The midnight streetlights seep into the high windows just above stacks of Styrofoam sheets. Rippled floor boards cave in to the underground, and stairs lead down to a door graffitied with Sex 101. Rows of lockers ripped from the walls lie in the corridor and an unrolled paper canvas shows a witch's figure in the light of a full moon.

This Knoxville, Tenn. school building rots with mold and is littered with glass, and each footstep necessitates the kind of caution shared by the thousands of explorers across the globe who venture into the urban, abandoned or blockaded spaces around them.   

Maria Stehle, an assistant professor in modern foreign languages and literature at the University of Tennessee, classifies urban exploration as "a very deliberate" but "aimless" wandering through urban environments.

She says it raises questions about race, gender and class. Why are there boarded up windows and dilapidated homes one street down from a thriving middle class neighborhood? Who used to live here and why did they leave?

Stehle looks for answers to the questions inspired by her observation. She says there are stories in all urban space that motivate thinking and change.

"Unless we want to be a complete pessimist, we would have to say thoughts are change," she says.

I don't understand when people don't want to do [urban exploring]. Maria Stehle, assistant professor, modern foreign languages & literature

Stehle, a German native, has done the majority of her research in Berlin, a city she and a group of UT students visit every other year for her "History of Berlin" class.

She says urban exploration is conceptually more prevalent in Europe, especially in France, where artistic theories of urban space crystallized from the 19^th century French poet and art critic Charles Baudelaire. He coined the term "flâneur" following the 1848 French Revolution to mean "a botanist of the sidewalk" and "a person who walks the city in order to experience it."

Stehle finds it easy to talk about the academic implications of studying a space and its inhabitants (or lack thereof), but hesitates when asked about the intrigue of urban exploration.

"I don't understand when people don't want to do it," she finally says. "I'm hesitant crossing fences that I'm not supposed to cross. But I am tempted sometimes. And they make me think about why there's a fence there and why I can't go into certain places."

Anders Westergren, a Swedish 24-year-old business owner and photographer, readily crosses fences. He says urban exploration is a good way to become interested in the history of a society.

He created the "Urban Exploration" group on Facebook in April 2011 that now has over 1,600 "likes." It contains photo albums of mine villages, ammunition storage facilities and power plants. The photos feature abandoned sites sprinkled around Sweden, many littered with warning signs threatening punishment for entering. These signs are not so much the concern as they are the intrigue, the rush and inspiration to mentally and physically cross visible and invisible barriers.  

"When it's that kind of sign you know it's probably something interesting." Westergren says.

His interest in urban exploration started as a child during family car trips through the mountains of northern Sweden, when Westergren wondered about the doors in the mountainsides and what was inside them. Now a history buff, he says Sweden is an interesting place to explore because of its rich mining and military history and the prevalence of abandoned hospitals used to treat tuberculosis patients during the 20^th century.

Even though it's rediscovery, it still feels like you're finding something new. -Josh Hackett, senior in English

Westergren's explorations began about eight years ago, when he and his friends snowmobiled through the woods about sixty kilometers from his small town of Piteå, Sweden to a copper mine used in 1936.

"That was pretty awesome," he recalls. "We were going into the mine, into the darkness, and you could hear the water dripping from the ceiling."

But Westergren says it's rare to discover a new place from nowhere and he usually explores areas he's heard stories or read history about.

Josh Hackett, a senior in English at the University of Tennessee, says this "rumor" factor draws him closer to urban exploration, a hobby of his since high school.

"You have to find out for yourself," he says. "There aren't really very many landmarks that people haven't discovered...[But] even though it's rediscovery, it still feels like you're finding something new." 

Although his curiosity has put him within gunshot's range, he has never been arrested or seriously injured.

"We went to this huge abandoned furniture factory and there were homeless people in there. They thought we were gonna steal their drugs, and this crazy homeless lady said she would shoot us up with her needles. We bolted after that."

Hackett defines urban exploration as "not breaking and entering, but finding beauty in an empty space." He finds this beauty in faded wallpaper, a structure's architecture, or the foreign nostalgia of forgotten toys or film. In the Knoxville school building, he climbs on dusty, wooden shelves and pulls down books and torn magazines, admiring their vintage contents.  

On the chalkboard inside one of the rooms, Hackett shines his flashlight on large cursive letters that read "Richard [Johnson?] . . . My Homeroom." The alumnus signed his name on September 23, 2011 -- 35 years after he walked out the open front doors now bolted shut. Outside the room and down the hallway, one plant, green and growing, pokes out of the rubble, trying to reclaim what it once was.