Solis, for example, first began adventuring when she was a child in Hamburg, Germany, but it wasn't until a few years ago that she started feeling compelled to document her explorations. She goes exploring several times a month and throws Dark Passage events -- often based on a historical novel or film -- in which she guides groups into forbidden places. Solis sees herself as "a little bit of an archaeologist, a little bit of a historian." She specializes in the numerous abandoned lunatic asylums and hospitals that dot the Northeast, gothic monstrosities still cluttered with abandoned equipment, letters, furniture and records. "You are finding artifacts from a whole different way of life that you never would normally see," she says. "I find file cabinets full of records and look at documents and try to figure out what went on in a place, reconstruct a story."

The destinations are often doomed buildings on the brink of being demolished; many infiltrators feel driven to make a written or photographic record of historical places that will soon be lost forever. Photographer Shaun O'Boyle, whose Modern Ruins Web site is full of stunningly evocative images of abandoned hospitals, shipyards and factories, believes that "ruined buildings have an interest that goes beyond any interest that building may have had when it was occupied. It has become abandoned space; it no longer functions as it was designed to. The building is no longer sheltering anyone. The slow crumbling and decay make it less and less like architecture and more and more like shapes and forms, masses and planes for their own sake, much like sculpture."

The infiltrators are not preservationists, however; rather, they are observers and chroniclers. Kevin Walsh, a 43-year-old copywriter for Macy's, maintains the Forgotten New York site to document the smaller lost detritus of his lifelong home: lampposts, moldering signs and forgotten alleys that once were thriving roadways. It's a trove of unnoticed ephemera: doorways in subway stations that lead nowhere, weed-encrusted station houses on the abandoned Rockaway rail line, long-forgotten sidewalk art. "I want to get there before the city notices that they are there and gets rid of them," he says. "That's why they're still there, because people don't notice them."

Many infiltrators shy away from press coverage -- such as Ninjalicious, the infamous founder of Infiltration.org and one of the movement's heroes -- for fear of trouble with authorities or encouraging too many newbies. But those like Solis and Deyo want to convey the message that the movement isn't about crazy kids breaking, entering and vandalizing; after all, only a certain kind of person would dare to venture into the blackened basement of an abandoned lunatic asylum and brave the invisible ghosts simply to observe and understand.

Perhaps that person is just a little bit crazy, but the urban explorers have a unique perspective on the places around them. Consider them the secret keepers of the cities, above and below the ground. As Deyo puts it, "People have their own lives to lead and don't feel much of a need to look up at architecture, which is a shame and is part of the reason why we're doing it -- it forces us, if no one else, to view the city as more than just a milieu for the mundane aspects of our lives, a place to work and live. It's also an environment, and like any environment it can be explored."

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