Geothermal energy may be approaching its Mitchell moment. George Mitchell, a scrappy independent oilman, is known as the father of fracking. Nearly three decades ago, he defied Big Oil and the conventional wisdom of his industry by making practical the hitherto uneconomic technique of pumping liquids and sands into the ground to force out gas and oil from shale rock and other tight geological formations. The enormous increase in productivity that resulted, known as the shale revolution, has transformed the global hydrocarbon business.