Before Leonardo da Vinci painted "The Last Supper," Tibetan craftsmen were creating stunning artistry of their deities in the remote Himalayan kingdom of Mustang. In Lost Treasures of Tibet, NOVA goes behind the scenes with the first conservation team from the West, as it undertakes the painstaking restoration of these ancient masterpieces and the beautiful monasteries that house them.Located in present-day Nepal, Mustang contains some of the last remaining relics of an almost vanished world of ancient Buddhist culture. Across the border in Tibet, Chinese occupiers have destroyed thousands of monasteries since taking control of the country in 1950. Therefore, the survival of Mustang's monasteries or gompas is more important than ever. But preservation is extremely difficult because of the centuries of neglect, weather, and earthquakes that have brought many buildings to the brink of collapse. Inside, their exquisite murals are in a near-ruined state.In the course of their restoration work, conservators from the West come face-to-face with a thorny problem of culture clash: local people want missing sections of the murals completed. Westerners are aghast at the idea, but their hosts are equally shocked at the thought of worshiping unfinished deities.The program follows the struggle of an international team headed by British conservationist John Sanday to restore the greatest gompa of all - Thubchen, the royal monastery in Mustang's capital of Lo Monthang. The first order of business is fixing Thubchen's roof - no small feat since 200 tons of dirt have been piled on its flat surface over the centuries to seal out leaks. To bear that much weight, the hidden ceiling beams must be more than two feet thick, an apparent impossibility considering that Mustang is virtually treeless. Sanday solves this riddle when his team excavates down to the beams and discovers an elaborate jigsaw puzzle of construction that uses interlocking small timbers to create a lightweight, load-bearing structure.