Of the 245 musical instruments, only one remains unplayable to musicians-a curved lute with one string. Zheng has recorded 20 instances of the lute appearing in murals painted across different dynasties. He couldn't find any other information about it except for what is shown in the murals at the Mogao Caves, which were built by Buddhist worshippers between the 4th and 14th centuries.

"I think it was created by painters. It never existed in real life," says Zheng.

Both the curved lute and the lotus-like ruan are depicted floating in the sky with ribbons.

Zhu Xiaofeng, a researcher with the Dunhuang Academy, says there are hundreds of musical instruments that appear to be floating in the murals. Also, bands of musicians playing different instruments are illustrated.

Among the bands, the most-seen instrument is the pipa, which is depicted as having more than 600 shapes, according to Zheng.

In the 1980s, Zheng and his team counted each individual image of a musical instrument that appears in 492 caves to get accurate statistics, taking photos of them as they went. They found that the pipa was the most painted.

"Since the Han Dynasty (206 BC-AD 220), the pipa has been a symbol of music in China. Painting it means painting the music scene," explains Zheng.