At first glance the wall of this cave looks a little strange... caked with mud or adorned with a thick-growing fungus. But, on closer inspection, it is actually covered by hundreds of thousands of slumbering bats.
The stunning photographs are the work of American photographer Stephen Alvarez, who has photographed caves and wildlife for Time and National Geographic magazines.
Despite the mind-boggling numbers of the bats in this cave alone, Mr Alverez explains that the entire species is threatened with extinction because of a fungus that has already claimed the lives of millions.
Fast asleep: Members of Nature Conservancy examine hibernating grey bats in Hubbard Cave, Tennessee. Photographer Stephen Alvarez went along to help document the numbers of the bats, which are under threat from a killer disease
Don't interrupt me, I'm counting: Part of the work of Bat Conservation International and Nature Conservancy is to estimate the remaining numbers of the bat population
He was called in to help document the numbers in the caves, where millions of bats seek refuge from the harsh winters in north and south east America. The tiny creatures are barely recognisable as they huddle together and hibernate for up to four months on the walls inside the caves.
Alvarez said he and the others in the expedition had to tred very carefully while inside each cave, so as not to wake the bats from their slumber.
He added: 'We spent around three weeks visiting and photographing the various bat caves. We started in Tennessee and worked our way north. There can be up to half-a-million bats in the cave, so you have to tread carefully to try not to rouse them.
'It's astounding to witness, there's so many of them, it looks like a bear skin rug on the wall of the cave but it's actually hundreds-of-thousands of hibernating bats. We had to estimate and count as fast as we can, as it's important to spend as little time in the caves as possible.'
Up close and personal: Alvarez got as close as he dared to the bats to produce some extraordinary shots (the bat in the middle, above left, looks like it could be a snorer!)
It's not a matter of scaring the animals, or being himself scared by a sudden flight of hunderds of thousands of individuals.
Alvarez explained that a bat disturbed from its hibernation would not be able to survive the harsh conditions that it was trying to shelter from.
Despite his wealth of experience in photographing subterranean chambers and caves, Alvarez said this was a special event for him.
He said: 'I very seldom go into a cave to photograph the animals, it's usually to photograph an amazing waterfall or landscape, so this was a unique experience.'
Another world: Alvarez captures the almost alien environment of bats flying above a fresh-water pool inside the Cueva de Villa Luz, Mexico
He took these photographs while on an expedition with experts from Bat Conservation International and Nature Conservancy. The aim of the expedition was to count the bats in each cave in order to estimate the remaining population of the endangered bats, which are currently under threat from a rare disease.
They are in rapid decline following the emergence of 'white-nose syndrome', a fungus responsible for the deaths of more than 5.5million bats so far.
Alvarez said: 'The bats are being wiped out by a disease which causes them to come out of hibernation. Most then die in the winter because they cannot find food and water.
Tragic consequences: White-nose syndrome is a fungus that forces the bats to wake from their slumber. Hunder and thirst will drive them out of their caves into the harsh conditions outside, where they almost certainly perish.
'The ones that survive the winter often have wings so damaged by the fungus that they are unable to fly well enough to forage in the coming year. Most of the bat caves we visited had a mortality rate of 90 per cent.
'Some of the bats were flying out of the cave into the snow and dying at our feet... this was an important expedition and something I've been wanting to do for a very very long time. And something I might never get another chance to photograph and document.'
'To see the bats in situ in the caves is awe-inspiring and to think this might never happen again is devastating.'
They are in rapid decline following the emergence of 'white-nose syndrome', a fungus responsible for the deaths of more than 5.5million bats so far.
Alvarez said: 'The bats are being wiped out by a disease which causes them to come out of hibernation. Most then die in the winter because they cannot find food and water.
Tragic consequences: White-nose syndrome is a fungus that forces the bats to wake from their slumber. Hunder and thirst will drive them out of their caves into the harsh conditions outside, where they almost certainly perish.
'The ones that survive the winter often have wings so damaged by the fungus that they are unable to fly well enough to forage in the coming year. Most of the bat caves we visited had a mortality rate of 90 per cent.
'Some of the bats were flying out of the cave into the snow and dying at our feet... this was an important expedition and something I've been wanting to do for a very very long time. And something I might never get another chance to photograph and document.'
'To see the bats in situ in the caves is awe-inspiring and to think this might never happen again is devastating.'
