A strange circle of ice formed on the River Otter, Devon, during the recent freeze (report, January 14). The perfect circle of ice was 3m (10ft) across and spun in a complete rotation every four minutes. It is thought the ice may have formed in an eddy in the river. But other types of ice circles found in North America have appeared as thin ice, or a hole in the ice, in an otherwise frozen waterway, pond or lake.

In 2002 a large circular hole opened up in a frozen lake in Brainerd, Minnesota, measuring 2,130ft (650m) long and 400ft wide. At first it was thought to have formed from warm thermal springs, but thorough investigations failed to detect any trace of warm water or unusual currents below.

In February 2001 a circle of thin ice was seen forming on a frozen pond in Greenbelt, Maryland. Office workers from a nearby block had an ideal view of the thin ice circle gradually appearing shortly after the water froze over.

Ice circles have been reported occasionally in Europe, but not in Britain. However, their origins remain unknown. Some people have suggested that – just like crop circles – aliens and UFOs are involved, although in most cases crop circles have been discredited as hoaxes. Scientists, though, can find no signs of fakery in the ice circles, and they are stumped for a rational explanation.