Deep below the streets of Mexico City, Julio Cesar Cu is hard at work swimming in dark sewer waters in a diving helmet and dry suit, surrounded by rats, feces and condoms.

It's a dirty job, but someone's got to do it.

For the past 30 years, the 53-year-old has plunged below the grimy surface to unclog drains with his hands, a crucial task to ensure the system runs smoothly for more than 20 million people producing 12,700 tonnes of waste per day.

Sometimes, he makes grim discoveries, like dead bodies floating down the tunnel.

"You can find anything you could imagine down here, from plastic bags to car parts," Cu told AFP before plunging eight meters (26 feet) below street level at a pumping station.

He became the mega-city's lone sewer diver when his two other colleagues quit for fresher air five years ago, though he now has two apprentices learning the ropes for a risky job that pays $480 per month.

"Someone has to do this work," the burly diver said as he geared up for his first plunge of the day. "The smell is unpleasant, but it's like everything, you get used to it."

It's a job that man can do better and faster than a machine, which would take 15 days to unclog drains.

Cu wears a yellow diving helmet and a red dry suit to protect him from slimy water. Because an oxygen tank would be too heavy, he breathes through a tube connected to the surface.

He swims under the surface or drags himself over waste, looking for any garbage that might stop the flow of wastewater and cause trouble in bathrooms above group.

Cu often swims blind since things can get pitch black just 10 centimeters (four inches) under the surface. Outside, three colleagues, including his two trainees, communicate with him via radio to make sure he is okay.

-- Risky swim in dark waters --

"A drop of water on the skin is a surefire infection for us," Cu said. The water carries other risks like nails, broken glass and syringes.

Cu has never suffered an accident, but he is haunted by the memory of a colleague who died after being swept away by the water of a reservoir 15 years ago.

So why would a man swim among other people's waste day after day, risking his wellbeing for a modest pay?

"My wife says that I work for the love of the art, but I really like my job. It's my passion," he said. "I'm motivated by the excitement because I never know what I'll find down below."

Sergio Palacios Mayorga, a geology researcher at the National Autonomous University of Mexico (UNAM), said the sewer diver became necessary because of a giant population that still has to learn to recycle and stop throwing trash on the street and in rivers.

"The diver job will still have to exist for a while longer. The need will lessen as the population learns to put trash in bins and not on the street, which fills up drains," Palacios Mayorga said.

-- The new sewer generation --

For the past year and a half, Agustin Isaias, a 32-year-old computer specialist, and Luis Angel, 23, have been preparing to become sewer divers.

Isaias said the city government should put more resources into the job in order to attract more divers.

"It would be nice not to lose this," he said, adding that he still needs a couple of years to begin diving without Cu's supervision.

Until then, Cu will take care of the trash "as long as my body can tolerate it."

"I don't want this diver job to disappear," he said, "because it's important."