The objective of underground mining is the extraction of ore below the surface in a manner that allows an optimum return on investment while maintaining safety of personnel and the integrity of the underground openings. The underground methods employed should be based on detailed drilling and sampling sufficient to characterize the distribution, grade, and extent of the ore. Entry from the surface to an underground mine may be through an adit, a shaft, or a decline. The mining method employed needs to allow access to the ore while minimizing the amount of waste rock that needs to be removed to excavate the ore. Underground mining is undertaken when the mineralization occurs below the surface and is confined to veins, fault zones or beds that tend to have a near-vertical planar or sheeted distribution that doesn’t accommodate open pit mining because the zone is too narrow and too much waste rock would have to be removed between, and on the exterior of the zones to extend the excavation to depth. Often these mineralized zones will consist of one or more sub parallel zones. Sometimes these zones coalesce or intersect as veins. When this occurs the grade of veins can often be enriched.   

A typical underground mine may have a number of nearly horizontal levels at a planned spacing to facilitate excavation of the ore. Commonly the ore will be drilled and blasted, and then moved or dropped to a haulage level from where it can then be moved to the surface. The mine will be planned to use gravity as much as possible to drop the ore and waste rock to a location from where it can then be moved to the surface via tracked locomotives and ore cars, hoisted up a shaft, or trucked to the surface by wheeled equipment. The underground mining will be performed in a pattern that leaves a honeycomb of openings (stopes) and supporting pillars. The mine plan will try to minimize the number of pillars containing ore that have to be left to support the mine openings. Sometimes the mine will be designed to fill the stopes with waste rock and reduce the amount of waste that needs to be hauled to the surface.

In general underground mining requires higher grades then surface mines because of the higher costs required for underground mining because the mineralization tends to be concentrated in planar zones and doesn’t extend in concentrations rich enough beyond the zone to be extracted and processed economically. Thus the amount of ore in an underground mine tends to be considerably less then a surface mine, but has significantly higher grades of ore.  

As a result of increased regulatory requirements, underground mining has become more attractive as an investment to mining companies because there is less surface impact, and its is now easier to permit an underground operation because there is less required reclamation expense.