What is Lake Vostok?

Quote from the Related Link: "Lake Vostok extends over about 14,000 square km (roughly the size of Lake Ontario) and is overlaid by 3,710 m (12,170 ft.) of ice. It has a maximum depth of 510 m, an average depth of 125 m, a volume of 1800 cubic km, and a bed that is 710 m below sea level."