On December 11, 1971, a shotgun-like blast killed twenty-two men working on a water intake tunnel beneath the bed of Lake Huron. The explosion was fueled by methane trapped within a layer of ancient Antrim shale. An inquiry determined that drilling a vertical ventilation shaft from the lake’s surface had released the gas, and a fallen drill bit ignited it. The blast produced a shock wave traveling 4,000 miles an hour with a force of 15,000 pounds per square inch, and witnesses saw debris fly 200 feet into the air from the tunnel entrance. The disaster led to stronger mining safety regulations and enforcement and became one of the deadliest industrial accidents in Michigan history. The tunnel was part of the Lake Huron Water Supply Project, begun in 1968 by the Detroit Metro Water Department to meet the needs of a growing population. The project built a submerged intake crib linked to a six-mile intake tunnel beneath Lake Huron. A mechanical mole bored the sixteen-foot-wide tunnel through bedrock at 150 feet a day, and the project excavated more than one billion pounds of rock. The water treatment plant pumped clean water into an eighty-two-mile network of water mains serving Detroit and Flint. Completed in 1973, the 123 million dollar system had a capacity of 400 million gallons a day.