Russian researchers say they have drilled through more than 2 miles of ice on Antarctica’s polar plateau and reached an ancient subglacial lake that has been sealed off for as many as 15 million years. Reaching the lake caps a two-decade drilling effort at Russia’s Vostok Station in Antarctica, which in 1982 recorded Earth’s coldest temperature of -129 degrees F. The chief of the Vostok Station, A.M. Yelagin, confirmed in a statement that Russia’s drilling team had reached the lake on Sunday, when water shot up from 12,365 feet below the surface and froze as it reached the -67 degree F air. Scientists are hoping that the lake could contain a wide variety of evolutionary secrets, including evidence of previously unknown prehistoric life forms. But Russian researchers said that clean water samples from the lake will not be taken until the next Antarctic summer, in December, since the water released over the weekend was contaminated by drilling fluids. Vostok is also the site of some of the deepest ice core samples ever taken on earth, revealing 800,000 years of climate data.