Fracking for natural gas

In this Jan. 17, 2013, photo, Universal Well Service fracturing engineering team leader Mike Michaelson, right, talks with technician Sean Cline before they start the 16 cylinder diesel engine that has been converted to run on a blend of diesel and natural gas at the Cummins Bridgeway facility in Gibsonia, Pa.

Credit: AP Photo/Keith Srakocic