Since 1977 an estimated 2 million acres of coal mine lands have been reclaimed in this manner. Underground mining methods are used where the coal seam is too deep or the land too hilly for surface mining. Most underground mining takes place east of the Mississippi, especially in the Appalachian mountain states. Coal production was once dominated by underground mining methods, but the growth of coal mining in the West changed that. Now, only 40 percent of our coal comes from underground mines.
Underground mines differ according to how the coal seam is situated with respect to the surface.
Where the bed of coal is relatively close to the surface, yet too deep to be recovered by surface mining, a slope mine can be constructed, with the mine shaft slanting down from the surface to the coal seam. The most common type is the shaft mine. To reach the coal, which may be as deep as 2,000 feet, vertical shafts are cut through the overburden to the coal bed, which is excavated by machines. In deep mines, the seam is mined in carefully engineered patterns that keep as much as half of the coal in place to help support the roof of the active mining area. The largest amount of coal taken from underground mines is produced using continuous miners.