Perhaps "ocean" is a too generous term for Antarctica's Lake Vostok. Located beneath the Russian Vostok station high on the central plateau of the East Antarctic, it is the largest of 70 known subglacial lakes buried under thousands of feet of solid ice. Vostok is the size of Lake Ontario and is 15 to 20 million years old. HBH With a maximum depth of perhaps 2,600 feet, it existed before the continent was covered by ice.
Isolated from the world for millions of years, the lake is in essence a proto-ocean buried under a two-mile thick blanket of ice. Subglacial water in the Antarctic was discovered many years ago using aircraft mounted, downward looking, low-frequency radars. First tested in Greenland in the 1970s, these systems can penetrate permanent ice cover to help define the land topography beneath.