It’s not really a lake, it’s more like an artificial lake. In fact, it’s the Berkeley Pit, a former open pit copper mine located in Butte, Montana, USA. It is filled to a depth of about 900 feet (270 m) with water that is heavily acidic. The pit is laden with heavy metals and dangerous chemicals, including arsenic, cadmium, zinc, and sulfuric acid. When the pit was closed, the water pumps at the bottom were removed, and groundwater from the surrounding aquifers began to slowly fill the pit. Since the pit closure in 1982, the level has risen to within 150 feet of the natural groundwater level.
In 1995, a large flock of migrating snow geese landed in the Berkeley Pit water and died, with 342 carcasses recovered.