Few have been able to explore the mysterious caves, since Upper Mustang is a restricted area of Nepal that was long closed to outsiders. Today only a thousand foreigners a year are allowed into the region.
In 2007 a team co-led by U.S. researcher and Himalaya expert Broughton Coburn and veteran mountaineer Pete Athans scaled the crumbling cliffs on a mission to explore the human-made caves.
Inside the caves, the team found ancient Tibetan Buddhist shrines decorated with exquisitely painted murals, including a 55-panel depiction of Buddha's life.
Looters have raided the caves over the centuries, cutting valuable artwork from the ancient texts. In addition, religious pilgrims have damaged the cave walls to collect souvenirs.
Still, the researchers were able to collect and document manuscripts from about 30 volumes, which were then moved for safekeeping to Mustang's central monastery.