The volunteers usually clear the underwater trash on weekends. They sort it on shore. The costs involved are shared among the team members.

Zhang said a large amount of the trash found underwater is not discarded directly into the lake; instead, it is discarded near the lake and carried into the lake by rain or wind.

Over the past five years, he has instructed more than 300 diving beginners. Zhang said before each diving class, he tells his students that to become a qualified diver, one has to first learn how to protect the environment.

Zhang said over the past five years, the volunteers have collected all kinds of waste – mobile phones, film roll, batteries, umbrellas and plastic food containers. And their cleaning efforts have paid off.

Zhang said that the water quality of the lake they have cleaned has improved a lot.

"We spotted freshwater jellyfish in the water, which are rarely seen because they are sensitive to water pollution," Zhang said. "In 2013, the jellyfish we found in the water were the size of a chopstick top and later, we sighted ones as large as a fingernail, indicative of the improved water quality."

Despite the achievements, Zhang admitted their efforts are not enough, given the vast size of the whole lake.

"We plan to make the water cleaning campaign a long-term charity program, and do our best to protect water environment," he said.