This strange almost perfect 'ice circle' has appeared on a frozen lake in Siberia.

While scientists have ruled out UFO involvement, they are puzzled as to how the mysterious, 2.5mile-wide geological phenomenon has formed in Lake Baikal.

The ice circle measures 2.5miles across and was spotted on Lake Baikal, the largest freshwater lake in the world

It was spotted at the southern edge of the lake while another was seen near the centre. The images were taken by astronauts aboard the International Space Station circling 220 miles above Earth. They charted the progress of the circles when they first appeared on April 5 until April 27 when the ice was beginning to split apart.

'Throughout April the circles are persistent,' a Nasa spokesman said.

'They appear when ice cover forms, and then disappear as ice melts. The pattern and appearance suggest that the ice is quite thin.'

Ice cover changes rapidly at this time of year on the Russian lake and can melt and freeze overnight. Scientists believe a spurt of warm water rose to the surface creating the distinctive pattern but are puzzled by the source of heat.

The two mysterious circles were pictured by astronauts aboard the International Space Station

Hydrothermal activity and high heat flow has been observed in other parts of the lake but one of the circles appeared near the southern tip, over relatively deep - normally cold - water.

Lake Baikal is in a rift valley and is the largest (by volume) and deepest (5,400ft) fresh water lake on Earth. The World Heritage site is also one of the world’s oldest lakes aged between 25 to 30million years old. Sediment deposited on the bottom is up to 4.3miles deep.

The lake contains rare fresh water seals and several species of fish found nowhere else on the planet.