The fears about copper-nickel mining start with sulfuric rock the metals are found in. When exposed to the air, these rocks can leech caustic pollutants like acid and metals.
Just west of Duluth, the St. Louis River spills through rocky channels on a final plunge to Lake Superior. Retired biology teacher Len Anderson said, not only is this area beautiful, it's key for the Lake Superior fishery.
"It also is the nursery for many of the fish that inhabit Lake Superior," Anderson said. "You know, over 100 river miles away from PolyMet, but this is where, I believe, the critical issue is going to come to a head."
The issue, he said, is methyl mercury - mercury in a form that can harm fish as well as the people and animals that eat the fish. The mercury is packed into sediment behind a nearby dam, but it's in a relatively safe form. Sulfates from mining, he said, would convert it into toxic methyl mercury.
Mining opponents, like Betsy Daub with the group Friends of the Boundary Waters, worry about sulfide mining's track record in western states.
"What we haven't yet seen is the mining companies - the industry - be able to hold up some shining star examples of sulfide mining where we haven't seen really horrifying pollution problems," Daub said.
Daub cites a 2006 study, co-written by Butte, Montana-based mining consultant Jim Kuipers. Kuipers studied two dozen projects, comparing what they said would happen with pollutants with what actually happened.
"In nearly every case where we had mines in close proximity to surface water and ground water, we saw that there was almost a 90 percent, if not greater, probability that the predicted water quality wasn't actually what we saw," Kuipers said.
Of 25 mines studied, he found 76 percent violated water quality standards. One of the worse cases studied is the Black Hill's Gilt Age gold mine. After multiple pollution spills, the mining company went out of business and South Dakota has spent millions trying to clean up the mess.
Some mines did much better. In Wisconsin, state regulators point to Kennecott Mining's Flambeau mine near Ladysmith.
"We don't see any evidence out there that it has severely adversely impacted the Flambeau River," said Phillip Fauble, Mining Program Coordinator with the Wisconsin DNR. "It seems to be acting as predicted in their models. We haven't seen that they've violated in the permit conditions for groundwater. You know, we've set conditions that they had to meet, and Kennecott showed that they could meet them."
The Flambeau mine site will be monitored at least 40 years to ensure it doesn't pollute.
The Minnesota DNR will play a key role in the Iron Range projects. The agency has learned plenty from a taconite mine called the Dunka pit. Sulfite rock exposed there in the 1960s was leeching metals into a nearby creek.
The state compelled the mining company to build wetlands that now largely absorb the metals. But the site requires monitoring for the foreseeable future. DNR mining researcher Paul Eger said you can't just walk away from a closed sulfide mine.
"We've learned a lot," Eger said. "New mines can operate with much less maintenance and much less unexpected water quality issues, but I do think that there is going to be some maintenance of these areas."
Mining expert Jim Kuipers said sulfide mining can be done well, with the best chance in a state with mining history like Minnesota.
"If we can encourage good mining companies, with good solid deposits to do the right thing, we might actually make some advance in terms of environmental protection, and things like that," says Kuipers.
But Kuipers said mining with poor deposits or weak financial backing often goes badly. And, he said, you have to balance mining with other values in the region.
"You know, you may have the perfect ore deposit sitting next to the last place in the planet you really want to mine," Kuipers said. "Then you really don't have a situation where I'm going to suggest you can do it right."
In Minnesota, the debate will focus on a region between Lake Superior and the Boundary Waters - an area some people might call the last place on earth you should risk polluting.