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The fracking process allows access to huge deposits of natural gas trapped thousands of feet below the surface in shale rock formations. The Marcellus Shale, which spans much of Pennsylvania, West Virginia and other states, contains one of the world’s largest known natural gas deposits. Proponents say it could satisfy the nation’s gas demands for 50 years or more and help break U.S. dependence on foreign oil.

To extract the gas, companies pump a mixture of water, sand and chemicals into the ground at tremendous pressure, fracturing the rock and allowing gas to flow freely. Companies install layers of protective piping and concrete to ensure the fracking fluid and gas come nowhere near water supplies.