Berkley Pit

From Wikipedia (http://en.wikipedia.org/wiki/Berkeley_Pit):
The Berkeley Pit is a former open pit copper mine located in Butte, Montana, USA, filled with water a mile wide and over 900 feet deep. The water 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 also turned off, which caused groundwater from the surrounding basin to leak into the pit at a very fast rate. The water seeped through the metal deposits and picked up their toxicity. The water is so loaded with metals (up to 187 ppm Cu) that "mining" of the water has been proposed. However, fluctuations in power prices have made this difficult.

Not until the 1990s were plans devised for solving the groundwater problem. The Berkeley Pit has since become one of the largest Superfund sites.