Coal mining has been performed since the arrival of the Industrial Revolution in the mid-18th century. Coal is used as a fuel primarily for steam-generated electrical power plants, as well as being a component of certain industrial applications, such as steel manufacturing. Coal is widely distributed over the planet, either as underground seams which must be mined, or as deposits located closer to the earth's surface. The latter can be extracted at the surface by removal of the topmost layer of earth and rock covering the coal seam.
More than two-thirds of coal extracted underground is done by a "continuous miner," a tractor with a mounted cylindrical grinder that breaks coal away from the seam. The continuous miner deliberately leaves undisturbed pillars of rock and coal in the mining area to create natural supports for the ceiling. This is known as "room and pillar" mining. When most of the coal seam has been extracted, the pillars then are mined one by one, allowing the roof to naturally cave in.
Twenty percent to 30 percent of mined coal underground is from longwall mining. This is performed by a mechanical cutter that shears coal off from a panel on the seam. The panel being worked on may be up to 800 feet in width and 7,000 feet in length. Mined coal is deposited onto a conveyor that moves the coal to a collection area. Hydraulically powered shields over the machine provide ceiling support. Longwall mining is more efficient than room and pillar mining, but the equipment is more expensive.
For extracting coal which lies close the surface, huge drag-line shovel machines remove the topmost layers of soil and rock, exposing the coal, which is then removed by smaller machines. Surface mining may involve removing sections of hills or top layers of a flat surface area. The layers of rock and dirt covering the coal are reserved for until the coal is removed, at which point the the dirt and rock is replaced, the mine covered over, and the environment restored as much as possible to its original condition.
Conventional mining employs crews of miners who use explosives and drills to extract coal, which is then loaded onto cars for transport to the surface. This method presents higher risks to miners because of the explosives. The coal dust generated by the drilling and explosives is also a health hazard when continuously inhaled. This is the oldest method of coal mining.