The United States is seeing a natural gas boom thanks to discoveries of abundant shale gas, and at the same time a groundswell of opposition from critics who say the environmental risks from drilling are too great.

At the heart of the issue is a drilling technique known as hydraulic fracturing, or "fracking," of underground rock formations by injecting chemicals and water to release trapped gas.

The natural gas reserves could supply US needs for 110 years, thanks in part to advances in horizontal drilling, according to the US Energy Information Administration.

But much of the gas is in areas which are not accustomed to drilling, including towns in Pennsylvania and around the Dallas metropolitan area in Texas.

Chris Tucker, spokesman for the industry funded group Energy In Depth, said that the Marcellus Shale over a wide area of the eastern United States, the Barnett Shale in Texas and others in the west could produce the energy equivalent of 87 billion barrels of oil.

And since the gas is close to population centers where energy demands are greatest, Tucker said, "you can produce it in the morning and have it in New York City by lunchtime."

Over 3,000 gas wells have been drilled in Pennsylvania alone in the past six years, according to industry figures, and 15,000 in north Texas. This has helped drive down the price of natural gas from around $13 per million cubic feet in 2005 to just over $4 today.