By Matt Wells
BBC News, Massachusetts
Bats are being woken up too early from hibernation
A mystery illness that has scientists baffled is wiping out tens of thousands of bats across the north-east of the US.
White nose syndrome, as it is known, cannot even be categorised as a disease.
"Did it spread? I don't know, because we don't know what it is," says government biologist Susie von Oettingen, on a recent fact-finding mission in the frozen woods of Massachusetts.
What is known is that the syndrome leaves small, white, fungal spots around the nose and mouth of the tiny nocturnal animals.
The bats have been woken prematurely from their winter hibernation and, with their fat reserves seriously depleted, their natural impulse is to forage for food.
The reality of the harsh New England winter is that there are no insects to eat so they starve to death.
BBC News joined several local scientists and conservationists in an area of disused mines and caves, close to the border with New York State, where bats are in abundance - including the endangered Indiana bat.
For years, state and federal wildlife officers have nurtured the population, but the apparent spread of the syndrome could undo all that in a matter of months.
Government biologist Susie von Oettingen is concerned
Despite the noon sunshine, which makes them a target for circling birds of prey, hundreds of hungry bats flew above our heads in various states of exhaustion.
"There's a bat crawling across the road," said Ms Oettingen, sighing deeply. "He should not be on the road on a sunny day."
"We have a lot of labs working at a lot of angles and we keep trying new ideas out. We're calling each other almost weekly to try and figure this thing out."
One possible link under serious consideration is the equally mysterious catastrophe that has affected the honeybee population.
Colony collapse disorder (CCD) ravaged hives across the US last year, although so far the bat syndrome is confined to the North-East.
"If, perchance, this was somehow involved with the spread of insects due to global warming or the use of pesticides - that's a possibility," said Ms Oettingen, who is a specialist in endangered species.
Whatever role human activity and behaviour is playing in the bats' plight, the outcome is sad to witness.
Dozens of bats huddle around the gutters of local homes, desperate for warmth. On the ground, dozens more shrivelled carcasses can be seen on top of the snow and ice.
Massachusetts wildlife officer Tom French led us to one of the mines where the bats should still be huddled together in hibernation. Some of the survivors showed clear signs of white nose syndrome, although others did not.
"It looks like you dipped it in powdered sugar," he said.
Tom French thinks the bats will start to develop immunity
"I have no doubt we will figure it out. But even once we figure it out, I suspect we'll never be able to do anything about it," he added, speaking in a whisper to avoid waking any more of the creatures.
He insists that is not the counsel of despair.
He believes that the bats themselves will begin to develop immunity, although nobody knows how far the syndrome will go.
Alan Hicks is the veteran government conservationist who first spotted the problem last year in a New York cave.
He is aware that bats lack the popular appeal of many other animals, but he thinks the syndrome is yet another environmental alarm bell.
"I don't want my grandkids saying, gosh, what was it like to have bats flying around? I want them to enjoy all the wealth and beauty that we have."