The Straits of Mackinac, which separate Michigan’s Upper and Lower Peninsula, are the site of one of the most famous water crossings in the United States – the five-mile-long Mackinac Bridge, which opened in 1957. But four years before, in 1953, Enbridge Inc., the Canadian energy transport company, installed two 20-inch steel pipelines across the straits to ship oil and natural gas liquids to refineries and processing plants in Sarnia, Ontario, and to southeast Michigan.