WESTBROOK, Maine (AP) — Think of it as a crop circle, but in a river.

A curious formation that's getting attention in Greater Portland is actually a spinning ice disk that is roughly 100 yards (91 meters) wide.

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.

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.

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.