The image above might suggest someone dropped a small planet in the middle of the Presumpscot River in Westbrook, Maine, but (shocker) that’s not the case. It’s actually a big disk made of ice that’s slowly rotating in place.
It would be reasonable to think the river's current is propelling that rotation, but research suggests the object may be spinning all on its own. Giant disks like this form not infrequently during the cold winter months, and in 2016 some physicists decided to get to the bottom of the phenomenon. The group created a miniature ice disk and floated it in a tank, where they determined that the rotational force was generated by a vortex that formed beneath the disk. Water, you see, is at its most dense at 4°C (that’s 39.2°F). As the ice of the disk cools down the fluid surrounding it, that water reaches the four-degree mark and sinks. It flows down and horizontally, creating a swirling vortex. And the larger the temperature gradient—meaning the warmer the river water is compared to the frozen disk—the faster the cooling water will sink. That means the disk spins faster, too. it’ll spin because the cooling water will sink faster.