In the frigid waters of Presumpscot River, a gigantic circular ice sheet with a 90-metre diameter spins on itself. Its appearance is surprising and could suggest an extraterrestrial intervention for the most imaginative minds.A circle of iceThis formation is what is commonly known as a circle of ice (or ice disc). Relatively rare, it usually appears on cold streams moving at low speed. It can reach a size ranging from a few centimetres to several meters, but one must admit that the one found in Presumpscot River is giant.'It could beat the world record if anyone were keeping count,' commented physicist Kenneth G. Libbrecht. When locals first noticed this strange iceberg last Monday, it had already been invaded by ducks, sitting on its surface like it was an improvised merry-go-round.A record player on the waterIce disks have intrigued scientists since at least the 19th century. In 1895, mention was made of a 'rotating ice cake' on Mianus River in the journal Scientific American. Since that first observation report, scientists have examined the conditions for the appearance of this strange structure and come to the following conclusions.They form at the base of a whirlpool. 'Since whirlpool water flows more slowly than the main stream, it is more likely to freeze, creating an ice disc,' said physicist John Huth of Harvard University. It is this same whirlwind that gives the disc its rotational motion.Check out the video above to see the incredible ice disc spinning for yourself!