The Michigan Supreme Court has directed state regulators to reevaluate a critical permit for a proposed tunnel intended to carry an oil pipeline beneath the Straits of Mackinac, a waterway connecting Lakes Huron and Michigan. The decision, issued on Friday by a 6-1 majority, casts further uncertainty over the future of the 73-year-old Line 5 pipeline, owned by Canadian energy transportation company Enbridge Inc.

Line 5 extends from Superior, Wisconsin, to Sarnia, Ontario, with a six-kilometer section running along the bottom of the straits. Enbridge has sought to replace the current underwater pipeline with a tunnel beneath the lakebed to reduce the risk of an oil spill. The Michigan Public Service Commission (PSC) approved the tunnel permit in 2023, but the decision prompted legal challenges from four Michigan tribal nations and several environmental organizations.

In its ruling, the Michigan Supreme Court found that the PSC did not adequately consider whether the tunnel project would extend the operational life of Line 5, potentially increasing environmental risks over time. The court also said the commission failed to consistently evaluate alternatives such as transporting oil by trucks or trains. Additionally, the ruling noted that the PSC did not sufficiently assess the tunnel's potential impacts on fishing, boating, and other public trust rights in the Great Lakes.

Opponents of the pipeline emphasize the environmental and cultural risks posed by maintaining and expanding the pipeline infrastructure in the region. The Straits of Mackinac hold significant spiritual importance for the Indigenous Anishinaabe tribes, who argue the construction would disrupt this sacred site for years. David Gover, an attorney with the Native American Rights Fund, stated that transporting over half a million barrels of oil daily through the Great Lakes—whether suspended in water or enclosed in a tunnel—is inherently unsafe.

Enbridge expressed disappointment with the court's ruling, describing it as another delay in a project under review for nearly ten years. Company spokesperson Ryan Duffy said the tunnel proposal is designed to enhance environmental protection in the Great Lakes while ensuring energy reliability in the region. The company initially proposed the tunnel following damage to the pipeline caused by a ship’s anchor, arguing that the tunnel would considerably reduce the chances of a spill. However, environmentalists have raised concerns about noise, vibrations, and permanent ecological harm during construction.

Separately, Michigan Governor Gretchen Whitmer and state officials have been engaged in legal efforts since 2020 to shut down Line 5’s segment beneath the straits, citing the pipeline’s potential threat to the Great Lakes ecosystem. The recent Supreme Court decision adds another layer of complexity to the ongoing disputes surrounding the pipeline’s future.