A Wisconsin Public Radio analysis found that a leak of natural gas liquids from Enbridge’s controversial Line 5 is the largest such incident in 50 years.

On Aug. 25, the pipeline leaked over a million gallons of mainly propane and butane, both types of natural gas liquids, in Iron County, in northern Wisconsin just south of Lake Superior. A subcontractor’s unoccupied truck rolled in the pipeline near the town of Saxon, causing a rupture that led an estimated 1.3 million gallons of natural gas liquids to vaporize into the atmosphere, according to Wisconsin Public Radio.

The outlet analyzed federal hazardous liquid accident data going back to 1970 and found the spill was the state’s largest during that period. The pipeline remains shut down, with the Wisconsin Department of Natural Resources asking that Enbridge stop work on the pipeline until better spill prevention measures are in place.

The 645-mile Line 5 pipeline system transports oil and natural gas liquids from Superior, Wisconsin, to Sarna, Ontario. The pipeline is notorious for a 2010 accident that released more than 20,000 barrels of heavy crude oil into a tributary of the Kalamazoo River near Marshall, Michigan, one of the worst inland oil spills in U.S. history.

An aging 4.5-mile section of the pipeline crosses beneath the Straits of Mackinac, which connects Lake Michigan and Lake Huron. A substantial spill into the Great Lakes could impact drinking water for up to 40 million people, including those living in large urban metros like Chicago and Milwaukee.