Marathon swimmer Penny Palfrey has powered through the Straits of Florida as she attempted a record 103-mile (166km) unassisted swim from Cuba to Florida that tests the limits of human endurance and the will of the high seas.
Slathered with sunblock and lubricant to prevent chafing, the British-born Australian bade farewell to onlookers, dived head-first into the calm, bathwater-warm seas off Havana and began stroking slowly northward with a kayaker as escort.
"There's a lot of work that's gone into this over the past year," Ms Palfrey said, thanking her husband, her team and Cuban officials who helped facilitate the trip. "It's all coming together, and so exciting to finally get to this point where we can get started."
A member of Ms Palfrey's crew was tweeting to fans, while a webpage updated her location every 10 minutes or so based on data from a GPS device worn by the swimmer. The site showed her making steady northward progress through the Straits of Florida, where the Gulf of Mexico and the Atlantic Ocean meet.
"Swimming strong. Great conditions. Some jellies, but not bad," read one tweet.
As she hit 10 hours in the water Ms Palfrey had travelled about 18 miles from Cuba, said Andrea Woodburn, who was managing her Twitter and Facebook accounts. Ms Woodburn said the evening forecast called for thunderstorms and some gusty winds, but otherwise their weather expert reported the "situation is about as good as it gets in the straits".
Ms Palfrey will have to fight through physical and mental fatigue while fending off dehydration, hypothermia and dangerous marine life. She estimates that it will take 40 to 50 hours to make the crossing, and the currents will determine where in Florida she comes onshore.
If Ms Palfrey succeeds, she will go in the record books as the first woman to swim the straits 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 made two unsuccessful cageless attempts last year on either side of her 62nd birthday, but had to call them off due to a debilitating asthma attack and painful Portuguese man o' war stings. She 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 was wearing a regular sporting swimsuit instead of a wetsuit, and planned to put on a porous, non-buoyant Lycra bodysuit that provides cover down to the wrists and ankles whenever jellyfish may be a threat.