Marathon swimmer Penny Palfrey has set out on a record bid to cross the Straits of Florida, a 103-mile (166-kms) unassisted swim testing the limits of human endurance.
Slathered with sunblock and lubricant to prevent chafing, Ms Palfrey bade farewell to onlookers, dived headfirst into the bathwater-warm seas off Havana and began stroking slowly northward.
She estimates that it will take her 40 to 50 hours to make the crossing.
The British-Australian dual citizen will have to fight through physical and mental fatigue while fending off dehydration, hypothermia and dangerous marine life.
If she succeeds, she will go in the record books as the first woman to swim the Straits of Florida without the aid of a shark cage. Instead she is relying on equipment that surrounds her with an electrical field to deter the predators.
Australian Susie Maroney made the crossing in 1997 at age 22, but with a shark cage.
American Diana Nyad has made three unsuccessful cageless attempts, including two last year just before and after her 62nd birthday. She called those off due to a debilitating asthma attack and painful Portuguese man o' war stings, but plans to try again this summer.
Ms Palfrey, a 49-year-old mother and grandmother, is more than a decade younger than Ms Nyad.
She is wearing a regular sporting swimsuit instead of a wetsuit, and plans to put on a Lycra bodysuit that provides cover down to the wrists and ankles whenever jellyfish may be a threat.
That is particularly the case at night, according to her support team of more than a dozen navigators, handlers, kayakers and medical personnel who are escorting her on the 44-ft (13-metre) catamaran Sealuver.