Ice disks can be anywhere from 1 to 200 meters in diameter, but will spin at about the same speed, no matter the size. The reason the disks spin (and, well, almost everything else about them) has thrown physicists and environmental scientists for a loop for more than a century. It was thought that ice disks spin due to eddies, which are little spinning currents that occur when water flows over rocks or in an enclosed space. This may be part of it, but eddies don't explain why small and large ice disks rotate at similar speeds.

So what's happening? A 2016 study published in the journal Physical Review E found that the spinning is caused by the water that melts off the disk itself. Because liquid water that is just above its freezing point is at its densest, the melted water sinks after it falls off the ice disk. But it doesn't sink straight down. Water spins as it sinks, just as it does going down a drain. As ScienceAlert reports, "the water underneath the ice discs was spinning as it went down, and this spinning was pulling the discs around. So these ice discs are spinning because they're melting, and that melting water spins as it sinks below the discs."

Although scientists have cracked the case of the spinning ice disk, they still haven't totally conquered the disk part. Why are ice disks circular? Who knows—maybe it is aliens after all.