The Berkeley Pit, Montana

Opened in 1955 as a way to mine for copper in Butte, Montana, the Berkeley Pit grew to a depth of 1,700 feet before it was closed down in 1982. Since that time the pit has filled with over 900 feet worth of groundwater and rainwater. Combined with the heavy metals and chemicals of the prior mining operation, the water has turned highly acidic and measures are taken to keep birds out of the water ever since a 342-bird flock of snow geese died inside the mile-long, half-mile-wide pit in the 1990s.