A massive subglacial lake under Antarctica's ice, sealed off for ~15 million years. Russian drilling teams reached it in 2012; possibly contains never-before-known life.
From Wikipedia
Lake Vostok is the largest of Antarctica's 675 known subglacial lakes and the 16th largest lake in the world by area. Lake Vostok is located at the southern Pole of Cold, beneath Russia's Vostok Station, under the surface of the central East Antarctic Ice Sheet, which is at 3,488 m (11,444 ft) above mean sea level. The surface of this fresh water lake is approximately 4,000 m (13,100 ft) under the surface of the ice, which places it at approximately 500 m (1,600 ft) below sea level.