R OCKPORT — A half-dozen middle-aged guys are standing in a backyard, in bathing suits, staring at a rubber tub filled with water and 120 pounds of ice from Market Basket, daring each other to go first. It’s possible the phrase “don’t be a wimp” is uttered.

This has become a Sunday morning ritual for this crew, a bracing bit of masochism known as an “ice bath.” Technically speaking, it’s been around for as long as water has been cold, but in recent years the practice has skyrocketed in popularity, thanks to wellness “influencers” who tout it as the cure-all for just about everything — from inflammation and weight loss, to depression and anxiety, to aging itself.

But while the cold is unmistakably hot right now, parsing out fact from salesmanship in the booming world of “cold therapy” is a challenge. The research is promising, but thin, while the positive anecdotes — “It will change your life!” — tend to dominate the conversation.

For this group of suburban dads, there’s certainly a lot of interest in the potential health benefits; they’re the sort who might use the phrase “I heard about it on a podcast.” But why they continue to subject themselves to a sensation that sets off alarm bells in the amygdala is more elemental: They like the way they feel after the painful plunge, the unmistakable sensation that some sort of reset button has been hit.

Or at least that’s how I explain it to the bewildered cashier when it’s my turn to go to Market Basket and fill an entire shopping carriage full of ice.

“The very first time I went into the cold water, my body instantly told me I was doing something good,” said Bill Purnell, who has quickly emerged as an ice-bath influencer. Just before COVID hit, he lost his job at age 58, was well overweight, and with gyms suddenly closed he “didn’t know what to do.”

Conveniently, strangely, weirdly, Purnell had a small water tank in his Medway backyard because he had a magic act with his wife and she would do underwater escapes. Anyhoo, one day that March, as the world was shutting down, Purnell filled the tank with frigid water, soaked for a few minutes, and hit his reset button. Since then, he credits his daily plunges with helping him lose 35 pounds and getting into the best shape of his life (helped by the habit of doing pushups to warm back up).

He’s since studied with Wim Hof, the Dutch guru who has done more than anyone to promote the benefits of the cold, and Purnell has become something of a guru himself: he runs a Facebook group called “Cold Water Transformation” and offers private coaching sessions for $125 to teach people his “Believe. Breathe. Achieve” approach to cold therapy. (He also likes to say “Be bold, get cold, and never grow old.”)

While athletic training rooms and spas have long used cold tubs as treatment for swelling and inflammation, it was the pandemic that finally pushed it into the mainstream. After lockdowns forced fitness into the backyard, many athletes and celebrities brought their cold tubs with them. (And then posted about them on social media.)

“With COVID, all of a sudden everyone wanted one in their backyard,” said Scott Burt, the owner of ColdTub, a Cape Cod-based company that started in the mid-2000s when Burt created the first commercial-grade tub for the Boston Bruins, after hearing a friend who was a trainer for the team complain about the difficulty of maintaining an ice machine and constantly changing the water and cleaning the tubs.

Burt said his company pivoted from large units built for entire teams to a new line of smaller units that cost between $11,900 and $15,900. One of the first people to request one was Lebron James, he said. ColdTub now has a rush of rivals marketing backyard cold plunges, so much so that The New York Times recently dubbed the competition the “cold wars.”

But while athletes and trainers have long sworn by the ability of cold tubs to make their muscles feel fresh, the evidence on the benefits is scant and often conflicting, and most everything that passes for a study involves only a tiny group of participants.

What we do know a lot about is the dangers of cold water, said Dr. Stuart Harris, chief of the Division of Wilderness Medicine at Massachusetts General Hospital.

“There are so many things that happen when you go rapidly into cold water, but generally there is a clamping down of blood vessels, especially in the skin, and a shunting of blood to the interior as your body tries to defend the core,” he said.

For people who fall into cold water unprepared, the biggest concern is that the “cold shock response” leads to gasping, which can quickly lead to drowning if the head is underwater. Even in situations where the plunge is intentional, there are concerns that the sudden cold can cause heart attacks and arrhythmias.

But Harris said that if you are healthy with no underlying conditions and safe about how you do it — never alone — then cold plunges shouldn’t be too much of a concern. Which is unfortunate because I was secretly hoping he would talk me out of it.

“It’s definitely Type II fun,” Elaine Howley told me. “It’s way more fun after it’s over, but the anticipation can be stressful.”

Howley, 44, a freelance journalist from Waltham, is one of the legends of New England ice swimming. She’s swum an “ice mile” — to qualify, the water needs to be 41 degrees or below — and is training for a second one this winter.

She’s also the ringleader of the L Street Ice Swimmers, a hardcore crew who congregate in South Boston to follow a regimen where they “swim down the temperatures,” meaning they go in the water as much as possible during the fall to let their bodies acclimate, though even that has its limits. It never stops being painful, Howley said, but it also never stops being a reset.

“You flood the system, do this crazy thing for a few minutes, and start over a little bit fresher,” she said. “Some people talk about feeling high.”

Which is the part that keeps me coming back, but it is also the toughest to explain. When I get back in the car after our Sunday ritual, I feel . . . Zen. Calm. Willing to let people cut me off in traffic without reaching for the horn or the finger.

And recently, those drives have caused me to look back differently on a childhood memory that I had perhaps blocked out.

Before there were the L Street Ice Swimmers, there were the L Street Brownies, a huge group of men who hung out at a bathhouse a few blocks from my house, and were famous for the fact that they swam year-round in nature’s cold tub, the North Atlantic, while somehow managing to stay permanently tan (“Brownies” was a gentler moniker than “shoe leather”). But looking back now, it dawns on me that they had one other important trait: they were all about a million years old.

They swore that cold water and sunshine were the fountain of youth. They also swore by being naked, which is a tricky visual for a kid of a certain height (see: “million years old” and “shoe leather”).

But let’s call that a Type II memory. Wasn’t fun to experience in the moment, but in retrospect there was something important I took from it.

So when I see the fellas this Sunday, I guess I need to go naked. I heard somewhere that it’s good for you. Probably on a podcast.

Billy Baker can be reached at billy.baker@globe.com. Follow him on Instagram @billy_baker.

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