Thousands of migrating snow geese have died in Butte after they made contact with toxic runoff from a former mine, officials have confirmed:

This was no ordinary pond, however. It was the 700-acre Berkeley Pit, a former mine now submerged in water as acidic as distilled vinegar. From 1955 until operations ceased in 1982, miners extracted nearly 300 million tons of copper ore from the pit. They left behind an immense crevasse, which filled with water 900 feet deep. Concentrated within the floodwater are arsenic, cadmium, cobalt, copper, iron, zinc and other inorganic compounds.

After it was abandoned, the pit became a federally managed Superfund site. It also became a tourist destination, where visitors observe the mine’s toxic, reddish water for an admission fee of $2. And microorganisms able to survive in the pit became an object of scientific study.

But snow geese, unlike extremophilic green slime, cannot tolerate acid water heavy in metallic compounds. Roughly 10,000 geese landed in the Berkeley Pit at the end of November [2016], turning the water “white with birds,” said a mine official with Montana Resources, which jointly manages the pit with the Atlantic Richfield Company, to the Montana Standard. On Tuesday, investigators could not give an exact measure of how high the death toll would go. But a preliminary estimate, via drone and flyover counts, found thousands of dead birds.

State and federal agencies in Montana are still investigating the snow geese die-off. Mine company Montana Resources environmental affairs manager Mark Thompson said that workers tried, and failed, to frighten away flocks of birds attempting to land in the water:

“I can’t underscore enough how many birds were in the Butte area that night,” Thompson said. “Numbers beyond anything we’ve ever experienced in our 21 years of monitoring by several orders of magnitude.”

The employees “did incredible things to save a lot of birds and they really put their heart and soul behind it,” he said. “They did everything they could think of.”

Early theories about the devastating event suggested climate change and attendant mild winters played a potential role:

University of Montana Western professor Jack Kirkley, who specializes in ornithology, told The Montana Standard that recent milder winters aren’t encouraging birds to head south as early and in some cases, are causing some to stay in places where they’ve never stayed the winter before. He notes that there are 4 million to 6 million snow geese on the continent, and there are some concerns that the population is too high … Driven to find new habitats, birds have been seen in areas where they were historically scarce.

Estimates place the number of dead snow geese near 10,000, and officials reported some ailing birds were rescued and treated:

The preliminary number released Tuesday is based on photos taken from drone and aircraft flights over the pit. The counting is not yet complete, said Thompson … Thompson said MR has directed Butte-Silver Bow animal control to “do everything possible” to try to save any birds found alive. Randall said the veterinarian treatment involves flushing the birds both inside and out to “get everything out of them.” The pit water contains sulfuric acid and heavy metals.

Locals expressed concern that the story would become inextricably linked with Butte nationally and globally:

Maria Pochervina, director of Butte convention and visitors’ bureau, said she would be surprised to see this tragic event negatively affect tourism this summer.

“People are fascinated by the pit,” Pochervina said. “When we tell the story of the value of minerals extracted, how it’s made our lives better, people are fascinated by that.”

But Dave Palmer, who takes office in January as the newly elected county chief executive and a longtime commissioner, said the die-off is not good news on the economic development front.

“It does make national news and that is unfortunate because that is what people look at — bad things,” Palmer said. “We could be doing a thousand good things in Montana and Butte itself and you never hear about them rise to the national level.

Larger discussion about the snow geese deaths also generated debate over addressing the disused mining pit more aggressively, with some residents saying that the pit should be drained. Although the former open copper pit mine is one of the largest of the Superfund sites, it remains a popular tourist attraction in Butte.