After more than two decades of drilling in Antarctica, Russian scientists have reached the surface of a gigantic freshwater lake hidden under miles of ice for some 20 million years — a lake that may hold life from the distant past and clues to the search for life on other planets.

Reaching Lake Vostok is a major discovery avidly anticipated by scientists around the world hoping that it may allow a glimpse into microbial life forms, not visible to the naked eye, that existed before the Ice Age. It may also provide precious material that would help look for life on the ice-crusted moons of Jupiter and Saturn or under Mars' polar ice caps where conditions could be similar.

"It's like exploring another planet, except this one is ours," Columbia University glaciologist Robin Bell told The Associated Press by email.

Valery Lukin, the head of Russia's Arctic and Antarctic Research Institute (AARI), which is in charge of the mission, said in Wednesday's statement that his team reached the lake's surface on Sunday.

Lukin has previously compared the Lake Vostok effort to the moon race that the Soviet Union lost to the United States, telling the Russian media he was proud that Russia will be the first this time.

Although far from being the world's deepest lake, the severe weather of Antarctica and the location's remoteness made the project challenging.

"There is no other place on Earth that has been in isolation for more than 20 million years," said Lev Savatyugin, a researcher with the AARI. "It's a meeting with the unknown."

Savatyugin said scientists hope to find primeval bacteria that could expand the human knowledge of the origins of life. "We need to see what we have here before we send missions to ice-crusted moons, like Jupiter's moon Europa," he said.

Lake Vostok is 250 kilometres long and 50 kilometres across at its widest point, similar in area to Lake Ontario. It lies about 3.8 kilometres beneath the surface and is the largest in a web of nearly 400 known subglacial lakes in Antarctica. The lake is warmed underneath by geothermal energy.

The project, however, has drawn strong fears that 60 metric tonnes of lubricants and antifreeze used in the drilling may contaminate the pristine lake. The Russian researchers have insisted the bore would only slightly touch the lake's surface and that a surge in pressure will send the water rushing up the shaft where it will freeze, immediately sealing out the toxic chemicals.

Lukin said about 1.5 cubic metres of kerosene and freon poured up to the surface from the boreshaft, proof that the lake water streamed up from beneath, froze, and blocked the hole. The scientists will later remove the frozen sample for analysis in December when the next Antarctic summer comes.

Scientists believe that microbial life may exist in the dark depths of the lake despite its high pressure and constant cold -- conditions similar to those expected to be found under the ice crust on Mars, Jupiter's moon Europa and Saturn's moon Enceladus.

"In the simplest sense, it can transform the way we think about life," NASA's chief scientist Waleed Abdalati told the AP by email.

Scientists in other nations hope to follow up this discovery with similar projects. American and British teams are drilling to reach their own subglacial Antarctic lakes, but Bell said those lakes are smaller and younger than Vostok, which is the big scientific prize.

Some scientists hope that studies of Lake Vostok and other subglacial lakes will advance knowledge of Earth's own climate and help predict its changes.