Petroleum is a complex mixture of solid, liquid, and gaseous hydrocarbons normally found a few miles beneath the earth's surface. It, along with coal and natural gas, is one of the fossil fuels. That name comes from the most common scientific theory that these three materials were formed between 10 and 20 million years ago as a product of the decay of plants and animals. Petroleum is sometimes called crude oil or, simply, oil, although some authorities give slightly different means to these terms.

Most commonly, petroleum and natural gas occur together under dome-shaped layers of rock. When wells are drilled into such domes, gas pressure forces petroleum up into the well, producing the "gushers" that are typical of successful new wells.

Petroleum has been known to and used by humans for thousands of years. Collecting the liquid that seeped to the surface, the Egyptians, Babylonians, and Native American Indians...