Mountaintop removal mining (MTR), also known as mountaintop mining (MTM), is a form of surface mining that involves the mining of the summit or summit ridge of a mountain. Coal seams are extracted from a mountain by removing the land, or overburden, above the seams. The land may be dumped back on the ridge and compacted to reflect the approximate original contour of the mountain. It is the predominant method of coal mining in the Appalachian Mountains in the eastern United States. Explosives are used to remove up to 400 vertical feet (120 m) of mountain to expose underlying coal seams. Excess rock and soil laden with toxic mining byproducts are often dumped into nearby valleys, in what are called "holler fills" or "valley fills." Less expensive to execute and requiring fewer employees, mountaintop removal mining began in Appalachia in the 1970s as an extension of conventional strip mining techniques. It is primarily occurring in Kentucky, West Virginia, Virginia, and Tennessee - Source: Wikipedia