Nine Native American tribes in South Dakota, North Dakota and Nebraska are suing the federal government in a bid to stop exploratory drilling for graphite near a sacred site in the Black Hills.

A small group of opponents has been demonstrating at the drilling location and at the mining company’s headquarters in what they call a land defense effort since they learned ground was broken on the drilling project in late April.

The tribes filed their federal lawsuit Thursday in South Dakota against the U.S. Forest Service and U.S. Department of Agriculture, alleging the agencies violated federal law by greenlighting a project near a site called Pe’Sla, a meadow in the central Black Hills used for tribal ceremonies, prayer and youth camps year-round. Buffalo regularly graze at the site, the suit said, adding the project poses a threat to wildlife.

Graphite has many industrial uses, including in batteries, lubricants, certain auto parts and in blast furnaces, according to website of the European Carbon and Graphite Association.

The project is the latest point of tension between tribes and mining interests in the lush pine and spruce-covered Black Hills, which encompass more than 1.2 million acres, rising from the Great Plains in southwest South Dakota and extending into Wyoming.

The region is sacred to Sioux tribes who call the area He Sapa and consider it “the heart of everything that is,” according to the complaint.

Some of the landscape was altered by an 1870s gold rush that displaced Native Americans. And more recently, a new crop of miners driven by rising gold prices have sought to return to the landscape.

The 1868 Treaty of Fort Laramie granted the Sioux Nations rights to the Black Hills, but the U.S. broke the treaty after gold was discovered. Though the Supreme Court ruled the Sioux were owed compensation, they have not accepted it and consider it unceded territory.

The complaint said the project by Rapid City-based mining company Pete Lien & Sons would impact the use of Pe’Sla for traditional, cultural and religious purposes by the tribes, and that the Forest Service did not consult with the tribes before approving the project.

Tribes bought parts of Pe’Sla in 2012, 2015 and 2018, and an agreement between the tribes and the Forest Service established a two-mile buffer zone on public lands around the site, according to the complaint.

Because Pe’Sla was not included as an affected area and no environmental review was conducted, the approval violates the National Historic Preservation Act and National Environmental Policy Act, the lawsuit alleges.