US swimmer Diana Nyad said this will be her final attempt at making the 103-mile journey from Cuba to Florida without a shark cage.
Ms Nyad said before beginning the gruelling swim, "I'm learning as I get older, and I'm really getting older now, that there's a fine line between having grace and letting go of something that you have no control over and you're just not going to beat, like Mother Nature maybe."
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Through the day, most of the cloud should break up to leave fine conditions for many with spells of sunshine.
The Eiffel Tower in Paris was plunged into darkness, with the Burj Khalifa in Dubia emblazoned with the Union Jack.
The major party leaders' campaigns for the General Election will remain suspended on Wednesday following the Manchester terror attack.