The Great Lakes (also called the Laurentian Great Lakes, or the Great Lakes of North America) are a series of interconnected freshwater lakes located in northeastern North America, on the Canada–United States border, which connect to the Atlantic Ocean through the Saint Lawrence River. Consisting of Lakes Superior, Michigan, Huron (or Michigan–Huron), Erie, and Ontario, they form the largest group of freshwater lakes on Earth, containing 21% of the world's surface fresh water by volume. The total surface is , and the total volume (measured at the low water datum) is . Due to their sea-like characteristics (rolling waves, sustained winds, strong currents, great depths, and distant horizons) the five Great Lakes have also long been referred to as inland seas. Lake Superior is the second largest lake in the world by area, and Lake Michigan is the largest lake that is entirely within one country. The southern half of the Great Lakes is surrounded by the Great Lakes Megalopolis.