A wooden board studded with rusty nails covered a shattered window, but a crowbar made it just an inconvenience. One by one, James Tantalo and four friends gingerly slid in.

On an overcast Sunday afternoon, the five adventurers switched on Maglites and started walking. Kate Gumbis, 20, of Wyandotte, Mich., offered dust masks to everyone. They declined, but she put one on.

It was pitch black inside the abandoned Fort Shelby Hotel.

Explorer Alan Pastor, 20, of Lincoln Park, Mich., led the group beneath a fallen ceiling, past empty soda cans and into the spacious lobby at the mouth of a staircase. Their flashlights showed only glimpses of the faded grandeur of the hotel, which opened in Detroit in 1918 and closed in the late 1970s.

Suddenly, someone's camera flashed and -- for only a second -- illuminated high ceilings, a reception desk and elevators.

"Cool," they gushed.

Tantalo, 23, of Plymouth, Mich., and his friends are urban spelunkers, a loose network, connected mainly through the Internet, of historians, photographers and amateur anthropologists who have found a playground among the world's abandoned buildings.

Tantalo has the luxury of Detroit's vast collection of empty pre-Depression buildings and factories. Counterparts in other cities make do with sewer tunnels, underground subway systems, forgotten train stations and empty mental hospitals.

"Detroit is a special place," said Tantalo, a Ford Motor Co. computer support technician. "There's always something to discover. It's like an archeological dig."

The city is considered the top destination for building spelunking, according to explorers.

"For fans of abandoned building exploration, Detroit is the No. 1 city in the U.S., hands down," said the Webmaster for the popular Toronto-based urban exploration Web site, infiltration.org, who would only identify himself as Ninjalicious via e-mail.

The fascination with Detroit's buildings has turned into an illegal and dangerous hobby. Urban spelunking is trespassing. And anyone could be lurking in abandoned buildings. The building's infrastructure could wither without warning, and there is asbestos everywhere. And for owners of Detroit's empty buildings, explorers are a nuisance and a liability.

"Yes, trespassing is a problem," said Howard Hughey, a spokesman for Detroit Mayor Kwame Kilpatrick. "People can get hurt. There's definitely something wrong with trespassing, even if it's for an appreciation for what's there."

Dan Stamper, president of the Detroit International Bridge Co., which owns the Michigan Central Depot, said the company doesn't track trespassing arrests, but added that the number of visitors has declined since it hired a security company after the Sept. 11, 2001, terrorist attacks.

"The building is not open for people to walk through," Stamper said. The 91-year-old train station has been one of Detroit's most popular spelunking spots for years despite the barbed wire and no-trespassing signs.

"Just because it's a building that's fascinating, doesn't mean people can trespass in it," Stamper said.

But serious explorers are adamant about being distinguished from vandals, looters and graffiti artists. And they don't want to be mainstream. Urban spelunking Web sites have been abuzz about an upcoming Discovery Channel reality series about exploration. Parts of the pilot, "Forbidden Zone," were shot in Buffalo last summer.

Spelunkers add that the popularization of the hobby ruins the subversive nature of building exploration, said Paul Mush, a 20-year-old college student from Westland, Mich.

"We do it so other people don't have to," he said, smiling.

Joe Van Esley, whose Plymouth-based real estate company has been trying to sell the Fort Shelby Hotel for years, expressed frustration over repeated efforts by explorers to break into the building. The building owners have been scrutinized in the past by city building inspectors for not properly boarding up the building.

"We've boarded up the building a couple times," he said. "We try our best. We're the victims here."

As for safety, urban explorers say that's a priority -- just like in any other extreme activity such as rock climbing or whitewater rafting. Be prepared, do the research and don't be foolish, they say.

Each explorer has his or her own injuries, broken bones and cuts that require tetanus shots. Explorers said they almost never come across muggers, drug dealers or homeless people in downtown Detroit buildings. But often there is evidence of someone living there, such as mattresses, clothes and radios. In one building, someone is keeping a small and tidy book collection. Explorers said they are more likely to cross paths with other explorers.

Take only pictures, leave only footprints: That's the urban explorer motto.

The exceptions are small artifacts, such as the Fort Shelby's reservation cards or pieces of wallpaper. They would be committed to ruin otherwise.

"By taking these artifacts out and taking photographs, that ensures that it'll be around as long as we're around," Tantalo said.

Inside the Fort Shelby, the group found notes with phone messages and hotel keys still in cubbyholes behind the front desk. They ventured into the handful of crumbling and empty ballrooms on the first floor, then headed upstairs to the more than 20 floors of hotel rooms. Climbing a narrow stairwell near the fourth floor, they noticed that entire walls were missing, removed by looters seeking bathroom fixtures and piping.

Phone books left behind date to the 1970s, and Gumbis looked to see whether her grandmother was listed.

A rusted 17-gallon container labeled "Survival Supplies, Furnished by Office of Civil Defense, Department of Defense Drinking Water" sat near a skeleton of a wall. Broken glass was everywhere.

On some floors, light shined through the broken windows into the rooms and hallway. The group separated, but stayed within yelling distance. Near the 16th floor, they opted to head straight for the roof, skipping exploration of the remaining floors. Then Gumbis demanded that they stop -- the dust was affecting her breathing. The explorers paused, then forged ahead, upward.

Eventually, sunlight greeted them at the doorway. They had reached the top, where seven or eight small trees and some grass grow -- part of Detroit's stubborn ecology.

From the roof, they saw the Detroit River and several of the city's landmark buildings. They enjoyed the view for a half hour, snapped photos and discussed dinner. Then they headed back down.

Because of the secrecy of urban exploration, it's almost impossible to know how many people spelunk or their demographics.

Ninjalicious said he's had a gradual increase in interest since he started his urban exploring Web site in 1996 and magazine in 1997. He said his Web site gets about 2,000 hits a day.

"The growing interest in urban exploration, which is largely a pursuit of beauty and authenticity, is at least partly a reaction to the increasing ugliness and phoniness of cities and suburbs," Ninjalicious said via e-mail. "Some people think of urban exploration as an extreme sport pursued by macho adrenaline junkies, but it's actually more popular among introverted types."

Tantalo, who makes monthly trips into buildings, agreed. His group often includes Pastor, who works as a restaurant dishwasher, and Tantalo's girlfriend, Gumbis, who works at a hospital. They were introduced to spelunking by a group in the Toronto area.

Some groups have taken exploring beyond Detroit to an old paper mill in Monroe and a former mental hospital in Traverse City, Mich.

One site, the Northville Tunnels near Sheldon, Mich., drew explorers for decades until it was torn up in 1999 to build a golf course and houses. John Wagner, 35, an electrician from Dearborn, Mich., started going to the tunnels almost 20 years ago and now runs the Web site www.northville-tunnels.com.

Wagner and his colleagues said there are few Detroit spots they haven't ventured into, and he said they have pushed the limits of the hobby to include vacant portions of buildings still in use.

They mean no harm, he said.

"If these building owners cared, they would do something with their buildings," said Wagner, a married father of three. "For us, there's nothing like sitting on the roof and looking out onto the city."

From left, Tim Tantalo, Kate Gumbis and Alan Pastor pose outside the vacant Fort Shelby Hotel in Detroit, a city considered by some explorers to be a top destination for building spelunking.Tim Tantalo, left, Kate Gumbis and Alan Pastor treat vacant buildings as mountains to be explored.