As drivers travel down Route 6 through Mansfield, Pa., they quickly realize something has changed about the rural town. Trailers for energy companies are popping up like mushrooms, and traffic has become increasingly snarled as trucks carrying material to and from natural-gas drilling sites share the road with local cars. It's not quite a boomtown, but it is certainly changing, and the transformative agent over the past two years has been the discovery of an estimated 500 trillion cubic feet of recoverable natural gas locked in the Marcellus Shale Formation some 5,000 to 8,000 feet below the earth's surface. The
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.