The Berkeley Pit in Butte, Montana, is a massive, flooded former open-pit copper mine, once the heart of "The Richest Hill on Earth," now a Superfund site filled with highly acidic, heavy-metal-laden water that is managed as an environmental challenge. It's a mile wide, nearly 1,800 feet deep, and serves as a site for "toxic tourism" with a viewing stand where visitors can observe the toxic lake and its ongoing water treatment