Lake Michigan-Huron is a designation given to the body of water (part of the North American Great Lakes) traditionally considered to be two separate lakes: Lake Michigan and Lake Huron. Hydrologically, however, they are a single body of water: They lie at the same surface elevation, , and the flow between them through the Straits of Mackinac—which are wide and deep—sometimes reverses from eastward to westward. If designated as a single entity, Lake Michigan-Huron would be the largest of the Great Lakes, and indeed the largest lake in the world, in terms of surface area (though smaller than the Caspian Sea). Lake Superior still surpasses Michigan-Huron in terms of overall water volume, containing of water compared with Michigan-Huron's , which makes Lake Michigan-Huron the fourth largest freshwater lake by volume in the world (the first and second being Lake Baikal and Lake Tanganyika).
There were earlier variations of the lake during the last ice age: