Mexico's sprawling, polluted capital of some nine million produces a staggering 35,000 liters of waste water every second. It has four divers who make 400 dollars a month to take the fetid plunge at filtration-treatment plants.

Their mission: to keep the flow -- from millions of homes, hospitals, factories and businesses -- on the go.

Many of the kinds of problems they face will be up for discussion as Mexico City from Thursday hosts the World Water Forum, where experts will will look at local supplies and water treatment in a safer, cleaner setting.

The network of pipes and canals that empty into filtration plants here produce a surreal soup that has included human corpses, accoutrements of witchcraft, whole auto bodies, dead animals, and plenty of unloved armchairs.

Needless to say, the grates tend to get blocked up.

"We go down into the 'pond' every time something gets stuck in the grates. Down there, you cannot see a thing, you really only have your hands to see with. When you start, you are just asking for everything to come out OK," said Covarrubias.

In 1991, a diver struggling to unclog a grate finally managed to pull out a car tire, only to be sucked into the sewer system by water pressure, and be killed.

"So far, I am still scared every time I go in," confessed Julio Cesar Cu, the lead diver.

Their equipment is decidedly low-tech.

The crew works in plastic suits, and wears gloves they hitch onto the suit with tape. Top that with 10 kilos (more than 20 pounds) of headgear, and a set of rustic tubes for breathing and talking to the surface.

Then, its down, down, down into it all, as many as 30 meters (32.8 yards) deep.

"I am feeling wetness on my neck," says Covarrubias stepping in to check the depth of the sludge on the plant floor.

The slightest contact with this toxic flow could be enough to kill most humans.

But some of the crew are convinced that as the years go by, they have earned some immunity to peril, if not stench.

Outside one of the capital's 86 wastewater treatment plants, Covarrubios doffs his headgear, breathes in a big gulp of fetid air, and braces for the disinfection process.

In a developing country such as Mexico, that means he has a few buckets of tap water tossed over his head.

"We are not heroes; this is just a job like any other," says a convinced Luis, age 43. "It just has some risks that we know how to handle," he adds.

But the fear of perishing, being ground up to bits by plant machinery, drowning or catching a lethal infection, is so intense that Carlos Barrios, a former national-level swimming champ, believes that you can almost feel death in there.

"One time, when I was in there, I felt that something or someone was standing next to me and started pushing me, and it also made some strange noises as if it were murmuring something; and then when I got out, the guy who was running the audio conection also told me he heard those sounds," Barrios said.

"Those noises were not like static or some kind of interference on the line," he says, trying to explain himself, as fellow divers poked fun at him.

They prefer to focus on the need for better working gear.

Team leader Cu shows a clip from a magazine -- a diving suit that would ease his fears called an ADS Hardsuit. Problem is, he says, its costs 2.7 million dollars.

"None of us is young," Barrios says. "The city is growing so much and people pollute the pipes so much that future divers should start being trained.

"But authorities are going to have to make an effort," he said, pointing at the garbage-strewn "pond" and asking: "How are you going to tell a young guy to get in there?"

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