A 62-year-old endurance swimmer has set off on her fourth bid in three years to become the first person to swim from Cuba to Florida without a shark cage.

Diana Nyad set off on the 110-mile slog from Havana at 9am on Saturday. The Florida Straits is a dangerous, shark-infested stretch of water, where jellyfish stings often curtail such bids. It is also prone to sudden, violent storms.

If she succeeds, she will bag the world record for the longest unassisted open ocean swim.

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Nyad, New York-born and Florida-bred, tried the journey three times in 2011 and 2012. Her last attempt was cut short amid boat trouble, storms, unfavourable currents and jellyfish stings which swelled her face.

Australian Susie Maroney successfully swam the Straits in 1997 with a shark cage. As well as protecting the swimmer from predators, a cage has a drafting effect that pulls a swimmer along.

In 2012, Australian Penny Palfrey swam 79 miles toward Florida without a cage before strong currents forced her to abandon the attempt, and in June her compatriot Chloe McCardel had a similar bid cut short by jellyfish stings.

Nyad also made an unsuccessful attempt in 1978 with a cage.

She said: "I admit there's an ego rush. If I - three days from now, four days from now - am still somehow bringing the arms up and I see the shore ... I am going to have a feeling that no one yet on this planet has ever had."

She said she'd understand if people asked "why would I come back to a place where maybe I'm lucky I didn't die before," and that this will be her final try - though she has said that before.

Likening her past despairing comments after failed attempts to the sadness after a failed marriage, she said: "Every person who's married, the day after they get the divorce they say, 'Never again!'

"But you need to heal, your heart needs to heal, and pretty soon not all men are bad again."

She hopes a new silicone mask will protect her from jellyfish this time, and will at times wear a full bodysuit, gloves and boots.

She will also use equipment that generates a faint electrical field around her, to deter sharks, and be followed by a 35-person support team in five boats.

"My adrenaline is pumping very hard," she said. "Which means in one half I'm excited. I did all the training. The body is ready. My mind is ready. On the other hand, I admit I'm scared."