Julia Solis looks very much at home perched on a high stool in the elegant glow of Smith Street’s Bar Tabac. Wearing a knitted jacket with fake fur trim, and makeup that darkens eyes and lips in an otherwise china-pale face, she has something of Marlene Dietrich’s willowy grace. Therefore, it’s quite a shock to see a photograph of her grinning, hair bedraggled, as she swims through the gloom of an old water tunnel hundreds of feet beneath 135th Street. The photo is one of many intriguing things about Ms. Solis, some of which are documented in her recent book, “New York Underground: The Anatomy of a City” (Routledge, 251 pages, $35), her personal tour of New York’s underground systems, illustrated with photos and drawings.

Ms. Solis is a renegade artist-explorer, an urban adventuress who seeks the soul of the city in its forgotten, derelict places. In other words, she finds strange, old abandoned parts of New York – tunnels, aqueducts, subway stations, theaters, mental hospitals, even sewers – and explores and documents them. They’re usually underground, they’re usually filthy, but they’re often grander than cathedrals and 10 times more mysterious. Is it dangerous? Well, once she ended up in the hospital emergency room after a long slide down a dark masonry tube. But she prefers not to dwell on that. Instead, there’s the thrill of identifying a hidden place through hours of research, or by word of mouth, then going out and finding it.

When she’s lucky, Ms. Solis gets to stage artistic events in interesting, forgotten places and open them up to the public, if only briefly. More often, however, she limns the margins of the law, entering and exploring dusty pockets of abandoned grandeur alone, or with fellow explorers. “I suppose this is what happens to a philosophy major,” she joked, sipping a vodka-tonic in the soft lit glamour of the bar.

Ms. Solis was born in Germany but moved to the Hollywood Hills when she was a girl, then studied philosophy at UCLA before moving to New York 10 years ago. She admits her journalist father took her on explorations of Hamburg’s ruins as a child, “but I’m not sure I can blame my family for this,” she laughed. It’s an obsession, a whole creative lifestyle that feeds an imagination long steeped in gothic fantasy (she read H.P. Lovecraft and E.T.A. Hoffmann avidly in her youth), and provides inspiration for her writing. “I always wanted to be a writer,” Ms. Solis said. “Writing and creating have always been my passion. Jobs and careers were always secondary to me.”

Perhaps the most dramatic of her public events so far was the exhibition held in November 2002 in a half-mile long stretch of abandoned subway tunnel under Atlantic Avenue. The exhibition, which had people forming lines three blocks long, was a collaboration with Bob Diamond, discoverer of the Atlantic Avenue tunnel, who used to run compelling tours of this forgotten part of New York’s dark history of transportation bungles. The November 2002 exhibition showcased the work of six photographers and four sculptors, and included live musical performances, sound installations, and videos, all against the eerie backdrop of a great 19th-century earth-floored cavern in the ground. It was the inaugural event of Ars Subterranea, an exploration group dedicated to showcasing and preserving unusual spaces in New York City, founded by Ms. Solis.

Ms. Solis founded Ars Subterranea in the summer of 2002, to replace a former group called Dark Passage that had staged several events in strange places. “I wanted to go further,” she said. The events of September 11 added an extra challenge to getting access to New York’s secret places, which are often part of or near to crucial infrastructure. Tightened security made it tough. “Obviously, the timing was bad, but I wanted to try to encourage people to explore more,” Ms. Solis said.

She is certainly holding true to her commitment. New York takes on a whole different aspect when you hand your curiosity and imagination over to Ms. Solis. I myself first met her at the base of the 3,600-year-old Cleopatra’s Needle behind the Metropolitan Museum at the commencement of Ars Subterranea’s inaugural treasure hunt last October.

Along with eight others, I spent a glorious autumn afternoon sleuthing around New York, solving “The Riddle of the Buried Stream.” The event showcased Ms. Solis’s imagination at work, aided by her five or six “core group” members, including longtime collaborator Chris Hackett. We’d already been directed to a Web site that explained how Egbert Ludovicus Viele, 19th-century cartographer and sanitation engineer, had made an elaborate map of the underground waterways crisscrossing Manhattan. But, Ms. Solis’s Web site revealed, Viele had deliberately left one of the rivers off the map; a secret river to be kept pure. And now we were supposed to find and chart its course. Our adventure found us accepting clues from mysterious strangers positioned in strategically weird structures in everyday places. (Who normally notices the tall obelisk sculpture at the corner of 33rd Street and Park Avenue?)

For weeks afterward, I found myself not only looking up to scan the oft-ignored canyon walls of Manhattan’s streets, but also sniggering quietly to myself when passing certain sites that had acted as tiny portals to a whole alternative New York.

New York is a city of separate yet concentric universes, and Ms. Solis’s is one of the more rich and bizarre – labyrinthine, half-hidden, hieroglyphic. Much of it is plainly visible, if you know where and how to look: Her favorite place in Manhattan is one of the most accessible – Grand Central Station.

Ms. Solis said her dream is to become curator for at least some of the city’s unusual spaces, such as the High Bridge, built in 1848 to carry water over the Harlem River from the Bronx to Manhattan. Or simply to get a good look at Water Tunnel No. 3, a huge new water conduit being built 800 feet under Manhattan, which is 25 years into its predicted 50-year span of construction. She’s also eager to start her own publishing house. “I’d like to be responsible for every aspect of publishing a book,” she said. “So if anything got screwed up, it would be my fault.”

Ms. Solis is currently taking her first foray into the world of self-publishing, with another book about an abandoned hospital in Jersey City. She has more than 2,000 photographs from eight photographers to edit, and an editorial to write before she publishes the book this summer. Meanwhile, the next Ars Subterranea scavenger hunt will happen in May or June, and there may even be some chances for those who subscribe to Ars Subterranea’s email list to explore urban ruins. Those interested should visit her Web site at www.creativepreservation.org, which includes an exhibition of photographs of abandoned spaces. Maybe it’s time to don a hard hat, and to remember that New York remains the most awe-inspiring, surprising, unpredictable city on earth.