Surface mining techniques are used when coal seams are close to the surface, or in the case of Appalachia, when the coal seams are thin and can’t be efficiently accessed via subsurface mining. In Appalachia, the forests are first clear-cut and then explosives are used to blast away part of the mountain. Heavy equipment then digs through debris, dumping the overburden (soil and rock) into the nearby valley, burying streams as the valley is filled in. The exposed coal is dug out and some processing is done on site. Coal sludge left over from processing is stored in ponds on the mining site.