Copper is found in the earth's crust and the oceans although the amount in the latter is thought to be negligible, amounting to no more than about eight months mine production at present-day rates. The upper 10 kilometers of the crust is thought to contain an average of about 33 ppm of of this element. For commercial exploitation, copper deposits generally need to be in excess of 0.5% , and preferably over 2%. The known reserves of higher-grade ore in the world amount to nearly 1 billion tons of copper. At the present rate of mine production, which is about 13.9 million tons (12.5 million metric tons) a year, known reserves of the element could be depleted in about 65 years. However, successful exploration for new mineral deposits, technological advances in mining and extractive metallurgy (which enable the exploitation of leaner ores, thereby enlarging the pool of known reserves) and copper uses (which permit copper to be used more sparingly where larger quantities were used in the past) and the continued recycling of scrap are likely to forestall indefinitely depletion of this valuable metal.
Underground mining is achieved by sinking shafts to the appropriate levels and then driving horizontal tunnels, called adits, to reach the ore. Underground mining is, however, relatively expensive and is generally limited to rich ores. El Teniente, in Chile, is the world's largest underground copper mine. Open pit mining is employed when the orebodies are extensive, low in grade, and relatively near the surface, where they can be quarried after removal of overburden.