The formation of the Great Lakes, which are also known as the Laurentian Great Lakes, began around 2 billion years ago after the forming of North American mountain systems and their subsequent erosion caused depressions that would eventually be flooded by seawater. Around 2 million years ago, the advancement of glaciers up to 6,500 feet in thickness caused more depression and the eventual retreat left massive amounts of water behind around 15,000 years ago. This glacial water is what forms the region that is now known as the Great Lakes basin, although their current areas and levels were only realized around 3,500 to 4,000 years ago.
The Great Lakes region consists of a series of five lakes, including Lake Superior, Lake Michigan, Lake Huron, Lake Ontario and Lake Erie. Together, these lakes make up 21 percent of the world's freshwater and span a total of 94,250 square miles. In 2009, the lakes contained 84 percent of North America's surface fresh water. Lake Superior is the biggest continental lake in the world by area.The lakes connect to the Atlantic Ocean via the Saint Lawrence River. The combined shoreline of the Great Lakes spans around 10,500 miles and provides a major source of leisure for residents and tourists alike.