June 14, 2013 -- Most of us don't descend into subways unless it's to catch a train, and very few people view skyscrapers as mountains to be conquered and climbed. But Moses Gates is not like "most of us."
Gates, a 37-year-old urban planner based in New York City, is part of a group of people dubbed "urban explorers" -- a kind of Indiana Jones of concrete jungles.
Urban explorers dive in the bowels and scour the heights of cities across the world in search of a new way to see what there is to see. For them, every skyscraper, sewer and scaffold is a space begging to be scaled.
"People are curious about their environments and people like to explore their environments, and it's a very natural thing. And it's natural if you're in nature and it's also natural if you're in cities," Gates said. "Half the world's population now lives in cities now. Cities are increasingly our environment."
The urban explorer "scene" is not an organized group, according to Gates, but technically, they are professional trespassers. The payoff of discovering a new vantage point, Gates said, outweighs the hazards of illegality.
"For every one neat thing you find walking around town there's twenty times you walk around town and find nothing that neat." he said.
Even a place like the catacombs of Paris, a 200-mile maze that holds the remains of six million people, making it one of the world's biggest mass tombs, isn't too scared for urban explorers.
"In Paris, going underground especially to the catacombs is just kind of thought of something that maybe not everybody does but people do and it's not a huge deal," Gates joked, adding that the catacombs will "always hold a special place for him" because it was the first place he went underground, just to look around.
"I've been seen coming out of a hole in the ground in Paris at 2 in afternoon on a busy street and I'll get a few funny looks," he said.
A few weeks ago, Gates visited another favorite spot of his, high up on a New York City skyscraper. But it wasn't Top of the Rock at Rockefeller Center or the Empire State building. It was an empty office floor on the 61st floor of the city's famed Chrysler building.
"It's such an iconic place," he said. "Everybody knows the [steel] eagles of the Chrysler building...it's an amazing experience and one that everybody should be able to have."
That "experience" of seeing a new urban point of view is well-known to Gates. He and his fellow urban explorers are not afraid to show-off their adventures. They post videos on Vimeo and upload photos to Facebook.
"I cannot, with any eloquence, describe why going into a whole in the ground and seeing what's there is cool," Gates said. "People never ask what possesses you to climb that mountain."
But as more and more people come to cities, regulations about where you can and cannot go is increasing, to the dismay of people like Gates.
Gates wrote a book about his urban adventures called "Hidden Cities." He invited "Nightline" to join him on an urban exploration around several "hidden" New York spots in the Bronx -- this time, without breaking any laws.
The High Bridge was the first stop.
"It's actually one of my favorite places," said Gates, walking up to the 114-foot bridge, the oldest remaining one that connects two boroughs in New York City.
The bridge is under construction and is set to reopen to the public in the next few years.
When asked if the posted "Danger" sign ever deterred him, or made him more determined to go beyond them, Gates likened it to asking a naturalist if woods are more enticing because of a Keep Out sign.
" The fact that there is a keep out sign there, is that necessary?" he asked. "I'm an city planner, and cities are my profession, they are my professional passion and I started [my exploring] just by walking around and walking the streets and seeing what I could see."