The formation in the Presumpscot River was widely shared on social media and has drawn comparisons to an alien spacecraft, a carousel and the moon.
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The oddity drew a crowd that stood mesmerized by the slow-turning disk Tuesday afternoon in Westbrook.
The ice formation is believed to have formed naturally in a part of the river where there's a circular current that creates a whirlpool effect, said Tina Radel, the city's marketing and communications manager.
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Locals say they've seen smaller ice disks before, but nothing like this one, Radel said.
Rob Mitchell, who owns a nearby office building, spotted the alien-looking disk Monday morning and immediately notified the city.
Officials say the disk is spinning slowly counter-clockwise, and has served as a big raft for ducks and other birds.
"The ducks were rotating on this big Lazy Susan," Mitchell said.