Physics causes some strange phenomena, like slowly-rotating discs of ice on a frozen river that resemble UFO saucers. But have no fear, science is here to explain away your excitement.

Michigan resident Jason Robinson spotted and filmed this eerie phenomenon in the Pine River in Vestaburg, Michigan, as reported Sunday on MLive.com. Similar discs of spinning ice have been documented before—the Associated Press spotted one back in 2013, for example. While your first guess might be aliens, last year, scientists learned that physics governs these spooky plates’ behavior.

Surprisingly, the physical behavior of melting ice cooling the surrounding river water—not the motion of the river itself—drives the turntable.

A team of physicists from the University of Liège recreated the spinning ice scenario in miniature by putting a 3.35-inch disc of ice in a one foot-wide temperature-controlled bath. In one experiment, the ice floated on its own, and in another, the researchers controlled things a bit more by embedding a nickel bead in the ice block’s center, with a magnet above the disc to hold it in place. In both cases, the warmer the water, the faster the little disc rotated—just like the spinning ice disc in the video above. The researchers published their results in the journal Physical Review E last March.

The spinning occurs because of a quirky property of water: It’s densest at 39.2 degrees Fahrenheit. In their experiments, the scientists measured the flow of the water beneath the ice, and found that the icy disc cooled the water surrounding it. When surrounding water hit the 39.2 degree point, it sank and formed a vortex. This vortex of water whirls the ice floating atop it.

The researchers’ findings can help explain why we don’t see spinning ice in certain environments, like deep lakes and icebergs. Deep lake water is generally already 39.2 degrees, meaning there isn’t a temperature gradient to pull water downwards as it melts off ice. In the case of icebergs, meltwater simply dilutes the saltier seawater rather than creating a vortex, the researchers say.

There are still some mysteries to be solved—for instance, scientists don’t know exactly why the ice forms a disc shape. They suggest two hypotheses: either smaller shards of ice gets caught in a vortex and accumulate into a larger disc, or the ice starts out more irregularly-shaped and is rounded from the spinning.

So the next time someone shares one of these videos and says “aliens,” just go ahead and ruin their day. With science!

[MLive]