Nation: About 386 million years ago, Pennsylvania received a great gift-or, depending on your point of view, a curse: the Marcellus Shale, a formation of black rock and limestone containing vast reserves of natural gas. The formation stretches from New York through the Appalachian Basin, but it's Pennsylvania that has played host to the most intensive plundering of those reserves since the mid-2000s, when energy companies began to use hydraulic fracturing to free up the gas trapped a mile below the surface...