The rock paintings mostly feature squatting human figures. [Photo provided to China Daily]

Mysteries remain

The pigments in the paintings have been mixed with minerals and oxalic acid derived from local plants to protect the red color from fading.

Describing the paintings, Zhu says: "Squatting human figures are common in prehistoric rock art all over the world.

"But nowhere else do you see so many squatting human figures as in the Zuojiang art. This could mean that the Luoyue people were very devout."

But the other possible explanation for the squatting figures, he says, could be their resemblance to frogs, which is typically connected with virility in such art. Worshipping a "frog deity" was also common among communities that cultivated paddy.

Despite the studies, many mysteries still remain.

For instance, why the artists suddenly stopped doing rock art and what happened to the creators is unclear. Also, how the motifs were drawn so high up－some in spots 60 meters above the river－is still being studied. And, while the locals see the site as a forbidden territory, outdoor adventurers have been drawn to it.

Wang Yuguang, who used to patrol the rock art sites for 20 years, says: "People from outside may not see the site in the same way that we do.

"They used to come and climb the rocks. They did not aim to destroy the paintings, but if we did find fallen pieces of rock, we would glue them back."

Wang, who retired in 2008, says he used to patrol the site four to five times a day to check the cliffs.

In 2009, the biggest rock art site in Ningming was closed to visitors, and visitors were allowed to view it only from an observation deck.

Separately, the State Administration of Cultural Heritage allocated 7 million yuan ($1.04 million) to set up a monitoring and data analysis system in 2015 as a better way to manage the area.