Lake Superior is the largest of the Great Lakes. The North American Great Lakes form a single watershed with one common outlet to the sea—the St. Lawrence Seaway. The total volume of all five basins is 6,000 trillion gallons (22.7 trillion liters) equivalent to about 20 percent of the world’s freshwater. Only Lake Michigan lies wholly within U.S. borders; the others share their boundaries with Canada. Some believe that Lake Huron and Lake Michigan are two lobes of one lake, since they are the same elevation and are connected by the 120-foot (36.5-meter) deep Strait of Mackinac, which is 3.6 to 5 miles (6 to 8 kilometers) wide. Gage records indicate that they both have similar water level regimes and mean long-term behavior, so that hydrologically they act as one lake. Historically they were considered two by the explorers who named them, but this is considered a misnomer by some.