Canada has been threatening to invoke a treaty that was signed in 1977 but has never before been used, barring actions that “would have the effect of impeding, diverting, redirecting or interfering” with “hydrocarbon in transit,” except in case of a natural disaster or operating emergency, the Washington Post reports. For years, Michigan has been saying a natural disaster is precisely what it’s trying to avert by taking action on the Line 5 pipeline.

“The clash over Calgary-based Enbridge’s Line 5, which carries up to 540,000 barrels of crude oil and natural gas liquids across Michigan and under the Great Lakes each day, is placing stress on U.S.-Canada ties—and raising questions about how the close allies, which have expressed a desire to work together to fight climate change, can balance energy security with the transition to a clean energy economy,” The Post writes. But Whitmer spokesperson Bobby Leddy said the governor is standing by her position that continuing to operate the pipeline past next Wednesday would be unlawful.

“These oil pipelines in the Straits of Mackinac are a ticking time bomb,” he told the Post. “Their continued presence violates the public trust and poses a grave threat to Michigan’s environment and economy.”