Beautiful, fractal-like patterns have set into the ice over ponds and lakes around the metropolitan area. How they form has been a small mystery for me for weeks.

They can be hard to see from the side, such as from a pond’s edge, but are unmissable up close: regions of darker, clearer ice that stretch outward from a hub in tendrils sometimes several feet long. These arms splay into smaller and smaller branches like the dendrites of a neuron or the roots of a vanished tree.

The shapes don’t seem to be cracks — they’re seamlessly encased in the paler ice around them. While eerily shaped, they also don’t seem to be tentacles reaching from the Upside Down of the Netflix show “Stranger Things,” as a friend of mine suggested.

I’ve found examples around the Louisville Swamp near Carver, in a pond at the Marjorie McNeely Conservatory in St. Paul and in several small lakes within walking distance of Southwest News Media’s office in Savage. One striking specimen featured dozens of the shapes arranged in an orderly row.

This was something I hadn’t seen before. I chalked that up to living for the first time in a place with long-lasting pond ice, but then some locals said they’d never noticed the shapes, either.

Something intriguing in nature that’s new to me and doesn’t have a widely known explanation is basically impossible for me to resist. What follows is my altogether unnecessarily thorough search for an answer.

A vague consensus from local park officials, ice fishers, other experts and Henry David Thoreau, of all people, seems to be that it starts with a gap or breach in the ice. But those holes could have multiple causes, and not all potential explanations require their presence.

Thoreau in his writings on Walden Pond in Massachusetts described “dark figures, shaped somewhat like a spider’s web, what you may call ice rosettes” in that pond’s frozen surface.

Thoreau wrote at one point the ice was covered in water from being pushed downward by the weight of some snow. He cut holes in the ice, and the water flowed down, “somewhat like cutting a hole in the bottom of a ship to let the water out,” carving channels in the ice as it went.

Dark shapes were left behind when those holes refroze, got doused by rainfall and then froze again with a new layer of ice on top, Thoreau said.

His explanation seemed fairly convincing, if a bit convoluted. This winter’s swerving from frigid to above freezing, from snow to rain and back might have given the right conditions. Maybe I’ve just been lucky to be around at the right time.

Joel Vos, a park ranger at the Minnesota Valley National Wildlife Refuge, and other refuge staff suggested a similar drainage process, likely through breathing holes carved by muskrats or beavers.

Vos pointed out clearer or darker ice often simply forms more slowly than the cloudier stuff. That could help explain why the shapes are different from their surrounding ice but still part of the same icy whole.

Another line of thought, however, supposes liquid water runs outward from a given spot in the ice rather than toward it.

An online search can find several such guesses from around the country. A few folks have noted the stars can form when water flows up and out onto ice that has an overlying layer of snow. The water might flow in this branching way because of a kind of wicking or capillary action through the slush.

Howard Mooers, a professor at the University of Minnesota Duluth specializing in glaciers and hydrogeology, said he and his colleagues guessed little upward currents of warmer groundwater might have affected a pond’s freezing in such a way that would leave stars behind.

Warmer water can float on colder water because it’s less dense, so the idea is the warmer fluid spreads outward under the ice, tracing the fractals as it gradually cools.

This might explain why faint lines in some ordinary pond ice sometimes follow the darker shapes’ curves, like iron filings around a magnet, suggesting the shapes and their surroundings formed simultaneously.

Phil Larson, who’s principal at Vesterheim Geoscience PLC and was also at Mooers’ office when we spoke, suggested little gas bubbles from the pond bottom could also play a role. Several of the shapes around Savage show air bubbles trapped in their centers.

The answer in the end could be one of these suggestions or all of them — maybe one situation holds here, another there, or they work in tandem. Maybe we don’t have it yet. The shapes are hidden under several inches of snow for now, but I’m willing to bet they’re still there.

Community editor

Dan Holtmeyer is the community editor for the Prior Lake and Savage papers. He grew up in Nebraska and worked as a journalist in Oklahoma, Missouri and Arkansas before coming to Minnesota in 2018.

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