A vast new exhibition space opened in New York City this summer, with a show 18 months in the making. On view are works by 103 street artists from around the world, mostly big murals painted directly onto the gallery’s walls.

It is one of the largest shows of such pieces ever mounted in one place, and many of the contributors are significant figures in both the street-art world and the commercial trade that now revolves around it. Its debut might have been expected to draw critics, art dealers and auction-house representatives, not to mention hordes of young fans. But none of them were invited.

In the weeks since, almost no one has seen the show. The gallery, whose existence has been a closely guarded secret, closed on the same night it opened.

Known to its creators and participating artists as the Underbelly Project, the space, where all the show’s artworks remain, defies every norm of the gallery scene. Collectors can’t buy the art. The public can’t see it. And the only people with a chance of stumbling across it are the urban explorers who prowl the city’s hidden infrastructure or employees of the Metropolitan Transportation Authority.

That’s because the exhibition has been mounted, illegally, in a long-abandoned subway station. The dank, cavernous hall feels a lot farther than it actually is from the bright white rooms of Chelsea’s gallery district. Which is more or less the point: This is an art exhibition that goes to extremes to avoid being part of the art world, and even the world in general.

The show’s curators, street artists themselves, unveiled the project for a single night, leading this reporter on a two-and-a-half hour tour. Determined to protect their secrecy, they offered the tour on condition that no details that might help identify the site be published, not even a description of the equipment they used to get in and out. And since they were (and remain) seriously concerned about the threat of prosecution, they agreed only to the use of street-artist pseudonyms.

Workhorse, in his late 30s, is a well-known street artist with gallery representation; PAC, younger by a decade, is less established but familiar (under a different name) to followers of urban-art blogs. The two came up with the idea for the Underbelly Project in 2008, a few years after PAC first saw the old station, led to it from a functioning one by an urban explorer acquaintance.

Abandoned stations like this — and there are a fair number of them in the city — are irresistible to those who search out hidden spaces in the city, despite or perhaps because of the fact that being there is illegal and potentially dangerous. PAC too found himself compelled after that first visit, and he began going back sporadically.

Seduced by the Abandoned

The place was pitch black, but standing with a powerful flashlight on a platform, PAC said, he had been able to make out a landscape of several more platforms, each lined with rows of columns, alternating with sunken track beds. The station, about the size of a football field, had clearly never been completed: no track had been laid in those beds, no escalators or staircases met the gaping holes in the platforms, and there was no electricity.

“I would hang out here for hours,” PAC said, enjoying “the solitude of being underground” and the architecture.

Then he met Workhorse, whose art often focuses on abandoned spaces.

“I told him I knew about a space that was pretty cool,” PAC said, and “brought him down here, and that night the idea for the project hatched.”

The difficult process of getting to the Underbelly space — which involves waiting at an active station’s platform until it’s empty, slipping from it into the damp and very dirty no man’s land beyond, and traversing that to get to the old station’s entrance — suggested to PAC and Workhorse how challenging the project would be. And the legal risks were obvious. Charles F. Seaton, a spokesman for New York City Transit, described such incursions as “trespassing, punishable by law,” and said “anyone caught defacing M.T.A. property is subject to arrest and fine.” Beyond that, Workhorse and PAC worried that given anxiety about terrorism in the subway, a large-scale, long-term project like theirs might even lead to more serious charges.