Mackinac, Straits of, the channel that separates upper and lower Michigan and connects Lakes Huron and Michigan. It is four miles (6.4 km) wide. The adjoining area is a summer resort. Mackinac Straits Bridge, linking St. Ignace and Mackinaw City, was opened in 1957. It is among the world's longest suspension bridges, having a total length of 26,444 feet (8,060 m) and a main span of 3,800 feet (1,158 m). , opening picture.) Mackinac, Round, and Bois Blanc islands are east of the bridge. Jacques Marquette built a mission at St. Ignace in 1671, and French and British forts were fur-trading centers in the 1700's.