The Berkeley Pit is a former open pit copper mine located in Butte, Montana, United States. It is 1 mi long by 1⁄2 mi wide with an approximate depth of 1,780 feet. It is filled to a depth of about 900 feet with water that is heavily acidic (2.5 pH level), about the acidity of Coca-Cola, lemon juice, or gastric acid. As a result, the pit is laden with heavy metals and dangerous chemicals that leach from the rock, including copper, arsenic, cadmium, zinc, and sulfuric acid.

The mine was opened in 1955 and operated by the Anaconda Copper Mining Company, and later by the Atlantic Richfield Company (ARCO), until its closure on Earth Day in 1982. When the pit was closed, the water pumps in the nearby Kelley Mine, 3,800 ft below the surface, were turned off, and groundwater from the surrounding aquifers began to slowly fill the Berkeley Pit, rising at about the rate of 1 ft a month. Since its closure, the water level in the pit has risen to within 150 ft of the natural water table.