Open Pit, or Open Cast Mining
Mining is a practical industry that seeks efficient ways to extracts mineral wealth from the ground. During the nineteenth century, precious metals deeper than about three hundred feet called for underground drift or hardrock mining.
Occasionally, ore bodies sufficiently close to the surface made open pit or open cast mining practical. Often regarded as a twentieth-century approach, open pit mining was actually widespread earlier. It was a logical extension of placer mining, which excavated into sand bars any number of feet. Digging deeper resulted in pits, a practice pursued with devastating effect in California where hydraulic mining eroded soil and silted the Sacramento River and San Francisco Bay.
In Nevada, open pits first appeared with the discovery of the Comstock Lode in 1859. Miners developed the Ophir Pit, site of the original Virginia City strike, immediately following the mining district's founding. Twentieth-century miners expanded the pit, but much of what is there today dates to 1860. Such excavations are often called "glory holes."
During the twentieth century, large heavy equipment and better engineered sidewalls resulted in open pits that dwarfed their predecessors. They are, nevertheless, based on an earlier, widely-employed prototype.