When Russia's harsh winter weather arrives and its lakes freeze over, most swans migrate to warmer climates. But more and more choose to spend their winters at a lake in the mountainous Altai region of Russia.

The air temperature here can drop to minus 40 degrees Celsius, yet the lake never freezes. This is because it is fed by underground hot springs, keeping it at five or six degrees above zero.

The name 'Svetloe' means 'Clear Lake' and it deserves the accolade – the water is so transparent you can see all the way down to the lake bed.

Whooper swans arrive at the lake every December. This is a fairly recent phenomenon – locals say the birds first appeared in 1967, and number only 15. Now there are about 350, with numbers rising every year.

When warmer weather arrives in March, the birds head to their nesting areas in the far north of Russia, and return to the lake nine months later with their chicks.

The lake, which forms part of a wildlife preserve, is also home to more than 2,000 ducks. A team of keepers protect the birds from hunters and ensure they have enough to eat.