Fuel formed under the earth's surface by the decomposition of organic matter is called:
Fossil fuel
Fossil fuels come from the very old life forms that have decomposed over a long period of time under the earth's crust. Fossil fuel is a general term for the buried combustible deposits formed from the decomposition of organic matter such as plants and animals. They were subjected to high heat and pressure in the earth's crust over hundreds of millions of years. The three most important fossil fuels are coal, petroleum or crude oil and natural gas.