An adventurer from Lincolnshire is attempting to swim 100 miles (160km) non-stop in Loch Ness in the Scottish Highlands.
Ross Edgley, 36, from Grantham, became the first person to swim 1,780-miles around Great Britain in 2018.
His Loch Ness swim, which he said would set a new record, is in support of a sea kelp conservation project.
"It's not just about records" the swimmer said. "There is a much bigger meaning behind this one."
Edgley is making the attempt to raise awareness of the Scottish kelp seaweed forests with support of Skye whisky distillery Talisker and Parley for the Oceans, a non-profit environmental organisation.
"I always say your reasons to continue need to be bigger than your reasons to stop - and with this particularly one we've got a pretty big reason to continue," he said.
"So, hopefully by making a bit of a swimming spectacle of myself - breaking this world record - it can raise awareness for that," he added.
Edgley has prepared for the non-stop swim, which could take up to 72 hours, by training for up to 12 hours a day in the waters around the Isle of Skye.
He has also consumed around 10,000 calories a day to help his body stay insulated from the cold.
After gaining 10kg, Edgley announced the arrival of the "Ross Ness Monster" on his social media channels.
Tomorrow WE GO 🔱— Ross Edgley (@RossEdgley) September 20, 2022
The #WorldsLongestSwim for ocean conservation! Tag friends & training partners & turn on notifications as we will be uploading updates across my social media channels through the swim:
Talking about the challenge, Edgley told BBC Radio Lincolnshire that although he was allowed a support boat, he was not permitted to touch land or the boat.
The crew were there "just purely to throw bananas at me" during the swim, he said.
"It's just me and the Loch Ness Monster."
Cyrill Gutsch, founder & chief executive of Parley for the Oceans, said: "Sea forests are the planet's underwater architects... that are crucially important for planetary health.
"The collective research by our scientific partners will provide vital evidence to support this process and to ensure kelp forests and their carbon sinks are protected in Scotland and around the world."
Edgley is already in the Guinness Book of World Records after completing a rope climb the equivalent height of Mount Everest in 19 hours in April 2016.
That was two months after doing a marathon while pulling a car.
But an attempt in 2017 to swim the 24.8 miles between Martinique and St Lucia while pulling a 100lb tree trunk ended in disappointment.