Bat cave
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Dorset - Spring has arrived on Mount Aeolus in Southern Vermont.

Bats are filtering out of a cave there by the minute.

But step inside and it's another story.

More than 20,000 dead bats are piled upon the cave's floor.

"It's pretty eerie and disturbing to say the least," said Ryan Smith of the U.S. Fish and Wildlife Dept. "You pretty much can't walk without stepping on them."

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Scientists fear New England's entire population will be wiped out and a mysterious disease called white nose syndrome is the culprit.

The disease is already responsible for the deaths of millions of bats in nine states.

The mortality rate in most caves exceeds 90 percent.

"If we lose 500,000 bats in Vermont, that could be two billion bugs a night that are not going to be consumed by bats," Smith said. "That's pretty scary stuff."

Dead Bats
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"The impacts of that, we don't even know what that means, but we're definitely going to find out," said Dave McDevitt of the Nature Conservancy.

Scientists now know the fungus comes from the Arctic but they aren't sure it's directly killing the bats.

The fungus acts as an irritant and it may be causing bats to awake from hibernation more frequently, depleting their fat reserves.

"So that may be the real cause. That may be the whole issue," Smith said.

Dead Bat
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Some students from Boston University hope to get to the bottom of the mystery.

They've spent the winter monitoring body composition and studying the bats' immune systems.

So far, it appears bats in deep hibernation are less able to fight the disease than bats who are frequently awaking.

"They may have to arise from torpor in order to mount an immune response, so if they're running out of energy or fat and they can't arouse, maybe they simply can't mount an immune response," said Marianne Moore, a graduate student at Boston University.

But answers are few and the questions many.

Scientists say a lack of funding is a major challenge and they fear these bats may be extinct by the time anyone is able to determine exactly what's happening.