The Berkeley Pit

A mile-long pit holding 900 feet of acid water full of metal

The Berkeley Pit is a former open-pit copper mine in Butte, Montana, a mile long and half a mile wide, about 1,780 feet at its deepest, and filled to a depth of about 900 feet with acidic water. It was cut into the vein system of the Butte hill, called the richest hill on earth, where mining began in 1864 and copper took over after 1881. Open-pit work started in July 1955 next to the old underground Berkeley Mine, which gave the pit its name, because it was cheaper and safer than going underground. Within a year the pit was taking out 17,000 tons of ore a day at 0.75 percent copper, and in all about a billion tons of material came out, with silver and gold besides. In the 1970s the Anaconda Company bought up whole neighbourhoods over the ore, Meaderville, McQueen, East Butte, Dublin Gulch and Finntown, and the pit swallowed them.

ARCO closed the mine on 22 April 1982 and shut off the pumps that had kept more than 3,500 miles of underground workings dry. Groundwater rose back toward its old level, and the pit filled with acid drainage from its own walls, the old mines, waste dumps and leach pads, rising about a foot a month. The water runs at a pH of 4.1 to 4.5, about as acid as beer or tomatoes, laden with arsenic, cadmium, copper, zinc and other metals dissolved from the rock. The EPA calls it the largest body of contaminated water in the United States.

The pit joined the Silver Bow Creek and Butte Area Superfund site in 1987. Under an $87 million settlement, ARCO and five other companies pump and treat the water to keep it below the protective level of 5,410 feet, above which it could reach Silver Bow Creek and the groundwater; a treatment plant opened in 2003, and since 2019 treated pit water has gone into Silver Bow Creek.

The water is deadly to birds. In the mid-1990s, 1995 by the EPA's account and 1996 by others, 342 snow geese were found dead in it, and on 28 November 2016 a storm brought down perhaps 60,000 more, of which three to four thousand died. Noise machines and propane cannons now ring the rim. Life has adapted all the same: Euglena mutabilis thrives in the water, and microbes from it have yielded compounds being tested against cancer cells. A viewing stand and small visitor centre let you look down into it.