With AFP Story by Veronique DUPONT: US-Energy-Gas-Environment A worker walks toward a part of a Consol Energy Horizontal Gas Drilling Rig exploring the Marcellus Shale outside the town of Waynesburg, PA on April 13, 2012. It is estimated that more than 500 trillion cubic feet of shale gas is contained in this stretch of rock that runs through parts of Pennsylvania, New York, Ohio and West Virginia.Shale gas is natural gas stored deep underground in fine-grained sedimentary rocks. It can be extracted using a process known as hydraulic fracturing "“ or 'fracking' "“ which involves drilling long horizontal wells in shale rocks more than a kilometre below the surface. Massive quantities of water, sand and chemicals are pumped into the wells at high pressure. This opens up fissures in the shale, which are held open by the sand, enabling the trapped gas to escape to the surface for collection.