The Berkeley Pit Lake, located in Butte, Montana, is a part of the largest Federal Superfund site in the United States. Selective underground mining in the Butte area began in the 1860s. The Berkeley Pit mine was eventually opened in 1955, when the area surrounding the mine was surface-stripped by the Anaconda Copper Mining Company to allow miners access to the depths of the mountain. The mines were worked until 1982, when the operation was abandoned. Over the twenty-seven years it was active, the Berkeley Pit Mine produced over one billion tons of ore, including copper, lead, zinc, gold, and manganese, leading to the nickname the Richest Hill on Earth. Once the pit was closed in 1982, groundwater started to fill the pit to the natural groundwater level. Because its water contains high concentrations of metals such as copper and zinc, the Berkeley Pit is listed as a federal Superfund site.