After the closure of the Berkeley Pit mining operations in 1982, pipes which pumped groundwater out of the pit were turned off, resulting in the pit slowly filling with groundwater, creating an artificial lake.
Only two years later the pit was classified as a Superfund site and an environmental hazard site.
The water in the pit is contaminated with various hard metals, such as arsenic, cadmium, and zinc.

It was not until the 1990s that serious efforts to clean up the Berkeley Pit began.
The situation gained even more attention after as many as 342 migrating geese chose the pit lake as a resting place, resulting in their deaths.
Steps have since been taken to prevent a recurrence, including but not limited to loudspeakers broadcasting sounds to scare off waterfowl.
However, in November 2003 the Horseshoe Bend treatment facility went online and began treating and diverting much of the water that would have flowed into the pit.
The Berkeley Pit is both a Superfund site and tourist attraction, viewable from an observation deck.
Per a 2014 report, scientists believe the Berkeley Pit may reach the critical water level—potentially contaminating Silver Bow Creek—by the year 2023.
Beginning in 2019, the Montana Resources and Atlantic Richfield Co. are ordered by the Environmental Protection Agency to begin treating water from the pit, which is to then be discharged into Silver Bow Creek at a rate of 7,000,000 US gallons (26,000,000 l) per day.
Nikia Greene, EPA project manager for mine flooding, assured in 2014: "The pit is a giant bathtub.
There's a hydraulic gradient into the pit.
We will never let the water reach the critical level."
After the closure of the Berkeley Pit mining operations in 1982, pipes which pumped groundwater out of the pit were turned off, resulting in the pit slowly filling with groundwater, creating an artificial lake. Only two years later the pit was classified as a Superfund site and an environmental hazard site. The water in the pit is contaminated with various hard metals, such as arsenic, cadmium, and zinc.
It was not until the 1990s that serious efforts to clean up the Berkeley Pit began. The situation gained even more attention after as many as 342 migrating geese chose the pit lake as a resting place, resulting in their deaths. Steps have since been taken to prevent a recurrence, including but not limited to loudspeakers broadcasting sounds to scare off waterfowl. However, in November 2003 the Horseshoe Bend treatment facility went online and began treating and diverting much of the water that would have flowed into the pit. The Berkeley Pit is both a Superfund site and tourist attraction, viewable from an observation deck. Per a 2014 report, scientists believe the Berkeley Pit may reach the critical water level—potentially contaminating Silver Bow Creek—by the year 2023. Beginning in 2019, the Montana Resources and Atlantic Richfield Co. are ordered by the Environmental Protection Agency to begin treating water from the pit, which is to then be discharged into Silver Bow Creek at a rate of 7,000,000 US gallons (26,000,000 l) per day. Nikia Greene, EPA project manager for mine flooding, assured in 2014: "The pit is a giant bathtub. There's a hydraulic gradient into the pit. We will never let the water reach the critical level."
Butte, Montana