But Li said not every evidence gathering mission is successful.

"Sometimes it is like looking for a needle in a haystack," he said. "The reasons are complicated, with the major one being that almost half the time suspects tell lies about locations.

"The job is full of potential risks. The underwater world in Beijing is muddy, typically with broken glass, metal or other things that can hurt you. And sometimes where we dive is not really water, but a dirty waterway with oil, waste and sewage.

He said that almost everyone in his team has experienced surfacing with a piece of excrement on their head and waste on their body. "It's not funny, but typical of our work."

However, "there must be someone to safeguard underwater security", Li said. "We are in the darkness, but we bring justice to light."