This is part of our year-end series, looking at where things stand in each of our major investigations.

This was the year that "fracking" became a household word.

It wasn't just that environmental concerns about the underground drilling process finally struck a mainstream chord -- after three years of reporting and more than 125 stories. For the first time, independent scientific investigations linked the drilling technique with water pollution, and a variety of federal and state agencies responded to the growing apprehension about water contamination with more studies and more regulation.

The most important development -- and perhaps a crucial turning point -- was in December. In a landmark finding, the Environmental Protection Agency concluded that hydraulic fracturing was the likely culprit in a spate of groundwater contamination that had forced residents to stop using their water in dozens of homes in central Wyoming. The agency had been investigating since 2008.

Earlier in the year, a study published through the National Academy of Sciences determined that in Pennsylvania, private water wells in close proximity to fracked gas wells were 17 times more likely to be contaminated with methane gas.

Those studies are separate from a national research project the EPA has undertaken to assess the risks fracking poses to water resources. The agency is examining five case studies across the country and is now estimating that some of its report will be complete by the original 2012 deadline and the rest will continue into 2014.

The study is meant to help Congress and regulators determine whether fracking should be regulated like other similar processes under the Safe Drinking Water Act and whether companies that frack should be forced to disclose the details about the chemicals they use.

Last winter, the Obama administration -- which has repeatedly referred to natural gas as a bridge fuel and encouraged its development -- urged the Department of Energy to conduct its own assessment of fracking's safety on a quicker timeline than the EPA.

In a matter of months, a DOE panel determined that the environmental risks were substantial and needed to be addressed in order to safely develop more natural gas resources. The panel raised concerns that pollution could have serious health consequences for those who live close to drilling operations.

Indeed, a report published by ProPublica in September found that residents in drilling areas across the country complained of serious health symptoms ranging from skin lesions to tumors, and that health and science organizations had yet to develop any comprehensive system for studying such problems.

While water pollution is one concern, many of the health effects reported are believed to be related to air pollution and emissions released in the natural gas development and drilling process.

Earlier in the year, a ProPublica investigation found that the EPA had grossly underestimated the amount of methane that seeps out of pipelines and drill rigs as gas is produced, and reported that the agency was doubling its calculations. Our analysis of the new emissions levels showed that they threaten to offset the relative advantages presented by cleaner-burning natural gas over oil and carbon in combating climate change and reducing carbon emissions.

In some cases, government officials didn't just debate fracking and call for additional study. They enacted real changes in how drilling is overseen.

The EPA announced that the drilling industry would have to comply with tough new industrial emissions standards. Then it said that it would issue new rules governing how wastewater from fracking is disposed of; this addressed concerns first raised by ProPublica in 2009 that in eastern drilling areas, which cannot inject waste into underground wells the way the industry does in the west, chemical-laden waste is winding up in river systems, and then drinking water. In December, Colorado implemented the toughest law yet requiring comprehensive disclosure of frack fluids, following similar but weaker laws in Texas and Wyoming.

This was also the year fracking went global. While France banned fracking outright and South Africa enacted a temporary moratorium, multi-national energy companies began exploring shale reserves in Poland, Argentina and China.

Closer to home, New York state officials continued to inch closer to allowing drilling to take place in the coveted Marcellus Shale. After a multi-year process and its own temporary moratorium on some fracking activity, New York finished up the latest version of its environmental review and has signaled that it intends to begin permitting more drilling early next year.

According to the state's environmental assessment, no fracking will be allowed on state lands, and the process will be severely limited within the New York City watershed.

Still, the state's chief environmental regulator, Joe Martens, told ProPublica he is confident the drilling can proceed safely, and that he does not expect there will be much to learn from the EPA's research into the issue. New York's draft plan is in its final stage of public review, and is expected to be completed on Jan. 11, 2012.

Staff reporter Nicholas Kusnetz contributed to this report.