The Berkeley Pit is a former open pit copper mine located in Butte, Montana, USA. It is one mile long by half a mile wide with an approximate depth of 1,780 feet (540 m). It is filled to a depth of about 900 feet (270 m) with water that is heavily acidic (2.5 pH level). The pit is laden with heavy metals and dangerous chemicals, including arsenic, cadmium, zinc, and sulfuric acid.

The mine was opened in 1955 and operated by Anaconda Copper 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 from the surrounding aquifers began to slowly fill the pit. Since the pit closure in 1982, the level has risen to within 150 feet of the natural groundwater level.

The pit and its water present a serious environmental problem because the water, with dissolved oxygen, allows pyrite and sulfide minerals in the ore and wall rocks to decay, releasing acid. If the pit water level reaches the natural water table, the pit water has the potential to seep into surrounding groundwater. The acidic water in the pit can carry a heavy load of dissolved heavy metals. In fact, the water contains so much dissolved metal (up to 187 ppm Cu) that the material is mined directly from the water.

In the 1990s plans were devised for solving the groundwater problem. Water flowing into the pit has been diverted to slow the rise of the water level. Plans have been made for more extensive treatment in the future. The Berkeley Pit has since become one of the largest Superfund sites.

The pit is currently a tourist attraction, with an adjacent gift shop. A $2 admission fee is charged to go out on the viewing platform.