The Great Lakes hold approximately 21% of the world’s surface freshwater.
Together, Superior, Michigan, Huron, Erie, and Ontario contain roughly 5,400 cubic miles of water. Their combined volume makes the system one of Earth’s largest accessible freshwater reserves, although the lakes represent only a small fraction of all freshwater when groundwater and ice are included.
The often-quoted 21% figure refers to the world’s surface freshwater, not all freshwater on Earth. That distinction matters because glaciers, ice caps, groundwater, rivers, wetlands, and atmospheric water are counted differently in global water inventories.
The lakes are also shared by two countries and support drinking-water supplies, shipping, fisheries, tourism, and industrial activity. Their size can make them resemble inland seas, but they are freshwater lakes connected by rivers and channels.