November 19, 1968, an explosion in the Farmington, West Virginia, Consol No. 9 mine kills 78 miners. More information here and here, and a recent NPR report here.
Major mining disasters such as the Farmington coal mine explosion in 1968 and the Sunshine Mine fire in 1972 led to the Federal Coal Mine Health and Safety Act of 1969 and the Federal Mine Safety and Health Amendments Act of 1977.