The Mogao Grottoes in western China have survived 1,650 years of sandstorms and rain only to face their biggest threat yet: tourists.
Record numbers of tourists have been swarming the narrow caves carved from a sandstone cliff near Dunhuang City in northwest China's Gansu Province.
The hordes of visitors have driven up the temperature, humidity and carbon dioxide levels inside the caves, which could damage the well-preserved but fragile Buddhist wall paintings inside.
"For example, the humidity level inside the caves should be below 62 percent. Increased humidity can accelerate the process of flaking and formation of salt deposits on the murals," said Chen Gangquan, chief of the cave monitoring center of the Dunhuang Academy.
"Such damage is a slow process and is difficult to notice with the eye,but it is irreversible," Chen said.
The academy is closely monitoring the temperature, humidity and carbon dioxide levels in all caves open to the public. If the data from certain caves exceed the limits, they will be closed temporarily.
To relieve the pressure, the Mogao caves have set a daily limit of 6,000 reserved tickets, plus an extra 12,000 emergency tickets to cater to the growing number of tourists since July 1.
The Mogao Grottoes have survived this long partly due to their remoteness. The site's Gobi Desert location has kept the caves dry and visitor numbers low.
Today, improved transportation and China's growing wealth have fueled a huge boom in domestic tourism. Dunhuang is no longer too far off the beaten path for the average traveler.
More than 1.34 million tourists visited the caves in 2016, a rise of 70 percent from 2014. In the 1980s, the figure was just around 10,000.
The academy expected the number of tourists in July to surpass 350,000, a new historical high.
The 735 caves carved along a cliff are home to a huge collection of Buddhist artwork, including more than 2,000 colored sculptures and 45,000 square meters of frescoes.
Guan Yanru, a tour guide with Dunhuang Academy, has only had two days off in the past two months. She talks to visitors for four hours every day in the scorching heat.
"People can bear the pressure from the tourist overload, but the artwork can't," Guan said.
The academy has taken measures to control the tourist flow and protect the sculptures and murals.
Since 2014, the UNESCO World Heritage Site has controlled the number of tourists during peak season. They are asked to register in advance and watch two 20-minute movies on the history of Dunhuang and the caves in a new exhibition center.
Later, they are guided to see caves that are open to the public. The videos have helped reduce hourly visitors from 2,200 people to 1,200 during peak hours, said Li Ping, deputy director with the Mogao caves administration committee.
"It helps relieve the pressure on the caves, and with fewer crowds, visitors can have a better experience," Li said.
Experts and officials from Dunhuang Academy and local tourism departments have traveled to cities, including Beijing and Shanghai, to promote low season visits to Mogao, when ticket prices are halved.
The efforts have proved effective. Statistics showed 34-percent tourist growth from January to April this year, an off-season period, compared with the same period in 2016.
Facing threats of natural erosion and human-induced damage, Dunhuang Academy has been working on a major digital archiving project since the 1990s.
Digital Dunhuang (e-dunhuang.com) was launched in April last year. It offers virtual views of 28 of the Mogao Grotto caves and two of the Yulin Grottoes.
Behind the online experience is a huge amount of work, said Wu Jian, head of the heritage digitization center of Dunhuang Academy. "More than 40,000 photos are needed to digitize just one small cave with 300-square-meters of frescoes."
"Dunhuang is a shared treasure of mankind," said Yang Xiuqing, secretary-general of the China Dunhuang Grottoes Conservation Research Foundation.
"It is our responsibility to share Dunhuang's culture with the world," Yang added.
"But we have stumbled into a predicament now," Chen Gangquan said. "We cannot let tourists down since they come great distances to see the treasure. However, if they all swarm into the caves, the site may be harmed."