The Berkeley Pit is a former open-pit copper mine in Butte, Montana, now filled with roughly 30 billion gallons of toxic, acidic water. Stretching over 1.5 miles long, 1 mile wide, and 1,600 feet deep, it is one of the largest Superfund sites in the United States and one of the most contaminated bodies of water on Earth. It is also, unexpectedly, a place where scientists have discovered organisms that may help fight cancer.
From Copper Mine to Toxic Lake
The Anaconda Copper Mining Company opened the Berkeley Pit in 1955, and within its first year, workers were extracting 17,000 tons of ore per day. The copper grade was about 0.75%, meaning the vast majority of what came out of the ground was waste rock. Over the mine’s 27-year life, roughly one billion tons of material were removed. Operations passed to the Atlantic Richfield Company (ARCO) before the pit finally closed on April 22, 1982.
What happened next created the environmental disaster that exists today. When mining stopped, the water pumps in the nearby Kelley Mine, located 3,800 feet below the surface, were switched off. Groundwater began seeping into the pit at a rate of about one foot per month. Decades of accumulated mining waste dissolved into the rising water, turning it into a stew of heavy metals and sulfuric acid. As of 2023, the water had reached over 900 feet deep.
What’s Actually in the Water
The Berkeley Pit lake has a pH that has consistently ranged between 2.20 and 2.95 since flooding began, with a long-term average around 2.63. For comparison, lemon juice has a pH of about 2, and battery acid sits around 1. The water is corrosive enough to dissolve metal and hostile to virtually all conventional plant and animal life.
The chemistry gets worse with depth. Near the surface, dissolved copper concentrations measure around 65 milligrams per liter, iron around 264, and zinc around 275. At the deepest point sampled (about 710 feet down), those numbers climb sharply: copper reaches 167 mg/L, iron exceeds 1,000 mg/L, and zinc approaches 600 mg/L. Arsenic, aluminum, and manganese are also present throughout the water column. Interestingly, the pH actually shifts toward neutral and even slightly alkaline at greater depths, reaching 8.3 at the bottom, but the metal concentrations there are at their most extreme.
The Superfund Cleanup
The EPA has been involved in Butte cleanup efforts since 1983, working alongside Montana’s Department of Environmental Quality and local officials across what is now called the Butte Superfund Complex. The Berkeley Pit falls under a specific cleanup unit focused on mine flooding and contaminated groundwater beneath the city of Butte and the neighboring community of Walkerville.
The primary defense against the pit’s water reaching surrounding groundwater is the Horseshoe Bend Water Treatment Plant. This facility can process up to 7 million gallons per day using a two-stage system. In the first stage, hydrated lime is added to raise the pH and neutralize the acid. This forces dissolved metals to form solid particles (metal hydroxides) that drop out of solution. A second stage repeats the process at a higher pH to capture metals that are harder to remove. A chemical called a polymer is added between stages to help the solids clump together and settle. A polishing facility and discharge system came online in 2019 to further refine the treated water before release.
The goal is not to drain the pit. It is to keep the water level below a critical threshold that would allow contaminated water to seep into Butte’s groundwater supply. The pit is continuously monitored.
Health Concerns in Butte
The pit’s contamination doesn’t exist in isolation. Butte sits on top of more than a century of mining activity, and the EPA has identified potential health threats from direct contact with contaminated soil, ingestion of contaminated water, and inhalation of contaminated air. Epidemiological studies from the 1990s and early 2000s found that cancer rates in Silver-Bow County (which includes Butte) were higher than both the Montana and national averages. Rates of neurodegenerative diseases like multiple sclerosis and Parkinson’s disease were also elevated.
Research into metal exposure among Butte residents has found elevated levels of arsenic and manganese in volunteers’ hair and blood. These metals are concerning because chronic exposure to even low levels of lead, mercury, cadmium, and arsenic can cause toxic effects. High copper combined with low zinc, a pattern consistent with the local environment, has been specifically linked to neurodegenerative conditions. Ongoing oxidative stress from redox-active metals can drive chronic inflammation, compounding health risks over time.
Life in a Deadly Lake
One of the most surprising chapters in the Berkeley Pit’s story is biological. Despite conditions that kill most living things (a flock of migrating snow geese that landed on the pit in 1995 died within days), researchers have found a thriving community of extremophile microorganisms in the water. These include bacteria, fungi, algae, and single-celled organisms called protists, all adapted to survive in highly acidic, metal-rich conditions.
For scientists, these organisms are essentially a library of novel chemistry. Of the six microbes that have been studied in depth, five have produced compounds with activity against specific cancer cell lines. A species of Penicillium fungus yielded a compound called berkelic acid that showed activity against ovarian cancer cells in National Cancer Institute screening. Another Penicillium species produced compounds active against non-small cell lung cancer and leukemia cell lines. A third, isolated from the secreted slime of an acid-loving yeast found 50 feet down in the pit lake, produced compounds being evaluated as inhibitors of enzymes involved in inflammation.
One fungus, a Pithomyces species, produced a compound that acts on the same brain receptor targeted by certain antidepressants and blood pressure medications. Another, Chaetomium funicola, yielded compounds that inhibit enzymes linked to both inflammation and cancer progression. None of these have become drugs yet, but the pit has proven to be a remarkably productive source of bioactive molecules precisely because the organisms living there have evolved unusual survival chemistry.
Visiting the Pit
The Berkeley Pit is a genuine tourist attraction, drawing visitors to a viewing platform on its rim. From there, you can look down into the terraced walls of the former mine and see the lake itself, which often appears a striking reddish-brown or deep blue-green depending on light and chemical conditions. The pit sits right at the edge of Butte’s residential neighborhoods, a reminder of how closely the city’s identity is tied to its mining past. Access is managed, and visitors stay well above the water on a designated platform overlooking the excavation.