For one of the largest mines ever conceived, the proposed Pebble Mine project in southwest Alaska hasn't gotten much press. The site, located about 200 miles from Anchorage and accessible only by plane, is said to contain half a trillion dollars' worth of copper, gold and other valuable minerals. For years now a group called the Pebble Limited Partnership has been working to start mining operations at the site, but for just as long, conservation groups have fought the plan, saying it would cause untold environmental disaster.

This week the Environmental Protection Agency came out in favor of restricting the mine, saying it "would cause irreversible damage to one of the world's last intact salmon ecosystems," according to EPA regional administrator Dennis McLerran, who added "Bristol Bay's exceptional fisheries deserve exceptional protection."

The move was not unexpected, and in fact it comes two months after the Pebble Limited Partnership filed a preemptive lawsuit against the EPA attempting to block the agency from taking any steps to regulate the mine. The EPA used preliminary plans filed in 2011 to make its decision; Pebble Mine says its plans are not final and accused the EPA of overstepping its authority.

Those 2011 plans — along with earlier plans filed in 2006 — tell a story of what could happen if the Pebble Mine is approved. It would create an open-pit mine 3.2 miles wide and 4,000 feet deep. The mining process would produce more than 10 billion tons of rock waste and require the construction of nine miles of dams to "impound" another 2.5 billion tons of toxic waste. Environmental groups such as the Wild Salmon Center say the extreme weather conditions and earthquakes that are common in the area could cause these dams to fail, "resulting in potentially catastrophic impacts to the Bristol Bay fishery." The group also says "the type of ore at Pebble is likely to produce acid mine drainage, which may lead to chronic contamination of surface and ground waters, having a severe detrimental impact on aquatic life."

The website Save Bristol Bay, run by Trout Unlimited, says this in particular would have a high risk of damaging the $500 million commercial salmon fishing industry in the region. The NRDC's web page opposing the project says the mine "would be gouged out of an American paradise — filled with salmon, bears, moose, caribou, wolverines and whales — that has sustained native communities for thousands of years."

The EPA's actions this week themselves aren't final, nor would they block the mine. The agency's proposals would, however, restrict "all discharge of dredged or fill material related to mining the Pebble deposit" if it would result in the loss of streams, wetlands, lakes, ponds, or other important salmon habitats. The EPA, which is using the Clean Water Act as its basis for action, will now hold public meetings and seek public comment on its proposals.

Although the Pebble mine has been in the works for more than a decade, it has gotten relatively little press, especially compared to more high-profile project such as Keystone XL. Aljazeera America recently called it "the biggest environmental controversy few have heard of." Hopefully this week's action by the EPA will help to turn that around.

Related on MNN:

EPA: Proposed Pebble Mine could destroy Alaskan salmon fishery
The mining project, one of the largest ever conceived, could hold $500 billion in gold and copper.