A team of six women are attempting to become the first ever ladies' relay to cross the North Channel twice in one sitting.
The women will attempt a two-way swim of the Irish Sea between Northern Ireland and Scotland, meaning that as soon as they reach Portpatrick they will turn back towards Donaghadee. The Ocean Walker Ladies' Relay team will be led by veteran swimmer Caroline Sims and will face a gruelling 36-hour, 70km swim through jellyfish-infested waters.
If they succeed, it will be the first time such a feat has been completed.
The Irish Long Distance Swimming Association will invigilate the swim to ensure its rules are met in order for the double crossing to count as a world record.
The swimmers, aged between 34 and 50, will each battle through what is widely regarded as one of the most difficult sea crossings.
The team are raising money for for RNLI, The Whale and Dolphin Society, and Parkinson's UK.
Captain Ms Sims said: "I think the reason it's never been done before is that it's very hard to do.
"It's very cold, the tides are strong and there are a lot of jellyfish. The challenge is there."