Coal is a fossil fuel that contains the stored energy of prehistoric vegetation. According to the World Coal Association, it dates back to 360 million years ago, and was formed when swamps and peat bogs were buried due to shifts in the Earth’s tectonic plates. Subjected to pressure and heat deep underground, the plant material in the swamps and bogs underwent a chemical reaction, creating coal. Coal is classified at various levels of carbon content, determining the amount and quality of energy it produces. The most commonly mined classifications are sub-bituminous coal and bituminous coal. The two other types of coal investors should be aware of are lignite and anthracite. Sub-bituminous coal is used primarily as fuel for steam-electric power generation. For the most part, bituminous coal serves the same purpose, but it can be used for heat and power applications in manufacturing too. Additionally, bituminous coal can be used in the production of coke, a key ingredient in steel fabrication.