Fracking is shorthand within the oil and gas industry for "hydraulic fracturing," a process in which drillers blast millions of gallons of water, sand and hazardous chemicals at high-pressure into sub-surface rock formations to create fractures that facilitate the flow of recoverable oil or gas. According to the Interstate Oil and Gas Compact Commission, 90 percent of all oil and gas wells in the U.S. are "fracked" to boost production. Fracking usually occurs just after a new well is drilled, but many wells are fractured numerous times to get as much production out of a profitable site as possible.

But after a series of accidents in Pennsylvania and elsewhere over the last few years, fracking has come under attack as dangerous to both human health and the environment. The most common problem involves the disposal of the toxic sludge that results from fracking. Texas-based XTO Energy, for instance, racked up 31 fracking-related pollution violations at 20 wells in Pennsylvania's Marcellus Shale in 2010 alone. But the fact that between 20 and 40 percent of the chemicals remain stranded underground-where they can contaminate drinking water, soils and other features of the environment that plants, animals and humans rely on-is perhaps even more troubling. According to the U.S. Environmental Protection Agency (EPA), a least nine different chemicals commonly used in fracking are injected into oil and gas wells at concentrations that pose a threat to human health.