Berkeley Pit: History of the Richest Hill on Earth

At the turn of the century, Butte was the largest city west of the Mississippi River. The Berkeley Pit mine was called the “richest hill on Earth”, and immigrants came from all over the world to work in the mines.

The city boomed with economic success. Copper wiring was essential in the electrical systems that powered the country in the beginning of the 20th century, and Butte was producing 15 percent of the world’s copper. The city was comparable to Chicago and New York, and had electric lighting before both of those cities. Though the copper kings were corrupt and Butte had its share of problems, it was a town of excess and abundance.

Miners extracted approximately 320 million tons of ore and removed over 700 million tons of waste rock from the Berkeley Pit. They dug deep and wide, but the copper eventually began to run out. The mine was no longer profitable enough to maintain, and the owners, British Petroleum-Atlantic Richfield Company (BP-ARCO), finally shut it down on Earth Day in 1982. Groundwater pumps were shut off and the one mile long by half a mile wide pit mine slowly began to fill with groundwater and rain. The groundwater flowed in through the pre-existing mining operations and became contaminated with highly toxic heavy metals: Copper, cadmium, iron, zinc, and arsenic. The ground also contains pyrite, which when combined with water creates corrosive sulfuric acid as a by-product and leaches metals into the water even faster.

Currently, the water is about 1,000 feet deep and is rising at a rate of approximately one foot per month. To deal with the rising water level, BP-ARCO and Montana Resources completed construction of The Horseshoe Bend Water Treatment Plant in 2003. The plant can treat up to seven million gallons of water per day, and currently uses liming techniques to treat a blend of Pit water and water used in active mining operations, effectively slowing the rate of fill. The water will continue to rise until it reaches a critical water level in 2023. Visit to learn more about the Berkeley Pit water level rise.

Mark Thompson of Montana Resources explains the water flow in the Continental mining operation surrounding the Berkeley Pit.

Click here to explore the Berkeley Pit in Google Earth.

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