CHARLESTON, W.Va. -- Government investigators evacuated Massey Energy Co.'s Upper Big Branch mine after encountering methane gas while searching for clues to the deadliest U.S. coal mine explosion in four decades.
Federal Mine Safety and Health Administration spokeswoman Amy Louviere says the evacuation occurred Monday.
She says methane started to accumulate while crews were pumping water from a flooded section of the southern West Virginia mine.
Methane occurs naturally in coal seams and poses one of mining's greatest hazards. The gas can explode at concentrations between 5 percent and 15 percent of the atmosphere.
MSHA suspects the Upper Big Branch explosion started with a methane ignition. The April 5 blast killed 29 coal miners.