They look too perfect to be natural—large circles of ice slowly spinning in rivers.
Video captured in 2016 shows a seemingly symetrical circle of ice in a river near Omsk Oblast, a region in central Russia that sits just north of Kazakhstan. At first glance, the ice seems to be barely moving at all, but closer looks reveal a slow, steady spin. Even more bizarre is the seemingly perfect symmetry of the disk.
According to the filmmaker (who did not provide their name), the spinning circle of ice measured an estimated 50 feet in diameter.
Early theories of why the disks formed centered on erosion. Ice disks observed in 1987 and 1994 were in the path of flowing river waters. A paper published in 1997 by the Royal Meteorological Society theorized that flowing river water created a whirlpool effect. As the ice spun, researchers theorized, the borders eroded into a circular shape.
A study of ice disks performed in March of last year modeled how this wasn't quite the case.
Published in the journal Physical Review E, researchers from the University of Liege in Belgium found that temperature changes—and not flowing rivers—prompted the spinning. As water warms it becomes less dense, and as it's cooled by surface ice, a vortex forms.
Researchers came to this conclusion by replicating in a lab how temperature changes water density.
They began by freezing tap water in a petri dish and placing it in temperate water, which resulted in the ice floating on its own. To add more control to the experiment, they conducted a second trial by embedding a nickel bead in the bottom center of the ice disk and placing a magnet at the bottom of the tub, which held the ice more effectively in place.
As the tub was warmed, the ice disk was observed speeding at a faster rotation. Warming temperatures created a vertical vortex that could effectively prompt spinning in a disk that had been still.
Researchers concluded that flowing river water was needed to stimulate the vortex into spinning, meaning the effect was unlikely to be seen in land-locked lakes or melting icebergs.