Thousands of mystery ice balls washed up in the Gulf of Finland in north-west Russia.

Experts were baffled by the strange 17cm long snowballs, which started to appear earlier this month in Zelenogorsk, Saint Petersburg.

Some passers-by described as resembling ''giant whale eggs."

Conservationist Ilya Leukhin said the ice balls may have been caused by oil pollution in the water.

She said: ''Seawater is a solution of a huge amount of salts, and its crystallization proceeds unevenly during freezing. Oil has a freezing point higher than water, so it could be those water molecules formed around the oil spots and formed a spherical body.''

Another explanation came from the Arctic and Antarctic research institutes that observed similar balls on the banks of the Ob River last year.

They said that after the snowfall, "ice fat" is formed, or scientifically, a sludge, and then during the thaw, it is blown away by the wind and the wave rolls into a ball.

Thousands of mystery ice balls washed up in the Gulf of Finland in north-west Russia.