The size of the pit, or cut, depends on the size of the ore body and how much overburden must be removed. Benches or steps are cut on the sides of the pit to stabilize surrounding rock and minimize erosion. Using a computer model of the mine, excavation is carefully designed by the mining engineer. When constructing an underground mine, it must sink vertical or have inclined shafts or bore horizontal tunnels or adits to reach the mineral vein. Mine construction can take several years and employ several hundred people.
Underground mining is virtually a world unto itself.
Productive silver mines in Idaho are over a mile deep and require very specialized people and equipment. Safety is paramount in the underground world. Once the rock is loosened up by blasting, shovel operators selectively load ore into large trucks bound for the concentrating mill. The most common form of concentrator is the flotation mill. After being crushed, the ore passes through a series of large grinding mills.