After a little exercise on his wrists and ankles, Ni puts on the diving suit weighing over 5 kilograms, as well as the 15-kilogram lead plate, and prepares to go down the well to work. [Photo/CFP]

In Shanghai, there are 130 "frogmen" whose job is to clean the underground pipelines in the city, and to inspect and maintain these pipelines. They also handle drainage incidents. Being a "frogman" is a very dangerous job, because they are exposed to dangers including hydrogen sulfide poison, suffocation from damage on the diving suit, stuck in the pipeline, and being washed away.

"We have our own code: Pulling the rope once, everything is normal; pulling the rope twice means more materials are needed; three times or more means the man underneath wants to come up, but if he does not show up in 2 minutes, something must go wrong," said Ni.

Besides professional diving licenses, these "frogmen" also have to be identified by professional organization for their underground pipeline techniques, and their licenses have to be renewed annually. In addition, the "frogmen" have to hand in official written application before they take a job. On this "invisible battle line", these "frogmen" are playing a vital role for the running of the city.