In cold climates, perfect circles of ice can form in slow moving water resulting in a very strange sight if you’re not familiar with the very rare phenomenon. This video of one such “ice disk” was recorded by hikers in the Scottish Highlands at the head of Loch Fyne.
‘We took a break to fill our water bottles from the burn by the track – that’s when we noticed the ice disk slowly spinning at the foot of a small waterfall,” hiker Dan Brown, 32, told Metro.
From a National Geographic article about another example spotted in Maine in 2019:
A 1997 paper in the Royal Meteorological Society theorized that river water created a whirlpool effect around a chunk of ice, slowly eroding it until its edges were perfectly smooth and circular.
But a 2016 paper added to this theory. Published in the journal Physical Review E, scientists theorized that river currents likely help such disks form initially but temperature changes are what keep them spinning. Warm water is less dense than cooler water, so as ice melts and sinks, it creates a vortex under the disk that causes it to rotate. The warmer the water, the faster the disk spins, they found.