The Berkeley Pit is a former open pit copper mine located in Butte, Montana USA. The pit began to fill with water after the mine was closed in 1982 and is currently about 900 feet deep. The water is very acidic (pH of around 2.5) due to dissolved sulfur compounds from the surrounding rock. The acid has leached heavy metals such as arsenic from the rock, creating a toxic brew. The concentration of copper in the water is so high that a chemical process is used to recover the metal from the water.
Wikipedia Entry on the Berkeley Pit
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