Wang Xinshu, is the oldest of the sentries. The 82-year-old was recruited by the local cultural relics protection department in 1987 and has guarded the Jianshanzi Grotto－among the Dazu's most ancient－ever since.

"There was no road to the grotto deep in the mountain," he said. "I had to walk uphill for an hour and a half to reach the site. I think it is my destiny to protect this grotto."

Zhou Yaode, 74, has spent 34 years protecting Douwanzhai Grotto at Fengshan village in Zhong'ao town. He is also a leader of a cultural relics protection team protecting the national treasures in five villages.

His wife, Zeng Kaibin, supports his protection efforts. The couple make a living by farming and raising livestock.

Zhou started guarding the grottoes at night in 2007, and has hardly slept at home overnight since.

Four years ago, Zhou wanted to quit because Zeng was very ill at the time, but she encouraged him to continue.

"You are doing the job quite well, why quit?" Zeng asked him then.

She died last year.

"I've never asked for leave, except last year when my wife passed away. I asked for a five-day leave and was sick for a while," Zhou said. "But I came back to the site as soon as possible because my responsibility (to guard the grotto) is huge."

Yang, the director of the protection center, said he was pleased with the volunteers' efforts.

"Their work has proved to be effective and reliable. The relics are in much better condition than in the past," he said.

"But I'm afraid the volunteers are a bit too old for the job now. Most of them are ages 50 to 80."

To assist them, the center has incorporated modern technology into the relics protection effort. The center now carries out monthly inspections by using a mobile phone app developed by Chongqing's Municipal Bureau of Cultural Relics. The inspectors can upload photos of the sites via the app to maintain records of the condition of the relics.