Petroleum, also referred to as "crude oil," is a yellowish black liquid with varying degrees of viscosity that occurs naturally in geological structures in the earth's crust and that consists of a complex mixture of different molecular weights of hydrocarbons plus other organic compounds. The most widely accepted theory of its origin is that it was formed from the remains of prehistoric organisms that became buried under several kilometers of ocean sediment and were subjected to intense pressure and heat in the absence of oxygen over a span of millions of years. An alternative theory is that it is instead continuously created by chemical reactions that combine non-biological carbon and hydrogen far below the earth's surface and thus is not a finite resource.
Petroleum has been the world’s main source of energy since the 1950s and is also used to make a vast array of other petrochemical products, including plastics, synthetic fabrics, and even pharmaceuticals. However, the consumption and disposal of the enormous quantities of fuel and products in which it is used has been one of the top causes of the earth's current rapid environmental deterioration, particularly the rising level of greenhouse gases and a massive proliferation of plastic waste. Thus, increasing effort is being devoted to developing less harmful replacements for petroleum, especially alternative energy sources and plastics substitutes.