What in the name of Neptune would make anyone splash into frigid Lake Michigan on New Year’s Day?

That’s what 150-plus like-minded cold water lovers did during the annual Polar Bear Plunge at Simmons Island beach in Kenosha.

Lainey Estrada, 16, of Kenosha, said it was a chance to boost her grade in Eric Herbrechtsmeier’s Algebra II class at Indian Trail Academy, along with seven of her classmates who accepted their teacher’s challenge.

“This is extra credit for me. (Herbrechtsmeier) said if we go in, it’s 30 extra credits. I’m not doing so good in that class. So, I kind of need it,” Estrada said as she shed her warm outer wraps to get down to her swimwear minutes before organizer Dan Vaccaro unleashed the plungers down a snow-cleared path in the sand at 11 a.m. for the run into the rolling waves.

The motivation for one of Estrada’s fellow Algebra II classmates, Heather Zons, 16? “Because I have an A-minus, and I want an A,” Zons said.

Herbrechtsmeier, it turns out, was there, too, and not just for moral support. He leapt into the icy waves with his students. “Oh, yes, I’d never ask my students to do anything I wouldn’t do,” he said with a laugh before doffing his outer winter clothing and joining the mad rush into the water.

“What’s incredible is all those students are currently passing my class. Everyone’s like a C or higher,” Herbrechtsmeier, 38, a native Kenoshan, said after re-emerging from the lake and thawing out. “I’ve been doing it since I’ve been at ITA, 12 years. We talk about creating community outside the classroom. This is part of that. That is a life lesson right there.

“That is the limit you’re body can go. It tests you,” he said. “It lets you know how much you can endure within a safe realm. You have to push your boundaries to see what you can achieve. That’s why we all do it.”

Justin Jabs, 17, of Kenosha, wasn’t motivated by academics, nor did it appear to be anything as lofty as community building. At 10:15 a.m., Estrada sent him a message on his computer telling him about the plunge. “So, I’m here,” said Jabs, who, like Estrada, was a first-timer at the event.

Mark and Theresa Lewenow, both 29, of Union Grove, are veterans of the goose bump gallop into the glacial drench. “I talked him into it. This is my 10th year, ninth for him,” Theresa said, hurrying to wrap herself in a heavy overcoat after returning to dry land. The 11-degree air temperature made her tiny two-piece bikini even less weather worthy than the 34-degree water.

“I’m fine, except for my feet,” she insisted. As for the slender plunger’s choice of swimwear: “The little suit is easier to take off when it’s wet.”

Husband Mark, the assistant chief of the Kenosha County Scout Rescue — who are always on hand to provide first aid at the plunge — apparently didn’t take that into account. “I just wore a pair of work shorts,” he said.

Whatever gear participants chose, all appeared to have a good time.

For Carlos Arcos, 38, of Kenosha, it was a self-baptism of sorts. “This is my first time. I’ve been here before lookin’. This time, I decided to go in. It was fun,” he said.

His wife, Megan, 33, opted to stay bundled up on dry land while Carlos dunked himself. “I’m not crazy. I need my feet. He said he wanted to start the New Year out clean. So, he did,” she said.

Melvina Klemm, 66, of Kenosha, waited until all the other plungers were scurrying back, wrapped in blankets and ducking into the warmth of waiting vehicles before she waded into the waves bedecked in what appeared to be a short-sleeved black half-leotard. Afterward, clad head-to-toe in Green Bay Packers green and gold, she linked her penchant for plunging to her parentage.

“I’m of Finnish descent, both parents. So, we’re supposed to go in the sauna, roll in the snow and jump in the lake,” Klemm said, laughing.

On a more serious note, Klemm, a paraprofessional at Gateway Technical College, said for her it was all about remaining vital. “I need to keep active. It’s part of my life strategy,” she said. “It was refreshing. I hope to keep going a while longer. If you don’t use it, you lose it.”

Vaccaro, owner of Diver Dan’s Scuba Center, has been doing plunges for 35 years, some of his first with siblings and other family members. This time around, an apparent virus ironically kept him shorebound. “For many of the people who come it’s a tradition. I see a lot of them here every year,” Vaccaro said. “When I first started in high school, it was a fundraiser for food pantries and things like that.”

The only donations suggested for this year’s plunge were to help Scout Rescue and defray the cost of hot cocoa offered to plungers free of charge. But with Keith Bosman and Jim Kreuser — new in office, respectively, as mayor of Kenosha and Kenosha county executive — Vaccaro hopes to lure them as participants next year and once again turn the event into a local fundraiser for a worthy charity.

The last time that happened, their predecessors, former Mayor John Antaramian and former County Executive Allan Kehl, helped bring in $27,000 in donations.