Twin Metals Minnesota, a wholly owned subsidiary of Chilean mining firm Antofagasta, has, for years, been trying to mine at the doorstep of the 1.09 million-acre Boundary Waters the most visited wilderness area in the country.

Opponents argue that allowing the project to move ahead could forever sully the vast network of federally protected rivers, lakes and creeks, often pointing to the hard-rock mining industrys long legacy of polluting water resources across the country. Extracting sulfide-bearing minerals like those Twin Metals is after could trigger the release of toxic sulfuric acid that can persist indefinitely.

In a legal opinion on Wednesday, Ann Marie Bledsoe Downes, the Interior Departments principal deputy solicitor, wrote that the Trump administration had improperly renewed the leases and violated department regulations by customizing lease terms specifically for Twin Metals.

The Trump family has ties to Andrnico Luksic, the Chilean billionaire behind the mine, and Twin Metals spent more than $1 million lobbying government agencies during Trumps tenure, according to federal filings.

Democratic lawmakers and environmental groups cheered Wednesdays announcement.

Some places are simply too special to mine, and it is our obligation to ensure these unique and valuable lands and waters remain intact for generations to come, Rep. Betty McCollum (D-Minn.) said in a statement.