Lake Vostok in Antarctica isn't like most of the world's other lakes. You can't see it from surface level, because it's been buried under two miles of ice for the past 15 million years. And unlike other bodies of water that recycle their contents fairly quickly, the water in Lake Vostok only recycles about once every 13,000 years from some unknown source. It's under too much pressure from all the ice above it to freeze, but it can't easily escape either, and it's uncontaminated by human activity or chemicals.
That's why scientists think it may be a treasure trove of extremophiles - that is, organisms which have evolved to live in the harshest, most inhospitable conditions.
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Studies of ice from just above the liquid lake have revealed dozens of types of bacteria and fungi, and also genetic fragments that might suggest the presence of fish and crustaceans, according to a 2013 Nature news article. A Russian scientist also reported that the ice contained a previously unknown type of bacteria, whose DNA was different from other known species. But other scientists disputed those finds, saying that the samples had been tainted by anti-freezing chemicals used to keep the drill working.