The Berkeley Pit in Butte, Montana
Water in the abandoned mine leaches metals and toxins from the surrounding rocks, threatening ground water and the Clark Fork River drainage. The water is rising about a foot every month.
Ok, pretty toxic, but more so than the D.C. Beltway?
The lake water is highly acidic and full of toxins. Necropsy of the geese showed the water burned their esophagus and internal organs.
But something damn interesting was found in the water: Extremeophile Euglena Mutabilis
RadioLab: What happened then...
The dead geese seeded the lake with a protozoa that can remove the toxins from the waters of the lake. This is not going to convert the lake to clean water anytime soon, but it is a very promising development in what was a mostly depressing situation.