device that uses a mild electrical current to ward off the predators.
The Florida Straits crossing was successfully completed in May 1997 by 22-year-old Australian Susan Maroney who swam in a cage to protect her from sharks.