Coal, oil and natural gas are called fossil fuels - they were formed deep under the earth millions of years ago from the decomposition of plant and animal matter.
Fossil fuels are used to generate electricity because they're widely available, inexpensive, easy to transport, easy to use, and the power plants can be located almost anywhere.
Coal is the most commonly used fossil fuel because it burns extremely hot for a long period of time, and it can be stored for future use.
Big increase in demand
Our dependence on fossil fuels grew during and after World War II. Prior to the 1940s, hydro power was becoming the dominant technology, with dozens of federally-funded hydroelectric plants constructed during the Great Depression.
The demand for electricity suddenly increased with the advent of World War II, and continued to increase over the following decades. The only way utilities could keep up with the demand was to build power plants.
The environmental impact
The utility industry met the explosion in demand for electricity without real knowledge of the environmental impact of using fossil fuels. When coal, natural gas or oil are burned, they release gases into the atmosphere:
In addition, the process of bringing fossil fuels to the surface, such as strip mining or oil exploration, can be detrimental to the surrounding habitat.
And while fuels like coal, natural gas and petroleum are still plentiful, the supply will eventually run out - once we use them, they're gone forever.