Over the last several years, vast amounts of natural gas have been found in the Marcellus shale that lies underneath vast areas of western New York, western and northern Pennsylvania, and parts of Ohio and West Virginia. If geologists are right, the Marcellus shale could be the world’s second-largest natural-gas field in the world. The natural gas is held tightly trapped in the shale, but advanced drilling techniques have made it economical to drill for the gas. Much of the activity is centered in the counties south of Pittsburgh.