"Many mountains may appear huge on the outside, but are hollow inside. As cave explorers, our first job is to find the entrance of caves," he says.

Yang first heard about caving in the late 1990s when he moved to Chongqing, where he opened a computer trading company. He was initiated by an American, who led caving activities in Chongqing.

"I was so nervous that I drank my water supply and everybody else's. But I became addicted to this sport immediately," says Yang, who was the only Chinese in the group.

As recent as two decades ago, most of the explorers of China's deepest caves were foreigners because of the country's lack of professional equipment and knowledge. Now, according to Yang, almost every province has its own Chinese caving team.

It is an activity that requires skills and proper gear such as harnesses, bags and packs, torch lights, suits and undersuits, as well as other accessories such as belts and ropes. Also known as spelunking in the United States and Canada and potholing in the United Kingdom, it is a physically exhausting and risky sport.

Caves that are formed in karst landscapes can be as deep as several hundred meters.

In 2009, Yang explored the second deepest cave in China, in Fuling district, in Chongqing, which is more than 840 meters.

"The most dangerous part is falling rocks," Yang says. "For safety reasons, we only allow one team member to descend or ascend each time, using a rope. But, falling rocks are unavoidable."

Yang recalls a cave exploration in 2005, when one of his teammates was severely injured by a falling rock and broke several of his ribs. It took them more than eight hours to rescue and carry him out of the cave. The deeper the cave, the chances of survival get slimmer because rescue efforts are more challenging.