Environmentalists have filed a second lawsuit to block a federal land swap for a hotly contested copper-nickel mine in northeast Minnesota, arguing that the transfer would give PolyMet Mining Corp. a windfall at the expense of the Minnesota public.

The suit, filed Monday by a nonprofit called WaterLegacy, says the U.S. Forest Service grossly undervalued the land it will give PolyMet for mining in exchange for private parcels owned by the company.

It is another in what could be multiple legal actions over a highly controversial open-pit mine proposed by PolyMet on Superior National Forest land near Hoyt Lakes on Minnesota's Iron Range.

PolyMet, a Canadian company one-third owned by the commodities giant Glencore, owns the rights to minerals beneath the surface. But the Forest Service doesn't permit open-pit mines on its properties. Instead, it agreed to give PolyMet 6,650 acres of Forest Service land at the mine site in a swap for 6,690 acres of what are now privately held acres elsewhere in northeast Minnesota.

The Forest Service said that a land swap would also avoid likely litigation from PolyMet over access to the ore.

The Forest Service priced its property at $550 per acre for its value as timberland, based on sales of similar forests in Michigan and Wisconsin, said Paula Maccabee, an attorney with WaterLegacy.

But, she said, the Forest Service violated federal rules because it should have valued the parcels as much more valuable mineral lands, since that's why PolyMet wants them. Maccabee said that in its own appraisal, the Forest Service said mineral lands in Minnesota were being sold at an average price of $1,645 per acre. In addition, she said, Kennecott Exploration Co. which is exploring for copper nearby, has paid up to an average of $3,885 per acre.

A spokeswoman from the Superior National Forest declined to comment.

PolyMet said in a statement the Forest Service "followed well-established federal guidelines by the U.S. Department of Justice. After years of review and analysis, the Forest Service has determined the land exchange is in the best public interest."

The earlier legal action, filed by the Center for Biological Diversity and Earthworks, served notice that they will sue the federal government for trading away wetlands and forest that provide critical habitat for wolves and lynx, two threatened species.

PolyMet has proposed an initial $650 million project that it says could generate up to 350 permanent jobs. After years of environmental review, it applied for a permit to mine, which state regulators are now considering.

The project is controversial because, unlike taconite mining, hard-rock mining generates acid runoff from tailings and waste rock that risks severe contamination to local water. PolyMet's mine could require water treatment for up to hundreds of years.