Despite many projects to reduce the runoff, the daily avalanche of detritus and 20,000 litres (5,283 gallons) of waste water pouring into the bay is drying it up and killing marine life.

Each year the 380-square-kilometer (147-square-mile) bay becomes shallower by five centimeters (two inches). Without all the waste from human settlements, it would naturally lose only 18 centimeters (seven inches) of depth in a century.

Already 15 percent of Guanabara Bay's surface area, or 60 square kilometers (23 square miles), has been lost to sludge, geologist Elmo da Silva Amador was quoted as saying in the newspaper O Globo.

At this pace, a third of the bay will be erased from the map in the next 100 years, and two-thirds in 200 years.

After five centuries the bay could fill up entirely, at which point the water's edge would be out past the legendary Sugar Loaf, the graceful outcropping which now marks the entrance of the bay from the Atlantic Ocean.

A nauseous odor oozes from the bay in Tubiacanga, a fishing village near Rio's international airport.

Even though the village has a sewage system and trash is collected three times a week, the reeking waste water floats near a collection of small fishing boats.

The problem, according to Paulo Cesar, vice president of the village association, is that "there is no structure for catching the rainwater, and so it washes trash and waste into the sea."

Most of the sediment and trash filling the bay comes from towns and villages upstream on the 55 rivers which discharge into the bay, Cesar said.

Many of these are connected to sewage systems. But in those close to the riverbanks, Cesar explained, "it is easier and cheaper to let the sewage run off straight into the sea."

Sergio Souza de Santos, 54, who has been fishing the Guanabara since the age of 12 in Tubiacanga, has seen his catch fall year by year.

"When I was young, we caught whatever we wanted. Now, we find everything in our nets except fish. The bay is shallower, and there is less oxygen for marine life. This job has no future, and young people will have to do something else for a living," he said.

An ambitious program to clean up Guanabara was launched in 1995 with 855 million dollars from the Inter-American Development Bank and the Japan Bank for International Cooperation.

But as reported in O Globo, much of the project remains on the planning boards, and what has been implemented has proven unusable.

Five waste-water processing plants were built, and three others expanded. But the waste water doesn't flow into them. A sewer network to collect the wastewater and runoff from regional cities has yet to be built, and so 75 percent of it goes into the bay without treatment.

"This project involves 16 municipalities. It is necessary to make arrangements with each of them. And that takes a huge amount of time," said a spokesman for Rio de Janeiro state's environment department.

"According to the program, at the end of the year 58 percent of dwellings will be connected to the sewer. The deadline will be respected," said the spokesperson.