The breath of millions of tourists is threatening the ancient frescoes of China's northwest caves. But help is at hand, writes Clifford Coonan

The light of the torch picks out the serene face of Buddha, a startling 1,000-year-old fresco on a dusty cave wall at the edge of the Gobi desert. It is one of thousands of paintings within a network of caves in the remote northwest of China, a place where merchants and travellers could pray before heading off on the treacherous Silk Road.

In the same network of grottoes is the cave which was filled from floor to ceiling with ancient manuscripts and paintings, until, 100 years ago, the British archaeologist Sir Aurel Stein persuaded their monk guardian to sell the artefacts and brought them back to the British Museum.

There are 800 caves altogether, of which about 500 had frescoes painted within. For 1,600 years, the caves have survived political upheaval, religious fundamentalists, clumsy Western adventurers, corrupt Qing dynasty officials, harsh treatment by White Russian soldiers and the unwanted attentions of Mao's Red Guards.

Now the breath of millions of tourists and the encroaching Gobi desert may be more than these paintings can bear, while wide-scale plundering of the manuscripts means these invaluable documents are dispersed throughout many countries.

Scientists from throughout the world are working on a major project to digitally record the murals and the manuscripts of Dunhuang and bring them all together online so that one of the world's greatest treasures can be seen by everyone. The project includes input from Ireland's Chester Beatty Library, which holds four Dunhuang manuscripts in Chinese and one in Tibetan, purchased in 1955.

"Dunhuang's caves are not just our responsibility, they are the whole world's. Dunhuang is such a great cultural site it should be shared by everyone, not just scholars and researchers," says Prof Luo Huaqing, a director of the Dunhuang Academy which co-ordinates the site.

"You can see the deterioration from these photos," Luo says animatedly as he goes through a sheaf of pictures taken by the French explorer Paul Pelliot nearly 100 years ago. "We need to save it now. In 100 years maybe they will have deteriorated further but at least the next generation will be able to see what they look like right now. It's most important, the job we have on our hands," says Luo.

The Dunhuang digital archive will include images from the caves, as well as frescoes and scriptures from the area that now reside in the world's museums.

The mammoth project involves scanning 45,000sq m of frescoes and minutely recording 3,390 Buddhist statues. The frescoes are spread among the 812 caves hewn out of the sandstone cliff wall which stretches for a whole mile. Behind the caves, the desert looms ominously in the shape of the Rattling Sand Mountain, and one of the fears is that the grottoes may be lost to the stretching desert.

And what treasures they are - true triumphs of religious art. The styles of painting are representative of many regions - the Silk Road transported ideas, as well as silk and tea, along the route.

Some murals show the sponsors who paid for them - local businesspeople, bureaucrats, traders. They came to this remote site to pray for safety before leaving and if they came back very rich they could build a cave by themselves. It took several years to finish a grotto and sometimes construction lasted decades. Some local officials were very rich and only painted themselves and their families.

In cave 259 is a figure with a smile very similar to that of Mona Lisa, although this mural is from the Northern Wei Dynasty (fifth century). Cave 244 has a thousand golden-faced Buddhas, while in cave 96 a 35.5m high Buddha from AD 695 dominates the room. Then there is a Tang dynasty Buddha nearby which is smaller, but appears bigger somehow - the head was made disproportionately large to give precisely that effect, showing an early understanding of perspective. This large icon was funded by one family over a period of 29 years.

Last year, there were 550,000 visitors to Dunhuang and that number is expected to rise this year, so something needs to be done urgently if the site is to be preserved while still functioning as a tourist attraction.

"With the digitisation project we want to gather good, general information about the murals and get an idea about the day-to-day deterioration of the images. The digitisation programme allows us to maintain the pictures in their current state and visitors who come can learn from them," says Prof Luo.

The earliest of Dunhuang's grottoes at Mogao date to the Northern Liang period (AD 366-439), when a wandering monk called Yuezun began the work, and continued until the 14th century.

Dunhuang was a busy stopping point on the Silk Road and caravans laden with silk and tea from China would stop here en route to the courts of Persia and Europe during the Sui (AD 581-618) and Tang (AD 618-907) dynasties. The desert made for harsh travelling - whole caravans would disappear beneath the sands, and desiccated corpses lined the Silk Road.

As well as being a centre of religious art, the grottoes were also a focal point for studying Chinese and Tibetan texts, written in ancient languages such as Sogdian and Tangut.

Liu Gang, director of the digitisation programme at the caves, says the physical act of taking the pictures is the hardest because of the size and often awkward shape of the murals.

"The murals are all very big and our digital cameras cannot take it all in at once, so we do it piece by piece. Each painting takes hundreds of photos. Our best camera can take one square metre. So it takes a lot of time," says Liu.

"We've done 30 caves so far. We can do three to five caves a year. There are around 500 caves that need to be digitised. When we started, the work was very slow and this kind of technology didn't exist. Digital cameras have improved a lot - in the past few years we've gone from six megapixels to 24. In future we'll have better cameras."

The people in the centre are skilful at using Photoshop software and their fingers whizz over the keyboards. Shi Yufang is a local from Dunhuang who trained in digitisation techniques this year.

"How long it takes depends on how flat the wall is. If there are bumps or curves, it takes longer. One wall can take up to four months," she says.

The images of the murals will form a virtual-reality tour for visitors to see before they enter the grottoes. The project is a collaboration with the Mellon Institute in Pittsburgh and it is estimated that it may take five years to finish the job.

The International Dunhuang Project is working to digitise the manuscripts - another crucial part of the programme. The artefacts that Stein brought back in 1907 included guides to feng shui, apologies for getting drunk and talking rubbish during a night out, suggested remedies for arthritis and period pains, as well as tax forms and military reports. Most of the scrolls and manuscripts are sutras, prayers for success along the Silk Road.

In 1908, the guardian abbot sold more to Paul Pelliot, who showed some examples of the manuscripts to Chinese scholars in Beijing. The following year, the Qing court sent envoys to pick up 10,000 manuscripts, but not before thousands had found their way into many different, foreign hands. Now the manuscripts are spread throughout the world.

The question of who should possess the artefacts is still hotly contested and visitors to the site are left in no doubt that China wants its treasures back. But, for the time being, preservation takes precedence.

There are 38,274 pieces in the British Library, of which 20,201 have been digitised, says researcher Alastair Morrison.

"There are so many influences at work and it's always nice to make people aware of the diverse history of the region," he says.

"The overall mission is to unite the Dunhuang project on the internet and to document everything. We've been working for four years.".

"We're putting all this stuff on the internet and it's there for everyone. The project was founded to solve problems and to share information and experience, to conserve and to keep them online. That's the priority for everyone," says Morrison.

Some of the documents were found in strange places. There were shavings of wood from the Han Dynasty in rubbish heaps, where people shaved off the old texts to write new ones. This detritus was brought back by Stein and has yielded valuable information.

One of the adventurers who has been strongly criticised is Langdon Warner, the Harvard art historian who used a revolutionary technique to remove one-square-metre chunks of mural from the wall in 1924, and brought them back to Harvard.

In one cave, numbered 328, you can see where Warner took a chunk out of an Early Tang mural from about AD 700. In cave 324, a beautiful painting of a barge is missing, and the sight of the empty square at the centre of the mural is still shocking. When Warner - who was one of the inspirations for Indiana Jones in the Steven Spielberg movies - came back in 1925 to take more he met stiff opposition from the locals, and withdrew.

The jury is still out on Stein. He was Hungarian by birth, but later became one of the great figures of The Great Game, a period when Russia and Britain competed for influence in Central Asia. However, he is definitely considered to have been more of a scientist than a reckless adventurer.

"It's thanks to his work that any of us know about that region and the Silk Road. He brought everything back, he didn't discriminate . . . He did things thoroughly in a scientific way. The vast majority of artefacts he brought back are in national museums, he didn't sell them," says Morrison.

There are plans to cut down the time visitors spend inside the caves, to help reduce the levels of carbon dioxide and moisture, emissions that break down the delicate dye-on-plaster of the murals and statues.

"But the Mogao caves will always be open to visitors. It's impossible to close the caves because we need the tourism revenue in a developing economy. Every country is developing tourism. Our principle is that we open the caves on condition the caves should not be harmed," says Prof Luo.