For a century and a half oil and gas drillers have known they can get more production if they apply force underground to push out liquid and gaseous fossil fuels.

In the years after the Civil War, an inventor developed a torpedo-like bomb to drop into wells and blast out more oil from surrounding rock. The torpedo made him wealthy.

Later, pressurized liquid became the preferred method.

An experiment in Kansas in 1947 showed that the liquid process, called hydraulic fracturing, can rupture tiny cracks in underground rock to release trapped natural gas. The process, now known as fracking, came into increased use over the following decades as oil and gas companies used it to make old wells more productive.

The process prompted environmental fear that cracking bedrock with foreign substances, including chemicals, would destroy water sources and contaminate aquifers. Fears developed that fracking might trigger serious earthquakes, that it causes natural gas to come out of people’s faucets and that it has resulted in major greenhouse gas leaks into the atmosphere.

Some places ban it and the federal government has increased regulation, such as restricting the type of chemicals used. But, to date, little definitive evidence of major environmental damage has came to light in studies.

The Environmental Protection Agency is currently involved in a nationwide study to evaluate potential environmental concerns.

In 2005, an exploratory fracking well changed the dynamics of energy production almost overnight. Workers sunk the well into the giant Marcellus shale beds in Pennsylvania, the state where America’s petroleum drilling industry began in 1859 when Edwin L. Drake and George Bissell struck oil near Titusville.

The Marcellus well spawned a natural gas boom throughout Pennsylvania that spread to shale deposits across the country.

Major shale gas areas are located between New York and Illinois, throughout parts of the Great Plains and Rocky Mountain region and in some portions of the Southern Appalachians, not including South Carolina which does not have any sizable shale gas formations.

The fracking boom in both natural gas and oil is expected to make the nation energy independent within a few years. For how long is a matter of debate: Some say for just 20 years. Others say the deposits will last a hundred years or longer.

At least one thing is certain: As fracking has soared and other countries joined in, the cost of natural gas has plummeted, by more than 80 percent at one point.

That ignited rapid shifts in electric power production.

As recently as 2005 coal formed the backbone of the nation’s power production, generating one-half of the output. By last year the percentage had dropped to 37 percent.

And many power companies that had expected to start building new nuclear reactors just a few years ago are now dropping those plans and building natural gas-powered generation units.