Coal is a black or dark brown combustible rock made primarily of carbon. It was formed millions of years ago when ferns, plants, and trees died and fell into swamps. The swamp conditions prevented the organisms from decaying completely, and after millions of years of intense heat and pressure, coal was formed. Coal is classified into four main types or ranks based on carbon and heat content: lignite, sub-bituminous, bituminous, anthracite. The general rule is that the higher the grade of coal, the cleaner it burns, and the more versatile its uses. Coal is extracted from the earth through underground mining or surface mining. The choice of mining method is largely determined by the geology of the coal deposit and its distance from the surface. Underground mining currently accounts for a larger share of world coal production than surface mining. Coal can be burned for heating or to produce electricity. To convert thermal coal to electricity, it is first milled to a fine powder which increases the surface area and allows it to burn more quickly. The hot gases and heat energy produced from combustion converts water into steam to run a turbine and generator. High quality coal is also a useful raw material. For example, it can be converted to Coke for steel-making. Coal can also be converted to liquid or synthetic gas by advanced chemical processes, making it a possible but costly replacement for natural gas or liquid fuels for transportation.