Chen Chao, who became a police diver in 2007, said a newcomer could learn diving techniques in just a year, but that was only part of the job, and the nature of the work meant that even a successful mission could leave squad members without a sense of accomplishment.

In 2011, Chen was assigned to search for the bodies of two children - a 5-year-old girl and a 6-year-old boy - who were killed and thrown in a river in downtown Beijing.

Chen, a sports graduate who learned how to swim at age 7, said it was the most haunting moment in his 11-year career and one he still remembers often.