How to explain the strange ice circle that rotates like a time-travel door?

In winter, rivers and lakes can freeze - this is not uncommon.

But if a circle appeared on the frozen river surface - perfectly round - and slowly rotated, it would be very strange indeed.

Recently, many people were surprised to see such a circle on the Betra River (the name of the river is written slightly differently in different sources, probably due to the local language) in the Republic of Bashkortostan, Russia. On the frozen river surface, a circle that looked like it was cut out precisely was slowly rotating.

Here is the video:

Source: Kirill Bakanov.

Netizens were amazed by this strange circle. Some commented that it looked like a door leading to another dimension like in a movie or a fantasy story. Others felt that this was a supernatural phenomenon.

So what is it actually?

This phenomenon is exactly what it sounds like : Ice Circles. During the winter, in very cold places, if the conditions are just right, this strange circle will appear.

Ice circles, also known as 'ice discs', 'ice pans', or 'ice cakes' , form in water when ice collects in the middle of a stream, right in the middle of a vortex, according to Tree Hugger .

An ice circle forms on the Presumpscot River in Westbrook, Maine, in January 2019. (Photo: Tina Radel/ City of Westbrook via AP).

As EarthSky explains, eddies in water tend to travel in circles. In the winter, ice crystals in slow-moving water can gradually coalesce, forming a circular 'carpet' of ice. The water holds this ice disk in place as it slowly rotates. As the disk rotates, it bumps into other pieces of ice and is essentially 'sanded' until it is perfectly round. The result is a circular disk of ice with surprisingly smooth, clean edges.

Here is a video of another ice circle:

(Source: JJ).

Ice circles are a very rare phenomenon. A report of an 'ice cake' on the Mianus River in the United States in 1895 is considered the first recorded instance of the phenomenon.