Coal mining is the industry most frequently associated with the region in outsiders' minds, due in part to the fact that the region once produced two-thirds of the nation's coal. At present, however, the mining industry employs just 2% of the Appalachian workforce. The region's vast coalfield covers between northern Pennsylvania and central Alabama, mostly along the Cumberland Plateau and Allegheny Plateau regions. Most mining activity has been concentrated in eastern Kentucky, southwestern Virginia, West Virginia, and western Pennsylvania, with smaller operations in western Maryland, Tennessee and Alabama. The Pittsburgh coal seam, which has produced 13 billion tons of coal since the early 19th century, has been called the world's most valuable mineral deposit. There are over 60 major coal seams in West Virginia, and over 80 in eastern Kentucky. Most of the coal mined is bituminous, although significant anthracite deposits exist on the fringe of the region in central Pennsylvania. About two-thirds of Appalachia's coal is produced by underground mining, the rest by surface mining. Mountaintop removal, a form of surface mining, is a highly controversial mining practice in central Appalachia due to its negative impacts on the environment and health of local residents.