Coal, oil and natural gas are called fossil fuels because they are derived from the organic remains of prehistoric organisms. Fossil fuels are hydrocarbons that emit energy when burned.
Fossil fuels are found within the Earth's crust. They are combustible, meaning they can burn, and it is through combustion that their energy is released. Fossil fuels power modern day society, meeting energy demands for commercial manufacturing, transportation, electricity and even steam generation. The fossil fuels present on Earth cannot be replaced, and the U.S. Energy Information Administration estimates that by 2030, the Earth's oil reserves will be depleted. The high demand for fossil fuels is leading to their exhaustion, which could create a global crisis in terms of energy production.