When you flip on a light switch, odds are, you're burning coal. But as the fracking boom continues to unleash huge quantities of natural gas, the nation's electric grid is changing. Power plants are increasingly turning to this low-cost, cleaner-burning fossil fuel.Bill Pentak stands in the middle of a construction site, looking up at his company's latest project towering overhead — a new natural gas power plant."This plant was sited precisely where it is because of its access to the abundant, high-quality natural gas that's found a mile to two miles beneath our feet," he says.