Line 5 of the Enbridge oil pipeline system consists of two steel pipes 20 inches in diameter that lie at the bottom of the Straits of Mackinac, a narrow strip of water separating Michigan's upper and lower peninsulas. The Canadian energy company Enbridge brought Line 5 online in 1953; today, the pipes help transport nearly 23 million gallons of crude oil a day between the US and Canada.

The five-mile stretch of pipeline under the straits is situated at the meeting point of Lake Huron and Lake Michigan — two of the five Great Lakes that together hold 90 percent of America's fresh water. The strait lies just west of Mackinac Island, a popular tourist destination listed as a US Historic Landmark. And the channel generates extremely powerful currents — they can flow east or west — that at times create a flow of water more than 10 times greater in volume than Niagara Falls.