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mixture of comparatively volatile liquid hydrocarbons (compounds composed mainly of hydrogen and carbon with some nitrogen, sulfur, and oxygen) that occurs in the Earth's crust and is extracted for use as fuel and various petroleum products. Because crude oil is a mixture of widely varying constituents and proportions, its physical properties also vary widely. For example, the specific gravity, as measured on the American Petroleum Institute (API) gravity scale, may range from 10 to more than 60 and the colour from colourless to black. Crude oil occurs underground, at various pressures from tens to hundreds of kilograms per square centimetre, depending on the depth. Because of the pressure, it contains considerable natural gas in solution. The oil underground is much more fluid than it is on the surface, because the elevated temperatures there (on the average, the temperature rises 1 C for every 33 m [108 feet] of depth) decrease the viscosity.