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