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. The word petroleum is derived from petra (rock) and oleum (oil) as it is mined from between the rocks under Earth as Petroleum was formed from organisms living in the sea. As these organisms died, their bodies settled at the bottom of the sea and got covered with layers of sand and clay. Over millions of years, absence of air, high temperature and high pressure transformed the dead organisms into petroleum and natural gas.