It was broad daylight in the middle of winter, and bats flew out of the mine about one a minute. Some had fallen to the ground where they flailed around on the snow like tiny wind-broken umbrellas, using the thumbs at the top joint of their wings to gain their balance.

They have plenty of company. In what is one of the worst calamities to hit bat populations in the United States, on average 90 percent of the hibernating bats in four caves and mines in New York have died since last winter.

“This is probably one of the strangest and most puzzling problems we have had with bats,” said Paul Cryan, a bat ecologist with the United States Geological Survey. “It’s really startling that we’ve not come up with a smoking gun yet.”