The U.S. Army Corps of Engineers has given PolyMet Mining Corp. the last major permit it needs to proceed with the state’s first copper-nickel mine.

The permit, announced by the Corps on Friday, allows PolyMet to fill in 900 acres of wetlands to construct the mine, which will be located near Babbitt in northeastern Minnesota. To offset that loss, the company will be required to buy credits from a federal wetlands bank that will be used to fund restoration projects in the same watershed.

The decision caps a review that lasted more than a decade. Calling the process “thorough” and complex, Corps officials said the permit saved 500 acres of wetlands from the company’s original proposal, which would have filled in more than 1,400 acres and involved a more uncertain mitigation strategy.

“This has been a very carefully weighed decision,” said Col. Sam Calkins, commander of the Corps’ St. Paul District.

The permit had been expected for several months, after PolyMet cleared the major hurdles of Minnesota state regulators. Still, the decision was celebrated by mining proponents and decried by environmental advocates as a milestone that pushes forward the project, which will be the first to try to pull copper, nickel and other precious metals out of the Iron Range.

PolyMet CEO Jon Cherry said that, with all required permits now in hand, the company will turn its focus to securing financing for the $1 billion project. “We couldn’t be happier with where we are today. This is a huge win for northern Minnesota,” Cherry said.

The mine is not, however, a done deal. Environmental groups have filed court challenges on most of the permits granted by the state, including those that regulate the safety of the mine and a tailings dam, air and water quality and the mine’s overall environmental impact, as well as the original land exchange that gave PolyMet mineral rights at the site.

Those appeals are working their way through the courts.

The Corps permit, too, could wind up in court. The Minnesota Center for Environmental Advocacy, a nonprofit environmental law firm, and its lawyers are reviewing the wetlands permit and deciding whether to appeal, said spokesman Aaron Klemz.

“This will be the largest permitted destruction of wetlands in Minnesota’s history,” he said.

Klemz said the group believes both PolyMet and the Corps are drastically underestimating the amount of wetlands that would be destroyed by the project. While the permit addresses areas that will be directly filled by mine construction, it appears to ignore the surrounding wetlands that will be drained once the mine is excavated, Klemz said.

“You dig a hole in the ground, it fills with groundwater,” he said. “That water comes from somewhere — it comes from wetlands. Our experts say that thousands of acres of wetlands are going to be drained into the pit.”

Cherry said the mitigation credits the company is buying will end up restoring more wetlands than the mine will drain.

“The process was very lengthy, very thorough, and it worked,” Cherry said. “We listened to the public comments and we made adjustments to our design.”

While the Corps’ review lasted more than 10 years, much of it was done outside of public view, according to the environmental groups. The last hearing about the potential wetland effects and PolyMet’s mitigation strategy was held more than four years ago. The project has evolved significantly since then, Klemz said.

The Center for Environmental Advocacy petitioned the Corps in 2015 to hold a second hearing. The Corps responded Thursday, denying the request a day before announcing the permit had been issued, Klemz said.

Proponents of the mine welcomed Friday’s decision, saying PolyMet will create needed jobs on the Iron Range. The Corps’ go-ahead will prove to be an economic jolt to the region for decades, said Republican U.S. Rep. Pete Stauber.

“Families in northeastern Minnesota have been waiting 14 years, throughout the longest environmental review in our state’s history, for PolyMet to begin mining,” Stauber said.