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 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 sulfide 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 is actually done.
In the 1990s plans were devised for solving the groundwater problem. 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.
The underground Berkeley Mine was located on a prominent vein extending to the southeast from the main Anaconda vein system. When open pit mining operations began in July 1955, near the Berkeley Mine shaft, the older mine gave its name to the pit. The open-pit style of mining superseded underground operations because it was far more economical: even very low-grade ore could be recovered using shovels and trucks, which were also less dangerous than underground mining.
Within the first year of operation, the pit extracted 17,000 tons of ore per day at a grade of 0.75% copper, meaning that about 127 tons of copper was produced from the ore. Ultimately, about 1,000,000,000 tons of material was mined from the Berkeley Pit Copper was the principal metal produced, together with subsidiary amounts of silver and gold.
Two communities and much of Butte's previously crowded east side were consumed by land purchases to expand the pit. The Anaconda Company bought the homes, businesses and schools of the working-class communities of Meaderville, East Butte, and McQueen, east of the pit site. Many of these homes were either destroyed, buried, or moved to the southern end of Butte with the residents being compensated market value for their lost property.
In 1995, a large flock of migrating snow geese landed on the Berkeley Pit water and were killed. 342 carcasses were recovered. ARCO, the custodian of the Pit, denied that the toxic water caused the death of the geese, attributing the deaths to an acute aspergillosis infection that may have been caused by a grain fungus, as substantiated by Colorado State University necropsy findings. These findings were disputed by the State of Montana on the basis of its own lab tests.
Nearby residents are also concerned about the fog produced by the pit and are wondering what it is doing to their health. The most recent development in the clean-up was the construction of a treatment plant on Horseshoe Bend. This facility is intended to treat and divert water coming from the Horseshoe Bend flow. In addition, it will be able to treat the existing Berkeley Pit water in 2018, or whenever the water level hits the critical point of above sea level. This number was set by Federal order and is intended to protect the ground water from being contaminated by the water in the pit.
New fungal and bacterial species have been found to have adapted to the harsh conditions inside the pit. Intense competition for the limited resources caused these species to evolve the production of highly toxic compounds to improve survivability; natural products such as Berkeleydione, berkeleytrione and Berkeley acid have been isolated from these organisms which show selective activity against cancer cell lines.