Some call it sport, some call it fun, others swear by its health benefits, rattling off a list that would make a snake-oil salesman blush.
Ice swimming will ease arthritis and relieve rheumatism, cure depression and keep colds and flu at bay, devotees say.
Headaches or sinusitis? Having trouble sleeping? They insist things will improve if you go regularly to the "avanto," the Finnish word for a hole in the ice.
A dip in a frozen lake during the Arctic winter might not seem the most appealing pastime, but for many in Finland it is a weekly habit, as regular as skiing or skating.
"It is really something very special, something excellent," said 85-year-old Leo Wanamo, climbing out of a hole cut through about half a meter of ice after a short dip in the sea water below.
The retired Finnish army colonel praises the invigorating effects of cold water as he dons a sheepskin coat against a -15 Celsius breeze.
While Wanamo does not make any specific claims for the health benefits of an icy dip, he delights in being fitter and healthier than his peers who have not been hooked.
"The first time is the worst. After that it is very, very fine."
Medical researchers say studies show ice swimming can help to treat some illnesses or rehabilitate injuries, but add there is not much proof of any effectiveness in preventing ill-health.
"In studies on rheumatic diseases, cold treatment reduces aches and pains, and in some cases, depending on the treatment, patients can do without painkillers," said Juhani Smolander, a medical researcher who is studying the clinical benefits. "It doesn't cure the disease, but it does relieve the symptoms."
But researchers have found a positive psychological impact: people who immerse themselves in icy water regularly say they feel better than those who simply stay indoors, peering out at the brave or foolhardy jumping through the ice on a frozen lake.
"My skin is smoother and softer," said Mariia Yrjo-Koskinen, organizer of the World Winter Swimming Championships in northern Finland. "And it is even good for my husband, or so it seems," said the 43-year-old.
About 120,000 Finns go ice swimming regularly, but about five times that number - roughly 10 percent of the population - have taken the plunge at least once.
Yrjo-Koskinen said for some people it is serious exercise, but also simple fun.
That was what attracted many of the 1,000 or so swimmers to the championships, which included Australians, Canadians and even some from Kazakhstan, racing against each other in a 25-meter, eight-lane pool cut through the ice.
"If you have a hangover, are angry, or have something on your mind, cold-water swimming wipes it clear," said Stephen Hodnett, a Dubliner racing at the championships in Oulu.
"It just clears it all. The only problem is that you get cold."
In subzero conditions, it is warmer in the water than out in the open air, which is below freezing for most of the winter in Finland.
As you descend into the avanto, first comes the shock of the cold, sudden and thorough. Then comes tingling and sometimes a slight dizziness, almost like vertigo, as the cold seeps well into your bones.
A mild numbness follows as you linger in the water briefly.
Then, as you climb the ladder out of the swimming hole, the cold wind envelops you, frosting the hair on the nape of your neck. Suddenly you feel quite dry, and a strange feeling of serenity and well-being starts to seep outwards.
Yrjo-Koskinen said that was why many people got into the habit: "It's like getting high, in a very healthy way."