About 15,000 years ago, lobes of the last Pleistocene glacier gouged the basins of the Great Lakes. Some 10,000 years ago, the melting ice sheet filled the basins and created both Lake Michigan and Lake Erie. Further melting filled the other basins to create the world's largest system of freshwater lakes. The largest Great Lake is Superior, the others are Huron, and Ontario.
In Illinois, Lake Michigan covers about 60 square miles and forms the border for the state on the extreme Northeast corner.