Fossil Fuels are of great importance because they can be burned (Oxidized to carbon dioxide and water), producing significant amounts of energy per unit mass. The use of coal as a fuel predates recorded history. Coal was used to run furnaces for the melting of metal ore. Semi–solid hydrocarbons from seeps were also burned in ancient times, but these materials were mostly used for waterproofing and embalming.
Commercial exploitation of petroleum began in the 19th century, largely to replace oils from animal sources (notably whale oil) for use in oil lamps.
Natural gas, once flared–off as an unneeded byproduct of petroleum production, is now considered a very valuable resource. Natural gas deposits are also the main source of the element helium.
Heavy crude oil, which is much more viscous than conventional crude oil, and tar sands, where Bitumen is found mixed with sand and clay, began to become more important as sources of fossil fuel as of the early 2000s. Oil shale and similar materials are sedimentary rocks containing Kerogen, a complex mixture of high–molecular weight organic compounds, which yield synthetic crude oil when heated (pyrolyzed). These materials had yet to be fully exploited commercially. With additional processing, they can be employed in lieu of other already established fossil fuel deposits. More recently, there has been disinvestment from exploitation of such resources due to their high carbon cost, relative to more easily processed reserves.
Prior to the latter half of the 18th century, windmills and watermills provided the energy needed for industry such as milling flour, sawing wood or pumping water, and burning wood or peat provided domestic heat. The wide scale use of fossil fuels, coal at first and petroleum later, to fire steam engines enabled the Industrial Revolution. At the same time, gas lights using natural gas or coal gas were coming into wide use. The invention of the internal combustion engine and its use in automobiles and trucks greatly increased the demand for gasoline and diesel oil, both made from fossil fuels. Other forms of transportation, railways and aircraft, also required fossil fuels. The other major use for fossil fuels is in generating electricity and as feedstock for the petrochemical industry. Tar, a leftover of petroleum extraction, is used in construction of roads.