Fossil fuel is a general term for buried combustible geological deposits of organic matter, formed from decaying plants and animals that have been converted into crude oil, coal and natural gas. by exposure to the heat and pressure of the Earth's crust over hundreds of millions of years. One form of fossil fuel created during this period was coal. At the time, the Earth's surface was mostly covered in swamps and foliage. When these plants died, they fell to the bottom of the swamps. Over the years, as the thick layers of plants were covered with soil and the pressure and heat increased, the plants turned into coal. Coal is the most common form of fossil fuel and is used by many leading countries, such as China and the United States of America..