Berkeley Pit: Montana’s Toxic Mile-Long Lake

Butte was once called “the richest hill on earth,” and in July 1955 the Anaconda Copper Mining Company began digging it out from the top. The Berkeley Pit grew for decades. In the 1970s, its expansion swallowed whole neighborhoods, including Meaderville, McQueen, East Butte, Dublin Gulch and Finntown. The mine was always fighting water. Pumps deep in the Kelley Mine, about 3,800 feet below the surface, kept groundwater out of the workings. On April 22, 1982, under owner Atlantic Richfield, mining stopped and the pumps were turned off. Water began pouring back in, at first about a foot a month. As it moved through the exposed rock, it picked up sulfuric acid and dissolved metals: copper, arsenic, cadmium and zinc. The pit, about a mile long and half a mile wide, slowly became a lake roughly 900 feet deep, with a pH of about 4.1 to 4.5. The site became part of a federal Superfund cleanup. Regulators set a hard limit: the water could not rise above 5,410 feet, the point where it could start flowing into the surrounding groundwater. Wildlife showed what was at stake. In 1996, researchers recovered 342 dead snow geese from the pit. In November 2016, a huge flock landed there, and thousands died. In October 2019, a treatment plant began pumping out pit water, cleaning it and releasing it into Silver Bow Creek. By April 2025, about 9 billion gallons had been treated, and the level had held steady at 5,356 feet, about 54 feet below the limit. The lake has stopped rising for now, but the pumps can never stop.