Lake Vostok, Antarctica
Lake Vostok is the largest subglacial lake on Antarctica. It is unique in that the lake's liquid water surface has been trapped underneath 13,000 feet of glacial ice for at least 15 million to 25 million years. Scientists thus believe that ancient extremophile microbes found here could provide analogues to how life might thrive on Jupiter's moon Europa or on Saturn's moon Enceladus, which are both also believed to contain liquid water oceans underneath their icy surfaces. (To see the location of Lake Vostok in relation to Antarctica, check out NASA's animation here.)
Russian researchers drilled to the lake's surface for the first time in February 2012, though the full experimental results from water samples are still pending. The entire process should offer invaluable clues about how similar technology might be used to explore Europa.