The Great Lakes hold about 21% of all the surface fresh water on Earth.

The Great Lakes contain about 5,439 cubic miles of water. Lake Superior is the biggest and holds more water than the other four lakes combined. These deep basins formed between 10,000 and 14,000 years ago. Huge glaciers called the Laurentide Ice Sheet moved across the land and carved out deep holes. As the ice melted, the water filled these holes to create the lakes.The lakes cover about 94,250 square miles of the Earth's surface. Their shoreline is over 10,000 miles long. This is longer than the entire Atlantic coast of the United States. More than 40 million people in the U.S. and Canada get their drinking water from this system. The lakes are so large that they can be seen from space.The massive amount of water also changes the local weather. Cold air moving over the relatively warm water causes heavy snow called lake-effect snow. Only 1% of the water in the lakes is replaced each year by rain or melting snow. This makes the water supply very difficult to replace if it becomes polluted. The lakes also serve as a major path for ships carrying over 75 million tons of cargo through the Soo Locks every year.