When a power plant's lights go out for good, a mental hospital is shut down, or a factory fails, the doors are locked, the windows are boarded, and time goes to work.

Over decades, rain pits marble floors, rusts metal beams, and peels paint from walls. Vines break through stone and creep toward ceilings.

Such are the decaying innards of thousands of abandoned Industrial Age buildings in the region. But to a 30-year-old maintenance worker who calls himself Goddog, there is beauty in the eeriest interiors, and he isn't easily kept out.

Goddog is an "urban explorer," one of an edgy breed of adventurers who risk injury and arrest to investigate the remains of modern history. In the Philadelphia area, they estimate their core number at 50, with possibly more than twice as many dabblers in a fast-growing pastime.

Whether it's a meatpacking plant in Camden, an electrical station along the Delaware, or a mental hospital with a notorious name like Byberry, chances are somebody has sneaked in, or tried hard.

As Goddog said, "This stuff is never coming back."

Their hobby is illegal, so he and some of the other explorers interviewed for this article would speak only under their exploring tags. Many have been caught by police at least once and charged with criminal trespass, typically punishable by fines of a few hundred dollars.

Seldom do they carry tools to cut fences or jimmy windows; possession can bring a more serious charge of breaking and entering - and jail time. Besides, getting in can be as easy as walking through a sprung door since metal scrappers, squatters, and addicts are often the trailblazers.

Most urban explorers see themselves not as outlaws but as vigilante historians. Every structure is "a story," said Sarah McConnell, 23, of North Philadelphia, a mechanical drafter for an engineering firm. "You want to know more. You want to know why."

Packing cameras and tripods, they gather up documents left in dusty offices, photograph the inner sanctums, and post the pictures on Web sites they've dedicated to the buildings they visit.

"I want to capture history," said Goddog, who lives in Northeast Philadelphia. "I want to go into an old building that was built in 1850, and I see this beautiful doorknob that's all detailed and copper. So I'll shoot it - and the rust and the cobwebs on it. . . . It's like a beautiful piece of history that's been capsulized."

Police and property owners do not share Goddog's zeal.

"When we use the term urban explorers, it's just a fancy way of saying trespassing," said Fred Maher, a spokesman for Exelon, parent company of Peco Energy Co., whose decommissioned power plants are magnets for explorers.

Uninvited visitors will be prosecuted, whether they're stripping copper pipes or shooting pictures, he said. "They're breaking the law by entering private property."

And taking a chance. Even closed plants, Maher said, could have electricity running through them.

But while authorities decry their adventures, urban explorers are collecting fans in unexpected places.

Preservationists have only recently discovered that they have younger and somewhat grungier allies fighting, in their illicit way, to save historic sites.

John Andrew Gallery, executive director of the nonprofit Preservation Alliance for Greater Philadelphia, called explorers "a pretty cool group" after he visited some of their Web sites. There is "fabulous" potential, he said, for collaboration between them and advocacy groups such as the alliance.

"There are a tremendous number of historical buildings in Philadelphia," Gallery said. "We need people out there finding these jewels."

In September, explorers were among the 300 people who met to discuss preserving Pennhurst State School and Hospital in Chester County. Abandoned since 1987, it is at the center of a dispute over its development.

Explorers have offered photos and copies of documents they found there. They've also kept project leaders aware of dumping, thefts, and vandalism on the property.

"There have been cases where the photographs the explorers took are the only thing we have left," said Nathaniel Guest, a third-year Temple University law student helping lead the fight to preserve century-old Pennhurst.

"Some of the documents may not have been saved had someone not said, 'These are important. I'm going to take them.' "

Seeking adventure

For the purists, urban exploring is a discipline, if an X-treme variety.

They check newspapers to find buildings that companies are vacating. Google Earth lets them examine structures before they go in.

It's not kids' play - though that's how it started for most of them. They took up the hobby as teenagers, looking for a dead-of-the-night thrill.

Dennis Coleman, now a 20-year-old pizza cook in Haverford, discovered it in 2006 when some friends decided they'd rather poke around an abandoned mental institution on the Main Line than hang out another minute at the Wawa. He headed home exhausted after walking the grounds for seven hours, using a lighter when their flashlight faded.

Built in 1964, Haverford State Hospital lacked the Gothic grandeur of classic asylums, but Coleman was fascinated by the horrific conditions that led to its closing in 1998. Books could not satisfy his curiosity.

Coleman met another suburban teen, Drew Scavello, in the morgue there.

Scavello was grieving the death of his father when he stumbled on Haverford. It was a quiet spot to think, where deer roamed among open buildings.

"I could go up here and be at peace," said the 19-year-old from Devon.

The property was posted with "No trespassing" signs, but so many people blew past them that Haverford Township police had to patrol daily, making numerous arrests, Sgt. Shant Bedrossian said.

"What they were doing on the property, who knows?" he said. Some were stealing copper, but "most were just walking around looking, maybe drinking or getting high."

In 2007, the township began tearing down the buildings to make way for housing and a recreation center.

"Watching it being demolished slowly, it really hurt," said Scavello.

Coleman set up a MySpace page dedicated to Haverford State Hospital - where tributes could be posted, as for a friend who died - but Scavello went a step further, getting "HSH" tattooed in black on his right arm. He also decided to study filmmaking at Delaware County Community College in order to capture overlooked history, such as the suffering of Haverford's patients.

"A lot of explorers, they get really attached to their first big place," Coleman said. "And they always, most of them, have to experience . . . losing that place."

He and Scavello have since expanded their exploring universe. But don't ask for their haunts.

Divulging new "locations" would jeopardize "the quality of a building, in regards to graffiti and other vandalism," Coleman said. It would also "draw unwanted attention from law enforcement and building owners."

And alert the amateurs.

Explorers who can't find their own places are derided as "re-explorers." Those who show up only at the most obvious spots are "tourists."

"The whole point is finding stuff yourself," said Coleman. "It's exploring, not taking someone else's tour."

The trailblazing Goddog

Coleman and Scavello talk about Goddog with utter reverence.

"Pretty much any abandoned building that you see driving around, he's been in it," Coleman said. "Or he's looked very closely at it."

"He's, like, the legend in Philly," said Scavello.

Online, where explorers often connect before meeting, Goddog exudes cool confidence, a love of architecture, and an irreverent sense of humor. In pictures, he's standing amid steam valves and pipes, smoking in a room where the walls are crumbling around him, or peering from a hospital rooftop, his face lit in red against the night skyline.

While many explorers travel far to find old industrial hulks - Detroit being a favorite destination - Goddog doesn't stray. Philly is his city.

Its eyesores are his playground, and the playground is crowded. A 2001 survey estimated as many as 25,000 vacant structures in the city, said Frances Burns, commissioner of the Department of Licenses and Inspections.

In person, Goddog is self-deprecating and nervous, at least around reporters. He jokes that he's too fat to enjoy running from the cops anymore, and says his desire to know every nook in an abandoned building and photograph it is fueled by obsessive-compulsive disorder.

His baby was Philadelphia State Hospital - the infamous Byberry, in the Northeast - and the exploring community came to know him through his extensive Web site dedicated to it.

Before the buildings were bulldozed in 2006, Goddog broke open a cornerstone and found a copper box full of documents and pictures, which he posted. When he came across the names of former employees, he called them to ask what they remembered of the place.

"People around me were like, 'Are you nuts? Do you really care that much?' " he said. "Yeah, I really do."

Near the end, Byberry had worn out its popularity, luring dozens of explorers - mostly dilettantes - at a time. While purists are loath to muck up historic buildings, apart from discreetly signing a wall, the crowds and Byberry's imminent disappearance led to what explorers called anarchy. Layers of spray paint coated the walls; some fires were lit, several explorers said.

Byberry remains Goddog's first love, immortalized on his left arm with a tattoo of his favorite building and the words "R.I.P. Byberry."

Such sentimental affairs with derelict buildings may swathe the hobby in romance. But explorers often run into trouble. Coleman and Scavello once spooked a scrapper who pointed a gun at them, but backed down. Goddog rattled some men in a vacant North Philadelphia school who were stealing metal and took him for a cop.

In the same school, he spotted someone lying motionless in a hall. "I thought he was asleep. I walked a little closer and saw his eyes were wide open," recalled Goddog, who didn't stick around long enough to check for vitals.

The buildings themselves pose significant hazards, such as the asbestos at Byberry and Haverford.

There also are dicey questions of property-owner liability if someone is injured even while trespassing, said John Culhane, a professor at the Widener University School of Law in Wilmington.

As the owner, "if you know the third floor is rotted out and collapsing, you could be liable," Culhane said. "These places are pretty unstable."

A 26-year-old Montgomery County software developer with the exploring moniker Chip R. Jones recalled almost falling through a rotted staircase at Byberry in late 2004.

In February 2005, James Lowe, a contractor inspecting the building for demolition, plummeted through the same gap and was killed.

Officially neglected

Preservationists themselves admit it: The region's industrial heritage has gotten short shrift from them.

It has been "neglected," said Gallery, of the Preservation Alliance. "All of these huge industrial companies used to have buildings here, and few remain."

A decommissioned electric plant along the Delaware River in Kensington is one structure both explorers and preservationists hope to save from demolition.

Designed in 1917 by John T. Windrim, architect of the Franklin Institute, the Peco Delaware Power Station reportedly once burned more than 300 tons of coal an hour to light up most of the Northeast.

Thomas J. Sugrue, a professor of history and sociology at the University of Pennsylvania, described it as "a Greek temple to electricity."

But since the mid-1980s, the plant, whose creators put so much thought into architectural flourishes to mesh utility with beauty, has stood mostly dormant.

"This monumental castle . . . was never supposed to go anywhere, and now it's obsolete," said Goddog. "This big monster - it's one of our favorite places."

Explorers aren't the only ones who've descended on the plant. Thieves have yanked out pipes and fittings, Exelon's Maher said.

Last summer, the company tore down a 1954 addition partly because of liability worries, Maher said, adding that there are no immediate plans for more demolition.

But Goddog and his compatriots expect that day to come, especially as the city focuses on development of the waterfront. When it does, they will mourn and move on.

In recent years, Goddog has watched many of the best venues disappear. As stocks soared and development boomed, one fabulously rotting structure after another was lost, converted to condos or knocked down for open space.

Now the nation's financial meltdown is putting the brakes on that, and Goddog sees the silver lining for explorers. "Maybe something good will come out of this troubled economy."

Find links to Goddog's photo galleries and Byberry site; explorers' sites featuring Pennhurst and Asbury Park, N.J.; and the e-zine Infiltration at http://go.philly.com/urbanexplorerEndText

Contact staff writer Joelle Farrell at 610-627-0352 or jfarrell@phillynews.com.