The Michigan Supreme Court has ordered state regulators to reconsider a key permit that allowed a planned tunnel to carry an oil pipeline under part of the Great Lakes to move forward.

The 6-1 decision further clouds the future of the 73-year-old pipeline owned by Canadian energy transportation company Enbridge that carries petroleum from Superior, Wisconsin, to Sarnia, Ontario.

The part of the pipeline in question, called Line 5, runs for 4 miles (6 kilometers) along the bottom of the Straits of Mackinac where Lakes Huron and Michigan meet.

Enbridge wants to bury the pipeline in a tunnel under the straits. Environmentalists and Michigan Gov. Gretchen Whitmer want to move the pipeline away from the waterway altogether saying a spill would be catastrophic to the Great Lakes and surrounding area.

The Michigan Public Service Commission granted a permit to build the tunnel in 2023. Four Michigan tribal nations along with several environmental groups sued.

The Michigan Supreme Court ruled Friday that when they granted the permit, regulators failed to consider if the tunnel project will extend the lifetime of the pipeline, resulting in more harm to the environment.

The ruling also said the Public Service Commission wasn’t consistent when it compared alternatives to the pipeline like using trucks or trains to move the oil and didn’t assess how the tunnel might impact fishing, boating and other public trust rights on the Great Lakes.

Opponents of the pipeline say this type of project has never been done anywhere else and would turn the Straits of Mackinac, which is a sacred site for the Indigenous Anishinaabe tribes, into a construction site for years.

“Whether Line 5 is suspended in the water or encased in a tunnel, it is simply a terrible idea to pump more than half a million barrels of oil every day through the middle of the Great Lakes,” attorney David Gover with the Native American Rights Fund said in a statement.

Enbridge is reviewing its legal options after the ruling and said it is disappointed for another delay in a project that has been under review for nearly a decade and would “enhance environmental protection of the Great Lakes while maintaining critical energy reliability for the region,” spokesman Ryan Duffy said in a statement.

Enbridge proposed the pipeline tunnel nearly a decade ago around the time a ship anchor dented part of the line. The company said the tunnel would massively reduce the possibility of an oil spill, but environmentalists said building the tunnel under the bottom of the Straits of Mackinac would cause noise, vibrations and permanent environmental damage.

In a different set of court cases, Gov. Whitmer and other state officials have been trying since 2020 to shut down the pipeline under the straits.

Jeffrey Collins, The Associated Press