Steven Daly, an expert in river ice hydraulics at the Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory in Hanover, N.H., said his agency generally got just one or two reports of rotating ice disks in the United States each year.

They’re not usually this big, though.

Kenneth G. Libbrecht, a professor of physics at the California Institute of Technology in Pasadena, Calif., who has studied ice physics, said that most ice disks he had seen were in the 20- to 30-foot range. Local officials estimated that the Westbrook ice disk was about 300 feet across.

“It might be a world-record size, if anybody were keeping track,” Dr. Libbrecht said.

Tina Radel, the marketing and communications manager for the city, filmed a drone video after Rob Mitchell, a local business owner, alerted her to the remarkable sight on Monday. After posting it, she spent Tuesday fielding calls from reporters around the country.

“It’s been an overwhelming reaction,” she said. “People are loving it.”

In fact, The Portland Press Herald noted the ice disk had Westbrook buzzing “almost as much as when city police spotted a giant snake eating a beaver in roughly the same location in June 2016.”