Beginning in 1968, the Detroit Metro Water Department built the Lake Huron Water Supply Project to meet the needs of a growing population, erecting a submerged intake crib connected to a six-mile intake tunnel beneath Lake Huron. A mechanical mole dug the 16-foot-wide tunnel through bedrock beneath the lake at a rate of 150 feet a day, and the project excavated more than one billion pounds of rock. The water treatment plant pumped clean water into an 82-mile system of water mains supplying Detroit and Flint, and when the system was finished in 1973, it cost $123 million and had a capacity of 400 million gallons a day. On December 11, 1971, a shotgun-like blast killed twenty-two men working on a water intake tunnel beneath the bed of Lake Huron. Methane trapped within a layer of ancient Antrim shale fueled the explosion after drilling for a vertical ventilation shaft from the lake's surface released the gas and a fallen drill bit ignited it. The blast created 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.