An ageing pipeline crossing part of the Great Lakes has led to a standoff between the US state of Michigan and Canada. The outcome of the battle over Line 5, which delivers energy to the US Midwest and central Canada, will be viewed by many as a bellwether of how North America will balance its energy future with its environmental commitments. The most contentious part of the Line 5 pipeline - which runs from Superior, Wisconsin, by way of Michigan to Sarnia, Canada - sits on the bed of the Straits of Mackinac. The narrow waterway connects Lake Michigan and Lake Huron - two of the world's largest lakes.