High Quality Content by WIKIPEDIA articles! The Straits of Mackinac is the strip of water that connects two of the Great Lakes, Lake Michigan and Lake Huron, and separates the Lower Peninsula of Michigan from the Upper Peninsula of Michigan. It is a shipping lane connecting, for instance, the steel mills of Gary, Indiana to the iron mines of Minnesota. Before the railroads reached Chicago from the east, it served as part of the path for immigrants into the Midwest and Great Plains. It is five miles (8 km) wide at its narrowest point where it is spanned by the Mackinac Bridge. Before the bridge was built, car ferries were used to cross the straits. Today passenger-only ferries carry people to Mackinac Island which does not permit cars, while passengers can take a vehicle ferry to Bois Blanc Island. Before icebreakers and year-round shipping on the Lower Great Lakes, the Straits would freeze over in winter. Islands in the Straits of Mackinac include the two populated islands, Bois Blanc Island and Mackinac Island, and the two uninhabited islands, Round Island and St. Helena Island. At 11 miles (18 kilometers) in length, Bois Blanc is by far the largest island in the Straits.