NE Sunday evening in February, Benjamin Deyo and three companions gathered to admire a side of Grand Central Terminal that most people never see.

Disguised in the commuter's uniform of crisp suit and buffed shoes, they entered an out-of-service platform on the lower level, walked to the end, hopped down to the tracks and climbed up into a hallway lined with pipes. For the next 100 yards these members of a four-year-old New York group called Jinx held their ties to their chests as they ducked under scalding blasts of steam from broken pipes marked "Warning: Asbestos."

Inside a dark chamber perhaps 80 feet below street level, as trains clunked and whirred overhead, Mr. Deyo, 30, skirted a pool of water, climbed a slope of Manhattan schist and took in the view. "This is a purely utilitarian landscape, and the people who built it never meant for us to see these structures," he said from his perch. "Yet it is so beautiful here. Look at all this steel, the tracks, all the work that went into this."

The group's feats are as illicit as they are daring: its members are courting injury and risking fines for criminal trespass.

Jinx is one New York version of a large and loosely organized global network of urban explorers — also called infiltrators — who penetrate tunnels, abandoned hospitals, missile silos and other forbidden places. The fact that these places defy traditional aesthetics only makes them more attractive to those who enjoy architecture's underbelly as much as flauting authority. (Building officials were shocked to learn of these activities.)

The more off-limits a setting, it seems, the better. One group in Minneapolis kayaks in storm drains. A man in Dover, England, goes into the area's World War II batteries and barracks. In Berlin, some have entered a bunker under the Alexanderplatz. A group of Paris women, the K-ta Nanas, sneaks among the catacombs beneath the city, sometimes navigating the wet passages in bikinis.

Abandoned architecture, from houses in Pompeii to the Mayan ball courts at Tikal, has always attracted the curious. But interest in illicit visits has exploded since the early 1990's, nourished by the Internet.

Some who visit these sites are content to be armchair explorers. Others like living out their Walter Mitty fantasies but say they carefully weigh the dangers. Nevertheless, it's easy for an outsider to think these infiltrators have abandoned common sense.

"What we are doing is inherently dangerous," Mr. Deyo warned. "Remember the laws of gravity. Listen for trains. And don't urinate on the third rail — the electricity will travel upstream and into your body."

Once inside a narrow Metro North tunnel, with a headlight bearing down on them, they scanned nervously for an exit, crossing the high- voltage third rail and ducking into an archway cut into a filthy concrete wall. Just feet away, oblivious Harlem Line passengers rolled on by.

A set of stairs led up to a door marked North Emergency Exit 13. Suddenly they were standing on 56th Street and Park Avenue, about 14 blocks from their starting point. Back they went for a different perspective. Mr. Deyo led his crew into the terminal through an unmarked door on Vanderbilt Avenue that leads to a bar. "Are you with the party, sir?" the coat check attendant asked. "Yes," Mr. Deyo declared as he continued up a marble staircase to an out-of-the-way elevator that took the group several stories higher.

Soon the four men were at the peak of the station's sloped copper roof, this time nearly 80 feet above the street, with a stunning view of the Chrysler Building and the modernist towers on Park Avenue.

Mr. Deyo, a co-owner of a multimedia design company, is famous among fellow infiltrators for having crossed this slippery roof to scale a statue of Mercury, which rises another three stories. Nearly two years ago he hung the Jinx flag, an exclamation point set inside a yellow triangle (the international icon for "Danger") on a black field, on Mercury's crown; a remnant still flapped in the wind.

"We could explore our whole lives and never make the history books, but at least we could say that we found this," Mr. Deyo said, spreading his arms exuberantly.

He has also scaled the Manhattan and Williamsburg Bridges and considers himself a specialist in heights. When it comes to exploring the New York underground, he bows to another infiltrator, Julia Solis, a writer and translator in her 30's, who is known for leading elaborate incursions, like the formal candlelight dinner party for 40 she held in an active subway tunnel in 1999.

Among those who have joined Ms. Solis is Suzanne Martin, 33, of Coopersburg, Pa., who came to New York last month to trek down an abandoned 19th-century water tunnel in the Bronx populated by hibernating bats. Ms. Martin said she had previously explored abandoned farmhouses near her home in Bucks County. "I'd poke around and kind of soak in that feeling one gets from long-empty places," Ms. Martin said. "It really hadn't occurred to me that others might share my interest or brand of aesthetics."

Lowell Boileau, 55, who began infiltrating abandoned buildings in Detroit in the mid-1970's, said he experiences the same wonderment standing inside the city's abandoned Michigan Central train station that he first felt at the ruins of Ephesus, in Turkey, in 1971. His Web site, www.detroityes.com/index.html, shows dozens of abandoned Detroit landmarks and attracts about 1,000 visitors a day.

Mr. Boileau warned that it may be best to enjoy the sites from a computer screen; he recalled one man who jumped down onto a roof covered with debris and crashed to his death through a skylight. "Anyone going into buildings should beware of structural weaknesses from scavengers and deterioration," he said. "I don't advise it."

For some infiltrators, of course, danger is appealing. But most say that they are more attracted by the haunting and beautiful environments. Adding to their allure are the evocative artifacts they discover, like coal bins, elaborate murals and decorative tiles. As much as they admire these features, they are careful to say they rarely, if ever, take them. Their unofficial code is look but don't touch. Ms. Solis's Web site advocates "a safe appreciation of ruinous landscapes that is respectful of other people and environments."

"Please do not tag, steal or vandalize," it says.

Aside from the infiltrators' own honor system, abandoned structures are generally not protected from vandalism by preservationists. For example, the New York City Landmarks Preservation Commission tends not to designate abandoned buildings because they cannot be adequately supervised.

Many infiltrators have been drawn to these places since childhood. Ms. Solis remembers growing up in Hamburg, Germany, and leading neighborhood children to see imaginary corpses in storm drains. "No one would go into the drains but me," she said. "I felt so at ease there."

Like most infiltrators, Ms. Solis routinely trespasses, risking trespassing fines that vary from state to state. She has had brushes with security guards but has never been caught. The maximum jail sentence in New York would be 15 days on trespassing charges, said Vincent E. Doyle III, chairman of the criminal justice section of the New York State Bar Association. And unless a property is clearly marked off-limits, he added, "there might not be any violation at all." In Minnesota, on the other hand, trespassers can get as much as 90 days.

Marjorie Anders, a spokeswoman for Metro North, was dumbfounded to hear about those who see the station as a 47-acre infiltration challenge. "That activity would be unwise and unsafe and unappreciated," she said. "There are live third rails and trains moving around and we don't want any fatalities."

In February, during her first exploration of an abandoned 876-acre mental hospital complex on the North Shore of Long Island, Ms. Solis was careful not to be seen, and seemed unperturbed by risks. She and her partner for the day, Chris Beauchamp, 24, searched for an entry point.

"There's a hole, and there's no cover on it," she called. He dropped down a ladder and lighted an old- fashioned miner's lamp attached to his hard hat. The three-inch open flame illuminated Ms. Solis as she followed gracefully in black boots, a Prada skirt and a short shearling coat. ("I always dress well, to get more respect if I run into guards or the police," she said.)

The miner's lamp left carbon marks on the seven-foot-high concrete ceiling. Asbestos insulation crumbled in the moist, still air. Raccoon prints the size of human hands marked the steam pipes.

Judging from an aerial photograph of the hospital campus, which she found on the Internet, Ms. Solis said she believed they were headed toward an old boarded-up Georgian- style building she had seen near the tunnel entrance. (An agreement not to disclose the location was a condition of joining her.) Ms. Solis is drawn to the mix of the grand and the utilitarian found in hospitals, and she often spends entire nights going from room to room by flashlight, occasionally terrorized by her own imagination.

After an hour in the dark, she led the way through a narrow brick passageway supported by steel beams dripping rusty stalactites. Several hundred yards later they climbed through a manhole into a 1960's medical building. Layers of colored paint flaked from the walls like confetti. A torn portrait of Lincoln looked up from the floor. The windows were small and escape-proof. In 19th- and early-20th-century hospitals she has come across elaborate bowling alleys, theaters and marble lobbies.

But even this plain, modern building had nice surprises. She climbed out a window onto the roof and tried to imagine how a mental patient would react to the wide blue view of Long Island Sound.

A quarter mile away she could see the tall, century-old building she had originally headed for. She knew there had to be a way in, and she was determined someday to find it.