Hong Kong (CNN) — "One minute you're in the heart of the city, with a million people," says Ghost. "Five minutes later you're in an old deserted site."
Ghost, not his real name, is one of a gang of so-called urban explorers whose sometimes perilous excursions regularly uncover corners of Hong Kong rarely seen by the vast numbers of of people who live here.
"The contrast is surreal," he adds, describing an excursion into an abandoned private hospital in Central, the teeming financial district of Hong Kong.
For the group -- known as HK Urbex -- it's these hidden gems that make urban exploration in a cramped and crowded city unique.
They never know when they'll run into a site lost in time in a jungle of modern and shiny skyscrapers.
HK Urbex was founded in late 2013 by Ghost and a fellow explorer, codename Echo Delta, after a visit to an abandoned TV studio got them hooked.
The members of the secretive group HK Urbex, who venture into abandoned buildings in Hong Kong.
Since then they've traversed Hong Kong's crumbling edifices and documented them with high-quality first-person-shooter style videos and eerie photographs.
Urban exploration in Hong Kong involves the same risks as it does in any other place.
The first step usually involves trespassing.
"Stationary guards are easier to skirt," says Echo Delta. "As for patrolling guards, we need to play hide-and-seek."
Once they get inside, they walk around -- scaling from top to bottom.
They insist they observe, document and leave without altering anything.
"Visiting the abandoned sites always evokes a lot of emotions and feelings," says Echo Delta. "It's like a child opening up a wrapped present, always curious what is inside the box."
Ghost adds: "I like the quiet, spiritual feeling of a deserted building.
"Sometimes I even do urban exploration alone. It's a one-on-one with the building, a very serene moment."
Nam Koo Terrace was reportedly a brothel for the Japanese army during the occupation of Hong Kong. It's said to be haunted by the spirits of the "comfort women" who were confined here.
Among their most memorable experiences was a late night visit to a notorious "haunted" house in Hong Kong's harborfront Wan Chai district.
"The building was totally empty -- there wasn't even a chair left behind," says Ghost. "The lights were on, and the interior was completely painted in red.
"It was like walking into a horror movie."
There have been other memorable experiences.
On one excursion, they clambered onto a capsized boat only to have it sink beneath them.
Another visit to a Hong Kong MTR metro railway construction site resulted in threats of legal action.
"But the adventure to a deserted iron ore mine was the most dangerous we've ever done," says Echo Delta.
"It hasn't been in use for a long time and was sealed off. If there were any accidents, no one would have known we were inside."
"We found a mysterious water pool at the lower level of the tunnel," adds Ghost. "The water is crystal clear, probably the clearest I've ever seen."
British military base and political prison
HK Urbex's members insist they're not just in it for the adrenaline rush.
They say they hope their documentation will help bolster an appreciation of Hong Kong's heritage.
Many of Hong Kong's most beautiful old buildings, remnants of British colonial times, were demolished in the 1980s.
Ongoing urban renewal programs continue to claim others.
One surviving old structure is Hong Kong island's abandoned Victoria Road Detention Centre -- often called "The White House."
Built during the colonial era, the long-forgotten and somewhat mysterious site has been in the spotlight because of plans to redevelop it as the Hong Kong campus of the Chicago Booth business school.
The historic complex of low rise buildings was reportedly once a recreational club for the British army. Then it became a detention center for political prisoners.
Subsequently it was a shelter for Chinese dissidents who fled to Hong Kong after the massacre in Beijing's Tiananmen Square in 1989.
It was also the filming location for Wong Kar-wai's "2046" and Ang Lee's "Lust, Caution."
"It is the most memorable exploration for me because of its history," says Echo Delta. "I wonder who had been held there."
"That is why we're doing urban exploration -- to immortalize the old buildings before they disappear."
The group has shared a few of their rare glimpses into the buildings on its website.
'Spirit of rebellion'
"As we go in, we are sort of turning the pages of this book, unraveling the stories behind," says Echo Delta.
"We're like ethnographers," says Ghost. "We study the environment, search for remnants, and try to tell the back stories."
Not everyone agrees, especially when it comes to trespassing.
"Lots of Hong Kong people think of us as troublemakers," says Echo Delta. "Young people are more appreciative, I guess this has to do with the spirit of being rebellious."
Both of HK Urbex's founders say their activities have changed the way they view the city around them.
"Everyone has their own mind map of a city," says Ghost. "I now look at the city in terms of the abandoned buildings. That is what feels authentic to me."
"Now when I go on a trip, I look for abandoned buildings," said Echo Delta. "When I see one, I kind of get giddy and want to go in."