Fossil fuels are buried deposits of plants and animals that have been converted to coal, petroleum, natural gas, or tar by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. The energy in fossil fuels comes from sunlight, either directly or indirectly.

Coal comes primarily from the remains of plants that were buried in anaerobic conditions (without oxygen). Coal ranges from 55 percent to 90 percent carbon mixed with water and other substances including compounds of nitrogen and oxygen. Coal is graded according to hardness and carbon content. The lowest grade, lignite, is soft and brown in color. The hardest, anthracite, is nearly pure carbon. It is so hard it can be polished like a gemstone.

Workers drilling for gas. Natural gas is usually found in varying amounts, along with crude oil and with coal.

Petroleum (crude oil) is a liquid containing...