By Gary Duffy
BBC News, Brazil
It is not a location every artist would chose for a display of their work
Whether this is art seems very much in the eye of the beholder
In the polluted tunnels beneath the streets of Sao Paulo, Zezao, one of Brazil's most famous graffiti artists, returns time and time again to produce his work.
This part of the city is dark and dangerous. The brown and smelly water is a foul mixture, bringing with it rubbish and sewage from above.
Zezao has had several vaccinations to allow him to continue his work.
On one occasion he had to run for his life when the tunnels were flooded after heavy rain. On another he stood on a nail.
But all along this grim and uninviting channel are signs of an unexpected artistic display - graffiti painted with care and attention to detail, even though few people will ever get to see it.
The work can be dangerous but Zezao says it is important
Zezao says he works here to make a point about the years of neglect that have caused this pollution.
"I did this work because I wanted to show the architecture and the conditions of tunnels of Sao Paulo, which is this - degradation, rubbish - everything that the rain brings," he told the BBC News website.
It is a journey you have to make with care, and the only audience in this gallery are the insects that made it their home. In the corner a dead rat is clearly visible.
It is not perhaps the image that everyone has of graffiti artists, and despite the risks, Zezao says it is worth it to raise public awareness.
"This place is a very dangerous place, contaminated, because we are in the sewer of Sao Paulo. I did suffer some accidents here doing this job, but as dangerous as it is, I know the importance of my work for humanity."
Across the city another of Sao Paulo's most famous graffiti artists, known as Titifreak to his fans, is hard at work.
His graffiti has been displayed around the world and he has also worked for major companies such as Nike, and he proudly wears a pair of the company's trainers with his name printed across them.
Zezao uses what little light is available to highlight his work
Decorating a shabby apartment building, with the owner's permission, he rejects the view of those who dismiss graffiti as vandalism.
"A person who says that graffiti like this one here which is more artistic - to connect that to vandalism I think is missing a little bit of literature and culture," he says.
"People get confused when they see someone with a spray in the hand, which seems too aggressive."
Not everyone in Sao Paulo is a fan of graffiti. Every day painters from the city authority are sent out to cover it up.
Sometimes it is just names and letters scrawled on walls that are painted over - something that is know here as pichacao, and which is often seen as vandalism.
However sometimes more elaborate works are removed as well. Among the public, opinion seems divided.
"When it is well done it is beautiful... but when it is a mess no, because it is dirty and ends up destroying the appearance of the walls," one woman says.
Titifreak has shown his work all around the world
"I think this prejudices the image of the city and makes it appear dirty - so I don't think it is nice," said a man who was passing by a new piece of graffiti. "I don't think it is acceptable."
But at the same time, Sao Paulo's Museum of Contemporary Art is paying its own tribute to graffiti artists from Italy and Brazil.
And far removed from the grim tunnels beneath the city Zezao's work is on display here - work the museum says deserves to be valued.
"Graffiti is a way of artistic expression in a urban environment," says Lisbeth Rebollo Goncalves, director of the museum, which is known here as Mac.
"If you in fact meet a great artist doing a work of art in your home you would take great care of it.
"But because it is an anonymous piece of work, people feel a bit hostile about it being on a wall in a public place, but this part of urban life, " she says.
Such is the fame of Sao Paulo's graffiti, that other artists have come from around the world to see it.
Pichacao involves just names and letters being scrawled on walls
"The artists that I see in Sao Paulo are much more individualistic," says Gary Baseman, an award-winning artist and illustrator from Los Angeles in California.
"Each piece is very unique, but in the States it is more like they have a very iconic image or particular mark that they repeat over and over again.
"Part of the reason that tourists come here is to see the street art that is all over the place, it is like the city is just one big giant canvass. To try to control it, in a way, you are almost destroying the culture that the World sees - the reason they are coming here."
Jonathan LeVine, a gallery owner from New York, believes graffiti in Sao Paulo has particular value.
"In a city like Sao Paulo where maybe people don't have so much money. It's a way for them to have access to art they wouldn't otherwise have access to, and it's a way for people to express themselves to the public."
"From city to city and country to country, the reasons that people do it are very different."
It does seem that graffiti is too well established a part of life in Sao Paulo to ever disappear.
And in his gloomy polluted corner of the city, it seems Zezao will continue to produce his underground gallery in the hope that his message will finally be heard.