He and his teammates explore a tunnel behind a waterfall at Yangpi Cave, Guiyang city, Guizhou, in January 2020.[Photo provided to China Daily]

As his skills grew, Zhou joined a China-France team in 2014 and visited the Miaoting cave, categorized as the biggest of its kind, by volume, in the world. It was jointly detected by Chinese and European scientists. Experts theorize that it is capacious enough that a Boeing 747 passenger plane could fly in it.

"The road to the cave is etched with gullies of differing heights, which is typical of the karst landform in Guizhou," Zhou says. They are the first challenge the spelunkers have to face.

"The ground is covered with green moss and is very slippery," he says. "You have to be there to see just how big the cave is."

Over the years, Zhou has made nine trips through different entrances to access the Miaoting cave.

Desire for more adventurous spelunking drove him to master technical diving and underwater photography. He also became a dive master certified by the world's leading scuba diver training organization, the Professional Association of Diving Instructors, headquartered in California.