The Berkely Pit

The Berkeley Pit is a former open pit copper mine located in Butte, Montana, United States. It is one mile long by half a mile wide with an approximate depth of 1,780 feet (540 m). It is filled to a depth of about 900 feet (270 m) with water that is heavily acidic (2.5 pH level), about the acidity of cola or lemon juice.  As a result, the pit is laden with heavy metals and dangerous chemicals that leach from the rock, including copper, arsenic, cadmium, zinc, and sulfuric acid.
Info courtesy of Wiki.

the view once you go through the entryway.

views looking around the pit