Beginning in June of 1930, three thousand men dug a three-mile hole through a sandstone mountain near the town of Gauley Bridge, West Virginia, for the Union Carbide and Carbon Corporation—The Hawk's Nest Tunnel. The company was building an electro-metallurgical plant nearby, which needed a large supply of power and silica. So they designed a dam to divert a column of the New River underground and down a gradually sloping tunnel to electrical generators; the ground-up silica harvested during excavation would be fed into the furnace at Alloy.
More than three quarters of the workers were migratory blacks from the South who lived in temporary work camps. Some of them developed silicosis in a matter of weeks, while others wheezed with the disease for decades. When stricken, the migrant workers either fled West Virginia for wherever home was, or they were buried as paupers in mass graves in the fields and woods around Fayette County. The death toll was an estimated, though impossible to confirm, 764 persons, making it the worst industrial disaster in United States history. More info from e-WV.