Armed with new drilling techniques, companies are spreading out across the United States, cracking open shale rock in search of vast new stores of natural gas. It's not an exaggeration to say that hydraulic fracturing, or "fracking," has revolutionized the U.S. energy industry. Cheap natural gas has become America's top source for electricity, displacing coal and bringing back jobs to once-decaying states like Ohio.

Here there be shale gas.

But the fracking boom has also led to plenty of environmental concerns. Local communities are worried that the chemicals used to pry open the shale rock can contaminate nearby drinking water supplies. (So far, there's scant evidence this is happening in places like Pennsylvania, but the science is still in its infancy.) Excess gas is often vented off, producing air pollution. And the disposal of fracking wastewater underground appears to be linked to earthquakes in places like Ohio.

Confronted with these worries, states have responded with a patchwork of different regulations. But there's a lot of variation between different states. And here's a good way to track what's going on: A helpful series of new maps, put together by Resources for the Future (RFF), gives an overview of how 31 states with significant shale gas reserves are treating different aspects of fracking.

Here, for instance, is a look at which states require companies to disclose the chemicals they use in drilling. (Fracking is exempt from federal disclosure rules under the Safe Water Drinking Act.) Some states, like Pennsylvania — which sits above the gas-rich Marcellus shale formation — now require a full disclosure of chemicals. By contrast, Kansas, which is just beginning to see widespread fracking activity, is further behind:

Meanwhile, the map below details how different states treat the "venting" or release of excess gas into the air. Just 22 of the 31 gas states have restrictions on this process, which can release both heat-trapping methane into the atmosphere as well as “volatile organic compounds” such as benzene that can produce smog and trigger health problems. Some states ban this practice entirely; others restrict it to emergencies or require that operators not harm public health:

There are many more maps on RFF's Web site, which is worth poking around on. In an introductory essay, RFF's Nathan Richardson notes that these maps still provide just a partial picture — the details of laws matter, and more importantly, different states may enforce their rules with different levels of vigor. But it's an invaluable resource all the same.

The regulation of fracking has become a low-level campaign issue, as well. The Obama administration is gradually putting forward federal regulations. The Department of Interior is drafting rules for fracking on publicly-owned lands (where about 38 percent of the country's gas reserves sit, according to the American Petroleum Institute). The Environmental Protection Agency, meanwhile, is slowly getting in on regulation and has proposed rules that will require all producers to phase out venting by 2015 and capture their waste methane instead.

Mitt Romney, by contrast, has criticized the federal approach. In his "Believe in America" economic plan (pdf), he warns that the EPA should not "pursue overly aggressive interventions designed to discourage fracking altogether." By contrast, Romney praises states for having "carefully and effectively regulated the process for decades." Indeed, many Republicans believe that fracking regulations should be mainly left to the states, which can issue rules more speedily and can tailor regulations to the specific needs of their communities. Environmentalists, by contrast, worry that this will create a race to the bottom whereby states pare back their rules — or enforce them weakly — in order to compete for business.

Both sides agree that addressing the public health and environmental aspects of fracking isn't costless. The International Energy Agency recently estimated that addressing all of the various concerns could boost the price of natural gas by roughly 7 percent. Yet the IEA also warned that if these rules weren't adopted, public outcry and protests could stop the shale gas boom altogether. Anti-fracking protests like those in New York state could become the norm. And that, the IEA notes, could prove even more costly to the gas industry.