The vast, rotating circle looks like a UFO or a moon, risen out of the blue water. But the explanation is entirely natural: it is the result of a very rare phenomenon that occurs in rivers in cold regions.
That doesn't make the images of the phenomenon any less astounding to look at. Photos and drone videos show the huge disc spinning slowly in the city of Westbrook, Maine, where it emerged this week.
The photos show the vast scale of the ice circle, compared to the vehicle and even multistorey car park that it are beside it. Local media reports suggest the disc is as much as 100 metres wide – and might even grow more with time.
Rob Mitchell, who owns a nearby building, said the rotating disc had become a home for local wildlife.
“There were ducks sitting on it," he told the local Press Herald. "The ducks were rotating on this big Lazy Susan. It was a big duck-go-round."
When the city government posted images of the disc to Facebook, a number of commentors joked that the it could have an extraterrestrial origin. "It's where the aliens landed," wrote one, while another asked if the city had checked for "space gremlins".
The effect is in fact the result of a set of strange physical effects that cause ice circles to form all by themselves, when the conditions are right. Usually they occur in rivers that are much further away from cities – and are rarely of such a grand scale as the one found in the Maine river – but such phenomena have been seen before.
Scientists say the phenomenon is related to whirlpools, and it might be easiest to think of the disc as a vast frozen whirlpool itself. They are thought to form when whirlpools begin, leaving an area of water that is initially free of ice but which then begins to freeze and capture more ice, thickening and growing as it goes, with the perfect circle staying because the edges are eroded when they touch against the ice at the border.
Such ice circles have been documented to stay around for a significant amount of time, rotating for months.
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