Ice disks have been blowing the minds of hikers and scientists for over 100 years. Under the right conditions, a thin circle of ice can be seen spinning on an eddy in a river. And thanks to some petri dishes, a magnet, and a small tank of water, a team of physicists has finally cracked why this mystery occurs. 

It was previously assumed that the spinning ice circles were caused by currents in the water. While this is sometimes a factor, there’s more to the story.

The ice disks are typically found in cold climates, most frequently in North America and Northern Europe. While they can be as small as a CD, there have been sightings of ones up to 17 meters (55 feet) wide.

The team of physicists from the University of Liége in Belgium created ice disks 8.5 centimeters (3.3 inches) wide that had a small ball of nickel at the center. They then placed them in a water bath and suspended a magnet above the disks to hold them in place on the surface of the water.

As expected, the disks spun despite there being no currents present in the still water. They then increased the temperature of the water bath, causing the disks to slowly melt. As they increased the temperature of the water, the disks began spinning faster and faster.

This prompted the researchers to find the link between the ice melting and the rotational spinning. At a certain point, as the water gets warmer, it also gets denser (which, conversely, is why ice floats on water). However, when the water melts off the ice disk, it doesn’t simply sink downwards. The researchers found that the water beneath the ice spirals slightly horizontally as it plumes downwards, much like when water drains down a sinkhole and sweeps around in a spiraling rotation.

The team's findings were recently published in the journal Physical Review E.