Mining is the means for extracting economically important minerals and ores from the Earth. Where the desired minerals lie near the surface, the most economic form of mine is the open pit. This usually consists of a series of terraces, which are worked back in parallel so that the mineral is always within convenient reach of the excavating machines. Strip mining refers to stripping off a layer of overburden to reach a usually thin mineral seam (often coal). The excavating machines used in open-pit mining are frequently vast. Soft minerals such as kaolinite can be recovered hydraulically – by directing heavy water jets at the pit face and pumping out the resulting slurry. Where a mineral is found in alluvium, buckets or suction dredges may be used. But where minerals lie far below the surface, various deep mining techniques must be used. Sulfur is mined by pumping superheated water down boreholes into the mineral bed. The melts the sulfur which is then pumped to the surface (see Frasch process). Water-soluble minerals, such as salt (see sodium chloride) are often mined in a similar way (solution mining). But most often, deep minerals and ores must be won from underground mines. Access to the mineral-bearing strata is obtained via a vertical shaft or sloping incline driven from the surface, or via a horizontal adit driven into the side of a mountain. The geometry of the actual mining area is determined by the type of mineral and the strength of the surrounding material. All underground mines require adequate ventilation and lighting, facilities for pumping out any groundwater or toxic gases seeping into the workings, and means (railroad or conveyor) for removing the ore and waste to the surface. As in open-pit mining, the rock is broken mechanically or with explosives. However, particular care must be exercised when using explosives underground.
Several occupational diseases (e.g., pneumoconiosis) are associated with mining and extraction metallurgy, particularly where high dust levels and toxic substances are involved.