Vast reservoirs of heat are locked in the earth's interior, untapped. The ground underneath our feet holds so much heat that tapping only 2 percent of it could satisfy current annual U.S. energy use 2,000-fold for each and every year of the foreseeable future, according to an analysis from the Massachusetts Institute of Technology. Fracking, the same technology used to drill for natural gas, may provide an economical way to get at that geothermal energy.

The idea is simple: Pump water or other fluids down deep beneath the surface. Hot rocks at depth boil the water into steam, which rises back to the surface to spin a turbine and generate electricity. In regions with hot rocks and plentiful water near the surface, building massive power plants is straightforward. But such optimal sites are few and far between. Fracking, which is the technique of fracturing passages in subterranean rock, can help. The same fractures that send natural gas streaming out from deep wells also allow geothermal heat to be tapped from practically anywhere on earth.

The U.S. Geological Survey estimates at least 500 gigawatts of electricity-generating capacity could be harvested this way—or 1.5 times more than the entire U.S. fleet of coal-fired power plants. That immense potential is why the Department of Energy invested $5.4 million to help geothermal specialists Ormat Technologies create the nation's first fracking-enhanced geothermal system (EGS). The company's Desert Peak power plant started spinning out electricity in April near Reno, Nev. Other EGS plants are already operating in Europe and Australia.

“The big prize is EGS,” enthuses Douglas Hollett, an oil industry veteran now heading the DOE's geothermal technologies office. “The key is learning how to do it in a reliable way, in a responsible way.”

As it stands, geothermal produces “less than one percent of global energy,” notes a recent perspective in Science. The reason for this dearth is simple: money. In addition to the years-long, multimillion-dollar expense of building a power plant on top of any viable wells, there is also the $6-million to $8-million risk of prospecting and drilling a dry hole—a well that does not produce steam. Drill too many dry holes, and you'll go bankrupt before generating and selling a single watt of power.

If technologies can be developed to reduce the risks, geothermal could play a more prominent energy role. With that in mind, the DOE is pursuing better methods for geothermal prospecting, drilling and fracking. Because much of that work could also benefit traditional drilling, the oil and gas industry may actually help foot the bill for enhanced geothermal technology. As Hollett says, “We know that has to tie into what we do.”