The Great Lakes is the world's largest system of freshwater lakes. Together, the lakes occupy an area of nearly 100,000 square miles. 20 percent of the world's surface freshwater is contained in the lakes. The lakes were formed about 10,000 years ago; at the end of the last ice age. As the massive Laurentide ice sheet retreated, glaciers left behind a large amount of water which filled in the basins they had carved. The Great Lakes are made of five large lakes: Lake Ontario, Lake Erie, Lake Huron, Lake Michigan, and Lake Superior. Lake Superior is the world's second largest lake. The remaining lakes are among the world's fifteen largest lakes.