It started with one family going for a daily dip in the Irish Sea off the County Down coast.
Now dozens have joined in, and they have formed a group known as the Donaghadee Chunky Dunkers.
They go into the sea every day, no matter how cold it is.
They only have one rule - no wetsuits are allowed.
By their own admission, they are all different shapes and sizes.
Most of them are from the Donaghadee area, but some travel from as far away as Dungannon at weekends to join in.
Many of the dunkers are women in their 40s, 50s and 60s.
"It's like a cheap, mid-life crisis really," said swimmer Lyn Magill. "It's not a sports car, it's just going into the sea.
"And it's not just about the good feeling afterwards, it's also about the social side, as you make so many friends."
Meanwhile, Sarah Holland said: "It's addictive. It just makes you feel amazing."
The time of the daily dip changes: The swimmers like to go in at high tide.
'A really good feeling'
Rather than front-crawl, most of the dunkers do some gentle breaststroke and stay in the water for 10-15 minutes.
Allison Allen said her brother set the trend back in 2012.
"I went in for occasional dips, but it was 2013 before it all really started," she said.
"In 2016, it started to grow and grow. Then a Facebook page evolved. Now around 30 a day go in. More at the weekends."
Ms Allen added: "It's cold, but you become accustomed to it.
"You just adjust the length of times you swim for, depending on what the water is like."
Lesley Aiken said the swim made her feel great.
"I'm cold when I come out but it's a really good feeling," she said.
"A few people have mentioned that they have a wee bit of depression and they feel that it lifts them."
On Saturdays and Sundays, numbers increase, sometimes approaching 100 people.
Although the organisers are not in favour of wetsuits being used, they actively encourage members of the group to ensure they can warm up quickly afterwards - by bringing hot drinks, extra clothes and even hot-water bottles.