Beneath 4km of Antarctic ice sits a lake untouched by sunlight for millions of years

Lake Vostok lies under nearly 4km of East Antarctic ice, kept liquid by geothermal heat despite the deep freeze above. Confirmed by seismic and satellite surveys in the 1990s, its water has been isolated from the atmosphere for an estimated 15 to 25 million years, making it one of Earth's closest analogues to the buried oceans suspected beneath Jupiter's moon Europa.

— Andrey Kapitsa et al., Large deep freshwater lake beneath the ice of central East Antarctica — Nature, 1996