There is startling scientific evidence which suggests a requirement to clean the city's polluted waterways in time for the Games will not be fulfilled.
With just over 500 days to go before the 2016 Olympic Games open in Rio de Janeiro, work is continuing around the clock to complete the sporting venues. But the BBC has seen startling scientific evidence which suggests a requirement to clean the city's polluted waterways in time for the Games will not be fulfilled.
From the air you can see Rio's Olympic Park, in the Barra zone to the south of the city, taking shape.
From a distance, it looks pretty spectacular but up close the view and the smell is much less appealing. With deadlines looming to complete the sporting venues, the waters around the park have been completely neglected.
The chemically overloaded lime green lagoon, which surrounds it on three sides, is fed by several small jet-black rivers, full of untreated sewage.
"Every river around here is practically dead, because they are full of sewage," says the biologist Mario Moscatelli who has long campaigned against the way these once pristine lagoons have been destroyed.
From the vantage point of our helicopter, Moscatelli points out islands of raw sewage and solid waste that have "grown" in the lagoons.
Flying just up the coast, past the tourist beaches of Ipanema and Copacabana, the situation is equally critical.
Guanabara Bay - underneath Sugar Loaf Mountain - is an iconic backdrop for Olympic sailing events but some parts of the bay are so polluted they're anoxic, devoid of oxygen and of life.
For the last year, Olympic hopefuls including the British sailing team have been training in these very waters, counting on a government promise to clean up the bay in time for the Games.
Among the Great Britain sailors I spoke to was Charlotte Dobson and her partner, Sophie Ainsworth, who race in the 49er class.
"There's a lot of debris in the water which is really bad for the racing because you get stuff caught around your boards," says Dobson. "It can really slow you down and it's really by chance, completely erratic."
Although the Rio State Government has repeated its commitment to treat 80% of the sewage entering the bay, as was stipulated by the International Olympic Committee, one state environment official recently admitted that target was not realistic.
It's not difficult to see why.
On the north side of Guanabara Bay, in the suburb of Niteroi, I sought out one of the state's much-publicised 'eco barriers'.
Expecting to find an elaborate, high-tech system I was shocked by what I saw - a small, tatty old net that stopped some of the detritus but most of the sewage and waste still clearly got through.
One of only seven such 'barriers' placed around this huge bay, it was a pathetic gesture and also surely raises questions about the huge amounts of public money the officials claim has been spent on pollution control.
Rodolfo Paranhos, from the Federal University of Rio, is a leading scientist, who regularly tests the water. A leading authority on the state of the bay he says levels of harmful chemicals in the water are now higher than ever.
"Nitrates, ammonia and phosphates are all there in much higher quantities," Parahnos tells me as he charts the latest water samples taken that morning.
"I'm afraid to say that overall quality is getting worse year by year. We've been doing this for more than 20 years and the trend is increasing. The levels of nitrogen are increasing, for example, by 25% every year."
When I ask the scientist if local officials are right when they say that they're cleaning the water by 45% or more and that all will be up to internationally acceptable standards by the time of the Olympics, he says "Of course not and they know they are not right!"
As was the case with last year's World Cup, finishing Rio's Olympic projects is a race against time. At a ceremony in Rio's Ipanema district to mark the completion of a section of the city's Metro, I questioned the Rio state governor, Luis Fernando Pezao, about the delays.
"It's not easy. There are always bureaucratic delays but the water will be fine for the Games", said the governor. But he avoided my specific question about guarantees to clean 80% of sewage entering the bay.
For homegrown Brazilian sailors, like Martine Grael, the condition of the waters means the Rio Olympics are not the source of pride she might have hoped for. To blame, she says, are those old age problems of corruption and a lack of planning.
"The city wasn't planned and it's not just the poorer areas, the favelas," says Grael. "There are condominiums not even attached to the sewage system but politicians don't want to take it up because it doesn't win them votes."
Some areas of Guanabara Bay are less polluted than others and Olympic organisers will be hoping that favourable weather will allow the sailing to take place in relatively benign conditions.
But the Games are also meant to be about legacy - leaving clean waters Rio's residents can use and be proud of.
BBC News - Wyre Davies, Rio de Janeiro correspondent