Butte, Montana was once the nation's most prolific copper-mining district. Miners first came to Butte in 1864 to mine placer gold. Hard rock silver mining began in 1874, then rich copper veins were discovered in 1882. The district quickly switched from silver to copper, and by 1887, Butte was the leading copper-producing district in the United States. The Anaconda Copper Mine was the world's most productive copper mine from 1892 through 1903, and intermittently for years thereafter. Open-pit mining began at the Berkeley Pit in 1955; the Berkeley Pit has been inactive for years, and continues to fill with acidic water. Through 1964, Butte produced 7.3 million tonnes of copper, 2.2 million tonnes of zinc, 1.7 million tonnes of manganese, 380 thousand tonnes of lead, 645 million troy ounces of silver, and 2.5 million ounces of gold. The only remaining active copper mine at Butte is the Continental pit, operated by Montana Resources LLP.