 Due to national security concerns, however, the Pipeline and Hazardous Materials Safety Administration does not divulge the materials actually carried on major US pipelines, including Line 5.Public concerns have particularly focused on the risk of a spill under the Straits of Mackinac, and the difficulty of controlling any spill that might occur.According to Enbridge, the pipes under the straits have never leaked, are monitored 24 hours a day, and are regularly inspected by underwater autonomous vehicles.