The United States sourced about 81 percent of its energy needs from fossil fuels. in 2015. Fossil fuels -- oil, natural gas and coal -- come from the decayed remains of the plants and animals that lived and died more than 300 million years ago. Buried and compressed under layers of rock and sand in the Earth and beneath the oceans, these remains became the different deposits drilled, mined, excavated and used as non-renewable energy sources in modern life.

More than 6,000 years ago, people living along the Euphrates River and ancient Egyptians collected a black liquid seeping from the ground -- oil. They used it as a medicine for wounds and burned it to provide light from lamps. In that same region, between 6,000 and 2,000 years ago, lightning strikes ignited gas seeps and introduced natural gas to the ancient Persians for the "eternal fires" of their fire-worship. More than 3,000 years ago, the Chinese discovered coal as a stone that burned: they used it to smelt copper.

When burned, oil, natural gas and coal produce the chemical energy that meets more than 85 percent of the world's energy demands. The demand for oil has progressed way beyond ancient medicinal use -- Native Americans waterproofing their canoes or the Revolutionary War-era treatment of frostbite. Petroleum products not only heat homes and businesses, they fuel transportation on land, sea and air and generate electrical power. Farm fertilizers, fabrics, almost all plastics and thousands of other vital and daily-use products come from crude oil.

For many years, coal was the mail fuel for heating homes and businesses, powering railroads and factories. Today, coal is the primary fuel for powering electricity. In 2015, coal accounted for nearly 32.3 percent of all U.S. electricity generated.

The natural gas industry, once a source for lighting in homes and street lamps, still remains a vital fuel source. Both public and private enterprises benefit from the modern technologies for retrieving it from the ground and distribution, supplying 32.7 percent of U.S. electrical needs in 2015. Natural gas remains a source for heating and air conditioning office buildings, schools, churches, hotels, restaurants and government buildings and fulfills the cooking needs for restaurants and other facilities. Used for waste treatment and incineration, natural gas also supplies power to furnaces in glass manufacturing and food processing.

Researchers and scientists argue that no fossil fuels will remain after about 2050, though that number keeps changing. Alternative energy sources include bioenergy, wind, solar, hydropower, geothermal, hydrogen and fuel cells and nuclear energy. Since the U.S. still relies on at least 81 percent of fossil fuels as an energy source, when these fuels are gone, the country must find look to other energy sources. Technology currently exists to deploy these alternative sources -- and some states already use these clean energy sources -- but several states block some or all of their use, and the federal government recently imposed tariffs on imported solar goods, which dampens research and raises the costs.