A 62-year-old American woman is attempting to swim across the Florida Straits from Cuba for the fourth time and without a shark cage.
Diana Nyad took to the water in the Cuban capital Havana on Saturday and is due to make land at the Florida Keys on Tuesday.
The swimmer has a 50-member crew with her for the 103 mile (166km) distance which is likely to take around 60 hours of swimming to complete.
A device that helps keep sharks at bay by generating a faint electric field that is not noticeable to humans is carried on a kayak that shadows Nyad.
A team of handlers is always on alert to dive in and distract any sharks that make it through.
The athlete has also had to cope with painful jellyfish stings on her lips, forehead, hands and neck.
Last September she was forced to cut short her attempt after a build-up of toxins from jellyfish stings.
Nyad's first attempt to make the crossing was in 1978, when she was 28-years-old.
Shoulder pain, asthma and ocean swells forced her to cut short another attempt in August 2011.
This time, the long-distance swimmer has a specially designed bodysuit to protect her against jellyfish that she wears at night, when the creatures are most active and when the accompanying boats and kayaks switch on red lights - instead of white, which attract them.
The suit does not, however, prevent all stings, including by dangerous box jellyfish.
Nyad swims backstroke at night to keep her face out of the water and avoid being stung, according to the crew, which works from five yachts and includes divers with shark experience as well as jellyfish experts.
The swimmer comes up to her Voyager escort boat, which she is not allowed to touch, around every 90 minutes to fuel up, sipping on a concoction of nutrients, electrolytes and calories through a hydration pack.
Fans can follow Nyad's progress online at www.diananyad.com.
Nyad, who turns 63 on Wednesday, set an open sea record for both men and women by swimming from the Bahamas to the Florida Keys in 1979 - a journey that is the same distance as the Cuba-Florida swim, but a feat she has described as far less dangerous.
And she set a record for circling the island of Manhattan at age 50, clocking in at seven hours and 57 minutes.
In July, British-Australian athlete Penny Palfrey, 49, failed to swim unassisted from Cuba to Florida and had to be plucked from the sea after nearly 42 hours in the water when she could no longer cope with a strong current.