The Great Lakes are a series of large interconnected freshwater lakes in North America, spanning the border between Canada and the United States. The five lakes are Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario. Hydrologically, Lake Michigan and Lake Huron are often treated as a single body of water because they are joined at the Straits of Mackinac.
Together, the Great Lakes form the largest group of freshwater lakes on Earth by total area and the second-largest by total volume. They contain more than 20% of the world’s surface fresh water by volume. Their sea-like size and conditions—such as strong winds, rolling waves, and great depths—have long led them to be called inland seas. The lakes drain primarily into the Atlantic Ocean through the Saint Lawrence River, and are also connected to the Mississippi River basin via the Illinois Waterway.
The Great Lakes began forming at the end of the last glacial period, about 14,000 years ago, as retreating ice sheets left basins that filled with meltwater. They have played a major role in transportation, trade, migration, and fishing, and support a diverse freshwater ecosystem. The surrounding region, known as the Great Lakes region, includes major cities such as Chicago, Detroit, Toronto, and Cleveland.