One of two things Butte, Montana is best known for along with the Berkeley Pit:
"The Berkeley Pit is a former open pit copper mine located in Butte, Montana, USA, about a mile and a half wide and about 1,780 feet deep. It contains about 900 feet of water that is heavily acidic, with a pH level of 2.5, and laden with heavy metals and dangerous chemicals such as arsenic, cadmium, zinc, and sulfuric acid.
The mine was opened in 1955 and operated by the Anaconda Mining Company and later by the Atlantic Richfield Company (ARCO), until its closure in 1982. When the pit was closed, the water pumps at the bottom were removed, and groundwater sourced from the surrounding aquifers soon filled the pit to the natural groundwater level.
This has presented an environmental problem in that the water, with dissolved oxygen, allows pyrite and sulphide minerals in the ore and wall rocks to decay, releasing acid. The acidic water in the pit can carry a heavy load of dissolved heavy metals. The water contains so much dissolved metal (up to 187 ppm Cu) that "mining" of the water has been proposed. However, fluctuations in power prices and metal prices have made this economically unfeasible at present.
In the 1990s were plans devised for solving the groundwater problem. The Berkeley Pit has since become one of the largest Superfund sites."
http://en.wikipedia.org/wiki/Berkeley_Pit
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