Thousands of snow geese have perished in Butte, Mont.'s Berkeley Pit after landing in the contaminated water of the 700-acre, open-pit mine.
The geese were forced to land there after nearby lakes commonly used by the birds became frozen over, Montana Standard reported.
Estimates so far, based on overhead counts, are that some 10,000 geese first landed in the pit's waters on November 28.
Berkeley Pit, managed by Atlantic Richfield (Arco) and Montana Resources (MR), was once an open-pit copper mine that is now managed as a federal Superfund environmental cleanup site. Its highly acidic waters are filled with chemicals from its rocks.
A similar mass-death of geese occurred at the site in 1995, when more than 300 of the birds died after landing at the pit and drinking the water. Officials expect the current death toll – still being assessed – to be higher than that incident.