An estimated 10,000 of the caves have been found in the former Kingdom of Mustang in North, Central Nepal.
Hidden within the Himalayas, 155ft from the ground, these man-made caves are one of the World's greatest archaeological mysteries.
Thousands of holes are carved into the fragile, sandy-coloured cliff in a gorge so large it dwarfs the Grand Canyon.
The astonishing number of caves, some dug into the cliffside, others tunnelled from above are thousands of years old but who built them and why remains a mystery.