Digitization, internet traffic used to ease number of visitors. The popularity of the Mogao Caves, which house the world's largest collection of Buddhist art, is proving their biggest threat.

The caves, in northwest China's Gansu province, already have received many more visitors this summer than they can handle, and this is altering their environment. But new technology may help save the caves from their own success.

In July, 350,000 people visited the caves, twice the monthly cap and 20 percent more than July 2016, according to officials at the Dunhuang Research Academy, manager of the caves, which have survived for centuries.

The single-day peak number of visitors in July exceeded 18,000, triple its daily maximum, said Li Ping, who is in charge of visitor reception.

"The humidity and temperature inside the caves increases with too many tourists," said Li, adding the moisture and the carbon dioxide they exhale cause irreparable damage to the caves' frescoes and painted sculptures. Sand and wind also pose a threat. Archaeologists say the caves are deteriorating more rapidly than in the past.

To reduce the damage, a maximum of 6,000 visitors in the caves are allowed per day. Tourists can book visits a month in advance on the internet. But this summer, reservations have been filled up quickly and an additional 12,000 tickets are sold every second day at the site to accommodate visitors who did not book online.

The caves, 25 kilometers southeast of downtown Dunhuang, have seen an increasing number of visitors since they were opened to the public in the late 1970s, especially after they became a UNESCO World Heritage Site in 1987.

The dilemma between preservation of the 1,651-year-old site and the demands of tourism has become more urgent, and the academy adopted a series of measures to try to strike a balance, aside from limiting daily visits.

Zhang Xiantang, deputy director of the academy, said the digitization of murals inside the Dunhuang caves is the most effective way to protect them. Before visitors start their tour of the real caves, they are led first to the visitor center, where 3D movies using digital technology are shown to display the frescoes and sculptures in some caves closed to the public for preservation.

This has helped visitors get better background knowledge and reduce their stay in the actual caves. The academy allows each visitor to examine eight caves a day and limits the combined time in the caves to 75 minutes.

Each cave has hundreds of figures on its walls, mostly of the Buddha, including palmsize artwork. Visitors cannot get a good look at mural details, staying in a cave, in dim light, for just a few minutes.

But with high-definition prints displayed in museums, people can see the details clearly, Zhang said. The project has enabled the "unmovable museum on walls" to be shown anywhere in the world with 3D technology, he said.

There are 2,415 painted sculptures and 45,000 square meters of frescoes inside the 735 caves, the earliest dating back to 366.

So far, the academy has finished collecting data on 119 caves, of which 30 can be viewed online, e-dunhuang.com, according to Yu Tianxiu, who has worked for years on the digitization of the caves. The website, which launched on May 1, 2016, will have an updated version by the end of this year offering more pictures and interactive experiences.

"It takes a month to photograph a painting and process the information online," Yu said.

The Dunhuang digitization began in the 1990s after it became apparent that the frescoes needed protection from heat, humidity and carbon dioxide.

"The odor of perfume can linger in a cave for days and harm the frescoes," said Bian Lei, a tourist guide at the caves. "The colors are fading, too. I can see that."

It will be difficult to complete the digitization of all the caves. A photo of a painting on a wall now needs 60 gigabytes of storage space, similar to that of a smartphone, because one photo often has more than several thousands of parts.

In July, Seagate Technology, a US data-storage company, provided the academy assistance and donated $78,000 to support the digitization of cave 307, which has many images of children.

"Lots of high-tech companies want to cooperate with us. But many soon leave because they underestimate the time and patience that is needed for the preservation," Yu said. "It's a lifetime career."

Apart from taking photos and processing them over months, Yu and his team also use the data to replicate the look of sculptures and murals through 3D printing technology. Visitors to the caves can go to the replicas of eight caves in front of the real caves. They also are part of the academy's digitization project, and have been exhibited in many cities in China, and also in the United States, Japan, Australia and Russia.