Coal is a natural dark brown-to-black solid used as a fuel formed from fossilized plants. Coal can be ignited and burned to produce energy in the form of heat. It consists of various organic and inorganic compounds. About 300 million years ago, much of the United States was swampland covered with giant ferns, reeds and other plants. When the plants died, they fell underwater. Underwater, the oxygen level prevented them from decaying completely, which resulted in the formation of peat. As the seas deposited layers of other materials over the peat through the centuries, the peat was tightly compacted and dried. The peat hardened in the final step of this natural process to become coal. Because coal is composed of once living material, it is known as a "fossil fuel" and is capable of being burned.