The Great Lakes are five interconnected freshwater lakes—Superior, Michigan, Huron, Erie, and Ontario—spanning the United States-Canada border and linked to the Atlantic by the Saint Lawrence River. Formed about 14,000 years ago as glaciers retreated, they became the largest freshwater lake group on Earth by surface area and the second largest by volume, containing more than a fifth of the world’s surface fresh water by volume. Their immense scale gives them sea-like conditions and made them vital to transportation, trade, fishing, migration, and urban growth. Connected by natural channels, locks, and canals, they remain a major waterway and one of North America’s most important natural and economic regions.