One click after another illuminates colourful scenes of hunters, Buddhas, flying deities, Bodhisattvas and caravanserais painted on the walls of the Mogao caves in northwest China, considered the epitome of Buddhist art -- and now in existential danger.

From the fourth century onwards the 492 largely hand-dug caves near Dunhuang, a desert oasis and crossroads on the Silk Road, acted as a depository for Buddhist art for around a millennium.

Unesco describes the World Heritage Site as "the largest, most richly endowed, and longest used treasure house of Buddhist art in the world".

"Dunhuang is where Chinese, Greek and Roman, Islamic and Indian arts meet," says Mimi Gates, a former director of the Seattle Art Museum who is helping to preserve the caves, and stepmother to Microsoft founder Bill.

The remote site in Gansu province saw 800,000 visitors in 2012, up 20 percent in a year. The recommended daily maximum is 3,000, but as many as 18,000 arrived on one public holiday last October.

"When tourists enter the caves, the humidity, the temperature and the carbon dioxide increase abruptly," said Wang Xudong, director general of the Dunhuang Academy.

The wall paintings themselves contain "a lot of natural salts that have come out of the rocks over 1,200, 1,500 years, the age of these wall paintings", he explained.

If the humidity goes above a critical value, "then the salts can suck the moisture out of the air and start the deterioration process".

But authorities must strike a difficult balance between limiting access to visitors and avoiding alienating them, particularly the hundreds of thousands of primarily Chinese tourists whose numbers are steadily rising.

"Every child in China in growing up learns about Dunhuang. It is one of the great sites, so when they grow up they want to come here, and they do now that they have the money to," says Neville Agnew, a project specialist at the Getty Conservation Institute who has worked with the academy for 25 years.

The digitisation project -- which has been running for decades -- is part of the solution, a grand project to transform the way visitors are received and cut the time they spend inside, even as numbers rise.

It is an immense task. The paintings cover 45,000 square metres (485,000 square feet) -- if set in a single mural three metres high, it would stretch for 15 kilometres.

Thousands of images are taken of each chamber, using specialised lights to avoid damage, and then laboriously computer-processed to create a precise cyber replica.

"Digitising the caves is very difficult," said Wang. "We began in the 1990s but at the time it was a failure. We continued in the year 2000 thanks to technological advances."

The key challenge is capturing the freshness of the colours, particularly natural pigments such as vermilion and malachite green, as well as any areas that are not flat, such as corners and sculptures.

"We have six working teams here on a very intense schedule. Each team must digitise three to four caves a year," said Wang. "Our goal is to reduce the amount of time that tourists spend in the caves, to minimise the damage that they cause."

Once the new system goes into effect next year visitors will be overseen as soon as they reach the airport -- essentially the only convenient entry point for Dunhuang -- and have to stick to a tightly controlled sightseeing circuit.

In a domed theatre currently under construction they will view high-definition images of the inside of the chambers, before taking a glimpse inside the real thing -- but only for a limited time.

"The typical tour might be 10 caves, and of those 10 caves there would be two or three of these so-called must-see caves," says Agnew.

Long after Dunhuang's heyday, the Silk Road eventually fell into disuse and it was largely forgotten by the outside world, with most of the caves abandoned.

In 1907 Hungarian-British archaeologist Aurel Stein led an expedition to the area and paid to remove large numbers of manuscripts, paintings and textiles. A French mission under Paul Pelliot acquired thousands of items the following year, and Japanese and Russian expeditions soon followed in their footsteps.

But the latest collaboration will preserve Dunhuang for future generations, say those involved, and make the masterpieces available to academics and amateurs around the world online.

"The painting is superb, the painting is unbelievable, the paintings are just masterworks of paintings, and in a variety of styles over the dynasties as they changed," said Agnew.

"History is here, art is here."