Cliff Bates was hiking the Appalachian Trail in Northwest Connecticut with his dog in 9-degree weather Jan. 1 when he saw slowly-rotating discs of ice on the Housatonic River that resembled UFO saucers.

The New Milford resident remembered seeing this strange phenomenon before in a video during which the ice just sat there in its own little cocoon. Recalling that it was a pretty rare occurrence, he thought about trying to get a closer look. Since the terrain was pretty steep, Bates had to settle for taking a photo from Bull’s Bridge in Kent.

“It was just down there in the was this kind of weird triangle and the ice chunks slowly circled inside that but never really left it,” Bates said, who forwarded Hearst Connecticut Media a copy of the photo.

It turns out these thin circular slabs of ice that spin freely are referred to as either ice discs, ice circles or ice pans. The slabs form in a water — such as a river — where accelerating water creates a force that breaks off a chunk of ice and twists it around. As each disc rotates, it grinds against surrounding ice — smoothing it into a circle, according to a Daily Mail article.

The discs are found in the cold climates of North America and Europe, Ryan Hanrahan, chief meteorologist for NBC Connecticut, said the past two weeks are the longest stretch on record of consecutive subfreezing temperatures. Given the extreme cold, he said it’s not a surprise to see things like the ice discs across the state.

Along with being rare, they are a more recently-documented phenomenon. Gil Simmons, chief meteorologist for WTNH said research has been ongoing for only 100 years, trying to understand the occurrence. One of the earliest recordings is of a slowly revolving disc spotted on the Mianus River and reported in an 1895 edition of Scientific American.

While it was more recently believed the ice discs spin due to eddies, little spinning currents that form when water flows over rocks or into an enclosed space, a study performed in 2016 modeled how it is caused by a different phenonmenon. Published in the journal Physical Review E, researchers from the University of Liége in Belgium found that the change in temperature — and not flowing rivers — caused the spinning.

Researchers came to this conclusion by recreating the spinning ice scenario in a lab. They first started by freezing tap water in a petri dish and putting it in temperate water, which resulted in the ice floating on its own. They then conducted a second trial with more control by inserting a nickel bead in the bottom center of the ice disc and placing a magnet at the bottom of the tub to more effectively hold it in place, the paper stated.

In both cases, the warmer the water, the faster the little disc rotated due to a weird property of water. Most substances get denser as they get colder, which makes their solid forms denser than their liquid forms. However, water follows this trend to a point, but then the density goes back down just before freezing. This is why ice floats; water is at its densest a couple degrees above its freezing point, according to the paper.

When the water melted off the discs, it was denser than anything around it, and it fell straight down. As the melting water would sink, it would spin, forming a whirlpool. This vortex of water is what whirls the ice floating atop it, the paper explained.

As Simmons concisely puts it: The melting water causes the ice disc to rotate in the body of water because of different temperatures and densities.