Thunder Bay National Marine Sanctuary and Underwater Preserve is a United States National Marine Sanctuary on Lake Huron's Thunder Bay, within the northeastern region of the U.S. state of Michigan. It protects an estimated 116 historically significant shipwrecks ranging from nineteenth-century wooden side-wheelers to twentieth-century steel-hulled steamers. There are a great many wrecks in the sanctuary, and their preservation and protection is a concern for national policymakers. The landward boundary of the sanctuary extends from the western boundary of Presque Isle County to the southern boundary of Alcona County. The sanctuary extends east from the lakeshore to the international border. Alpena is the largest city in the area.