Covering more than 94,000 square miles in the United States and Canada, the Great Lakes—Huron, Ontario, Michigan, Erie and Superior—are connected by a series of rivers, straits and smaller lakes, forming the world’s largest freshwater system.

With an estimated 6 quadrillion gallons of water (that’s enough to bury the 48 contiguous states under about 9.5 feet of H2O), the Great Lakes contain more than 20 percent of the world’s freshwater supply and more than 80 percent of North America’s freshwater.

Among the waterways linking the lakes are the St. Marys River, which flows from Lake Superior to Lake Huron; the Niagara River, connecting Lake Erie to Lake Ontario; and the narrow Straits of Mackinac, joining Lake Michigan and Lake Huron (hydrologically, the two lakes are considered a single body of water).

Of the five lakes, Superior is the deepest and largest in terms of surface area and water volume, while Erie is the shallowest and smallest by volume. Lake Michigan, the second-largest by volume, is the only one of the five lakes located entirely in the United States. Interspersed throughout the lakes are some 35,000 islands, the biggest of which, Canada’s Manitoulin Island in Lake Huron, is sizeable enough to have lakes of its own.

Numerous Indian tribes inhabited the Great Lakes region long before the arrival of French explorers in the 17th century. Etienne Brule, an interpreter and scout for Samuel de Champlain, is credited as the first European to discover the Great Lakes, around 1615.

The lakes later played a key role in the region’s development as an important industrial center. Since the opening of the Great Lakes-St. Lawrence Seaway System in 1959, large cargo vessels can travel from the Atlantic Ocean to Duluth, Minnesota, on Lake Superior–the farthest-inland freshwater seaport in North America—a journey of more than 2,000 miles.