Urban exploration - venturing into the hidden pockets of metropolitan landscapes - provides a unique window into a city's past.

On any given night, New York photographer and historian Steve Duncan can be found trudging through slimy rat-infested sewers, traipsing through dark graffiti-garnished subway tunnels or climbing up massive suspension bridges. Various forces drive urban explorers to venture into the unseen pockets of our metropolitan landscapes; Duncan is propelled by the urge to understand how cities develop and evolve over time.

“The thrill of urban exploration is the thrill of opening up a time capsule,” he described. “You can actually touch history.”

Four years ago during a trip to Italy, Duncan found himself tunnelling into Rome’s sewer system. He and his companion stumbled upon a few old entrances closed off by bricks. One entrance, though, remained open. They made their way inside, discovering a 2,000-year-old side drain that the local archaeologists were not even aware of. It was a small piece of the Cloaca Maxima, Ancient Rome’s sewage system – one of the first drainage structures ever engineered.  

It was one of Duncan’s most exciting moments as an explorer. “It is richly rewarding when you go off the beaten track and find something that exists within the contemporary city but vastly predates it.”

Of course not all his expeditions go that way. There have been plenty of times when he has lost his way, or failed to find what he was looking for, or just given up because the trek was too long.

There have also been times when things have gone terribly wrong. When wading through a storm drain near Jamaica Bay in New York, Duncan and a friend came very close to drowning. Unfamiliar with the infrastructure, they had not realized that when the high tide came in, no airspace would be left in the drains. “We were fleeing this flood and fighting our way through water that was rapidly rising,” he recounted. “We got to a manhole, but it wouldn’t open. My friend insisted that we keep going and try one more [manhole]. The second one was also unopenable. By this time, the water was already chest-high.” The third manhole did open up – right into the middle of a street. A minivan came toward them, but slowed to swerve around them.

“We didn’t know at all what the dangers were, so we were incredibly dumb,” he confessed.

Duncan has also been arrested multiple times, including once on top of a cathedral in Paris and once on top of a cathedral in New York. The latter incident, at the Cathedral of St John the Divine, triggered a response from an emergency swat team and more than 100 police officers. It turned out his camera and tripod had been mistaken for sniper weaponry. (The former arrest happened at Notre-Dame.)

So even a veteran like Duncan is quick to point out the risks of urban exploration. Sewers can contain dangerous gases, subway systems can have unpredictable train schedules, abandoned buildings can be crumbling or have asbestos, and usually it is illegal to access the sites.

Still, urban exploration remains a popular pastime, as it can be done in any manmade environment anywhere in the world. The activity’s one unwritten rule is: “Take only pictures, leave only footprints.” Urban explorers wish to preserve the hidden and abandoned parts of society because these places serve as unique windows into the past.

Like the New York underground, the Cloaca Maxima and Notre-Dame’s rooftop, there are fascinating destinations all over the world that beckon urban explorers and can transport them back in time – by a couple of years, a couple of hundred years or much longer. Of the sites below, only the Paris and Odessa Catacombs have small stretches that are legal to visit.

Tennessee State Prison, Nashville, United States
Nashville’s Tennessee State Penitentiary was open in its current location from 1898 to 1992. Although the prison was built with 800 single-occupancy cells, it received 1,403 inmates on its first day. Overcrowding and under-financing may have contributed to the prison’s eventual closing. Since it shut, the abandoned prison has set an eerie scene for films including The Green Mile and Walk the Line.

Today, explorers often bring asbestos masks to tour the cellblocks, the administrative area, and Death Row, which leads to the room where all executions in Tennessee took place from 1916 to 1960. You can still see the spot where the electric chair used to be. (Photos)

Loudoun Castle Theme Park, Scotland
A far less sombre place for exploration is in Galston, Ayrshire. The popular Loudoun Castle Theme Park went out of business in September 2010 after being operational since 1995. Most of its 25 rides remain suspended in time at the park, their paint gradually beginning to chip away. (Video)

Kings Park Psychiatric Center, New York, United States
Abandoned asylums, psychiatric hospitals and hospitals often become sites of urban exploration. It is natural to feel connected with people from the past when you walk through such facilities and stumble upon records and personal effects. On New York’s Long Island, Kings Park Psychiatric Center is a collection of abandoned buildings and structures, including patient accommodation, dining halls, steam tunnels and a cemetery. The centre treated patients from 1885 until 1996. In the “Group 4” patient building, explorers can find paintings and sketches done by patients, presumably as part of their therapy. (Photos)

Paris Catacombs, France
In 2004, Paris police discovered a secret movie theatre inside the city’s infamous catacombs, the underground tunnels where the dead were buried from the late 1700s to the mid-1800s. The purveyors of this cinema were urban explorers who just really loved the catacombs. Appropriately, they refer to themselves as cataphiles. Alongside the remains of six million people, they explore, they throw parties and on certain occasions, they watch movies. Catacomb exploration has such a cult following in Paris that there are special police officers -- known as cataflics -- whose job it is to guard the underground passageways. Travellers seeking a cataphile experience would be well advised to meet up with local groups or guides before heading down. (Audio slideshow)

Odessa Catacombs, Ukraine
Odessa is home to the world’s longest network of underground tunnels. The burial passageways stretch more than 1,500 miles. Created by stone mining when the city was being built, the catacombs have been used over the years as a hiding place for slave traders and as a military base. Today, a portion of this web of corridors has been converted into a museum. Contact the Ukraine tourism board for opening hours, or opt for a private tour. (Photos)