When 103 urban artists descended into the dark underground tunnels of New York City to create a hidden and totally illegal art gallery in an abandoned subway station, they didn't expect many people would ever see it in person.
The gallery's location was a guarded secret throughout its 18-month birth between 2008 and 2010, the handful of journalists invited to visit (who did not include this reporter) did not give up how to get there and the few accounts that emerged described slipping between active train lines, traversing dirty and dangerous platforms and enduring hours in a maze of pitch-dark tunnels.
But the project was never intended to be a mass-market draw. It was intended to become folklore. The dramatic and beautiful pictures they released of the gallery certainly did the trick - millions saw the pictures, and follow-up galleries drew large crowds.
So when curators Pac and Workhose (pseudonyms, naturally) came to discuss the possibility to do it again, it was basically a no-brainer. The only question was where.
Paris is a buried metropolis. There more than 170 miles of mines, catacombs and galleries are cut out of layers of sedimentary rock, forming a bubbling honeycomb beneath the streets. The grand reseau sud, a network of tunnels hollowed-out under the Ve, VIe, XIVe and XVe arrondissements, is the largest, followed by several other smaller complexes. The famous catacombs, which hold the organised bones of more than 6m people, have been on public show for more than 140 years.
Others are more well-hidden. An electrically powered restaurant complete with bar, cinema and kitchen, was discovered under the city in 2004. When police returned a day later, they found little left except a note reading, simply: "do not try to find us".
That doesn't even include its metro system, its sewers and other modern service ways.
What we're saying is that if you're looking for a city in which to bury an art gallery, Paris is a pretty good choice. So that's just what Pac and Workhorse did, setting about planning a complex, risky but unique gallery to carve out beneath the city.
"It's like a bank heist. You put a schedule together, you know the routines of people and you make sure you don't get caught," Pac told The Huffington Post UK, speaking on the phone from New York.
"Street art is a lot about using the city as your canvas, as your playground," he explained. "It really becomes about using your city infrastructure as this playground in which you can attach your ideas and communicate with people. While these works are bury they still exist in that city structure and are part of that playfulness."
"Ultimately this is the artists' work and they're allowed to put it wherever they want."
The details about how the gallery was built are a closely guarded secret - and again, in the interests of full disclosure, this reporter has not visited the site in person.
What is clear is that this time around the project was a more focused affair. Taking place over just 24 hours instead of 18 months, and featuring around 10 mainly European artists instead of more than 100, it was a quicker and more intense process.
"The execution of the project took less than 24 hours, yet the logistical planning took months. 10 artists, three organisers, two photographers and one writer from five countries descended on Paris for a few days to take part," said Workhorse, one of the curators of the NYC and Paris shows, in a statement about the project.
As for location all we can reveal is that it's somewhere beneath Paris, involves a long walk and possibly a climb.
"The drab grey subway shell quickly took shape, artists working side by side in near darkness, applying their creative vision to the surfaces of the abandoned hull," Workhorse described.
"The consistent hiss of spray paint was the backdrop of the project, a shuffling of feet and murmuring of voices that broke the silence. In less than 24 hours the walls were covered and we emerged above ground. The entrance was closed and as quickly as we arrived, we left to fly back to our respective homes."
It's a self-consciously romantic idea, but also a visceral one. Conor Harrington, one of the artists, said the motivation for participating was for him the simple sense of adventure.
"I suppose I liked the fact that it was all massively illegal," he said. "I did that side of things when I was younger, running around Ireland with spray paint, but I haven't done that in a long time so that appealed to me."
"Even if we were going to never publicise it I'd do it in a heartbeat. Just to go an paint somewhere unusual," he added.
"We were under quite a time constraint. We had a good few hours but I've never been under so much pressure to go in and get something done. Normally when I'm painting walls you can go back to it the next day or whatever. It was quite an operation. The doors of the tunnel and everything were sealed off so the fumes wouldn't escape, as if they escaped on to the track or up through the vents it would attract attention. We were kind of boxed in, 11 of us spray painting and nowhere for the fumes to go - so that was quite interesting."
Another artist who took part, sheOne, said that the thick fumes in the air created additional problems.
"Towards the end I couldn't really see my piece because the paint was so thick in the air," sheOne said. "So I was having to look at the photos on a laptop to see the details then go back up the ladder to finish it. That's a first."
Artists who put together and participate in projects like Underbelly are always asked if they mind their usually very public work being hidden away in private. It's a tired question, and a redundant one in the age of the internet when even a gallery buried underneath a city can be views by hundreds of thousands of people.
"I think it would be nice to see it through the lens of the photographers and videomakers that came with us," said C215, another artist who participated in the Paris project. "Since they are completely part of the project, and their sight adds another artistic layer to the thing, a filter that will turn it completely hallucinogenic."
"Organising a visit could lead to destruction of the work we did, while keeping the spot secret will create a big surprise in a few years, when discovered. We can call this urban archeology."
The organisers of the show insist both the NYC and Paris shows will remain hidden away. Which is not to say they can't be found if you know where to look. It's just not advisable.
In New York at least a few of those who have tried to see the show have been arrested in the attempt, and while Pac says such occurrences are "a natural part of [the work's] life" the work will likely remain a minority pursuit.
"When I hear about interesting abandoned spots I go out and see them with my own two eyes," said Pac. "But [the arrests] are not something I'm happy about. Our intention is obviously not to get people arrested, but that's the breaks."
But could there be an act three? Count on it. Though there is no telling where it will be.
"One can imagine that we started in New York, then we went to Paris, and there is probably another stop on the line. Where that might be I may know or I may not know," Pac said, laughing.
"What we're trying to do here is create a larger myth and so to do that it only makes sense to continue this project and bury these things around the world."