Petroleum is a complex mixture of hydrocarbons that occur on Earth in liquid, gaseous, or solid form. The term is often restricted to the liquid form, commonly called crude oil. But, as a technical term, petroleum also includes natural gas and the viscous or solid form known as bitumen, which is found in tar sands. Petroleum is a fossil fuel, meaning that it has been created by the decomposition of organic matter over millions of years. Petroleum is formed when large quantities of dead organisms–primarily zooplankton and algae–underneath sedimentary rock are subjected to intense heat and pressure.