Summer is the busiest season for staff at the Mogao Grottoes, as more than a third of the annual visitors to the more than 1,650-year-old UNESCO World Heritage site come during the season.

In 2018, the tourist attraction welcomed almost 2 million visitors.

Sitting in Dunhuang, Northwest China's Gansu Province, the Mogao Grottoes are home to a priceless collection of Buddhist artworks - more than 2,000 colored sculptures and 45,000 square meters of murals - in 735 caves carved along a cliff by ancient worshippers.

But only about 110 caves are suitable for visiting. "Too many visitors will affect the temperature, humidity and concentration of carbon dioxide in the cave. This will lead to repeated dissolution and crystallization of salt in the cave, causing the murals to decay," said Su Bomin, vice president of the Dunhuang Academy China, which manages the grottoes.

Multiple measures have therefore been taken to preserve the works. In 2014, it took the lead in opening ticket reservations and curbing the number of daily visitors. This summer, it also set a cap on the number of visitors allowed entry between 8 am and 3 pm.

In the meantime, countless antiquarians and experts have devoted their entire lives to the protection and restoration of the cultural relics.

Li Yunhe, 86, has been repairing ancient grottoes at Dunhuang since the 1950s. "I have repaired over 4,000 square meters of murals and more than 500 colored sculptures," said Li. 

A monitoring and early warning system also plays a key role in its conservation.

The Mogao Grottoes began monitoring the nearby environment as early as the 1960s. In 1989, a meteorological station was set up and in 2011, the current monitoring system was put into use.

Now the meteorological environment, cliff conditions, possible natural disasters such as floods and earthquakes as well as the interior environment of the caves are all under close surveillance, with the site becoming closed to tourists once the temperature, humidity and carbon dioxide density threaten to exceed safe levels.

"All data is automatically recorded every 15 minutes. We are studying the relationship between this data and the rate of the decay to explore appropriate preventive measures," said Guo Qinglin, head of the academy's conservation institute.

Extreme weather in recent years has also posed new threats to the grottoes. In July, the grottoes saw the biggest rainfall since 1989, when the academy began tracking records of precipitation.

Thanks to the flood control project constructed in 2013, the downpours caused only limited damage to the grottoes.

Despite all the efforts, it is frustrating that these ancient grottoes will inevitably vanish some day. But a digitalized Dunhuang Mogao Grottoes which is under construction will permanently preserve its glory.

By visiting the website "Digital Dunhuang," visitors can be immersed in the images of more than 4,430 square meters of murals across 30 caves.

The database was a result of more than three decades of efforts. In the 1980s, Dunhuang Academy China began partnering with research institutes and universities including Zhejiang University and Northwestern University in the US to digitalize the ancient cultural relics.

At least 40,000 photos must be taken to digitalize a 300-square-meter mural. "Now we have collected the digital data of 180 plus caves," said Yu Tianxiu who leads the digitalization work.