An unusual feature has intrigued and enthralled the world since suddenly appearing – or at least becoming Internet famous – this week in Maine.

A huge spinning ice disk has formed in the Presumpscot River in the city of Westbrook, and people looking for explanations are claiming everything from aliens and crop circles to a giant duck Lazy Susan.

Alas, it may not be aliens (we’ll let you know when it is), but it is a fabulously photogenic example of a rare natural phenomenon, one that stumped scientists for years.

This one is particularly large, stretching about 100 meters (300 feet) across. According to the New York Times, only one or two are reported a year in the US, and usually, they are comparatively diminutive at 6-9 meters (20-30 feet) in size.

“It might be a world-record size, if anybody were keeping track,” Dr Kenneth Libbrecht, professor of physics at the California Institute of Technology, said.

The perfectly round disk is spinning counter-clockwise at about the pace of a brisk walk, according to locals. It first made waves across the US thanks to incredible drone footage filmed by Tina Radel, marketing and communications manager for the city, alerted to the sight by local business owner Rob Mitchell. 

“There were ducks sitting on it. The ducks were rotating on this big Lazy Susan. It was a big duck-go-round,” Mitchell told The Portland Press Herald.

The disk appeared in the river on Monday and doesn’t seem to be going anywhere. Not only has it not stopped spinning, it seems to be growing in thickness.

So, what is actually going on here?

As Science Alert points out, the confusion of how these duck carousels form goes back to at least the 19^th century, with one of the earliest reported sightings dating to 1895, when a reader wrote into Scientific American having spotted a “revolving ice cake” in the Mianus River, New York.

The easy answer is that cold, dense air meets an eddy – a circular movement of water that causes mini whirlpools – in a river, freezes, and begins to spin. The water inside the eddy is moving slower than the current so is more likely to freeze, and as the ice turns, it bumps into rocks or more ice and gets shaved down into its circular shape. 

 

However, in 2016, scientists tested this out by recreating the spinning disks in lab conditions and found something else going on. They realized that if eddies were solely responsible, then smaller disks should rotate at a faster rate than bigger ones – only they don’t, they all seem to spin at around the same speed.

They also found that disks in still water still began to spin. It turns out, as water melts off the disk, it flows down to under the center of the disk, creating a little vortex that makes the solid ice rotate.

Personally, we’re sticking with duck carousel.