One day in this summer, three divers emerged from below the surface of a lake in Guiyang, Southwest China's Guizhou province, one of them holding a blue plastic basket filled with trash, such as plastic wrappings, metal pipes and glass bottles, that they picked up from underwater.

Zhang Xudong, a dive coach in his 20s, is the head of the dive cleaning team. Since 2013, he's been dedicated to removing underwater waste in the waters of Guizhou, along with other volunteers.

Coming from different social backgrounds, the volunteers have a shared goal: Restoring the underwater world to healthy cleanliness.

"At the beginning, we just wanted to promote diving and create a better diving environment for us. Gradually, we started picking up garbage underwater, and encouraged more and more people to follow suit," Zhang told The Mirror.

Zhang was born in a rural village in Guizhou. Since childhood he's enjoyed swimming, and he spent a lot of time swimming in a river near his home.

During his college years, he majored in outdoor sports at Guizhou Minzu University. During a winter vacation before graduation, he was offered a chance to learn diving in Sanya, a city in South China's island province of Hainan. That was the first time he saw the sea and dove in. He soon fell in love with diving.

Afterwards, he received further diving training in Southeast Asia, and became a certified dive instructor through the Professional Association of Diving Instructors (PADI).

In 2013, Zhang graduated and went back to Guizhou to establish a diving club.

"Guizhou has rich resources of fresh water and professional diving was not developed here. So I came back to promote this sport," Zhang said. "Overall, Guizhou has a beautiful environment. But when I dive, I can find some garbage underwater."

Diving in fresh water can be more dangerous than in the sea, said him, as there is more human waste in the water, which not only pollutes the water but also poses a danger to divers' safety.

Zhang recalled a dangerous diving experience.

One day in December, Zhang and several other volunteers dived into the water to collect garbage. Soon after they started working, one of them was discovered missing and couldn't be seen, due to low visibility in the water.

"We turned back to search for the missing teammate, and found him trapped in a fishing net and struggling to get out. Fortunately, we helped the man out quickly," Zhang said.