AT TWILIGHT the Gothic towers of an abandoned hospital loom like a haunted house. But the shadowy figures flitting along crumbling corridors are not ghosts, they are self-styled agents of a secretive undergound movement.

The six men who scaled a wrought iron fence and crawled in through a broken window call themselves urban explorers. Their quest is to infiltrate every forbidden corner of the land.

The intrepid adventurers climb bridges, explore disused factories and slither through dank sewers for the thrill of inspecting a hidden piece of the urban landscape. Trespass is their hobby and the more razor wire, warning signs and locked gates they encounter the more daring the challenge.

Trekking in the Andes or conquering Mount Everest hold no appeal for urban explorers who prefer obscure sites closer to home: an underground river beneath Croydon, derelict Tube stations or a network of tin mines in Cornwall.

“You get a chance to discover places where no one has set foot for maybe 50 years. These secret places exist all over the country but most people don’t even realise they are there or give them a second glance,” said Steve Higgins, 23, from Ilminster, Somerset.

Armed with torches and cameras to record each mission, Britain’s urban explorers live out their fantasies after dark. They convene at an arranged point and waste no time in entering their chosen site. Loitering or looking shifty attracts attention and urban explorers like to operate under the radar. Depending on the location, some wear suits and carry “credibility props” such as a briefcase or clipboard.

One veteran explorer, who goes by the name Laughing Boy, insists that a smart suit, purposeful stride and confident manner have allowed him to bypass security in most city buildings. Another tip he shares is that a significant number of alarmed doors are in fact disconnected. “There is always a moment of sheer terror when you turn the handle and wait for the alarm to go off but four times out of five, it doesn’t,” he said.

Often the problem is not security staff but curious passers-by who see a flickering torch beam or catch a glimpse of someone slipping over a wall. “Busybodies are the worst,” Laughing Boy said. “They are the ones who get us into trouble.”

The aim of any urban explorer is not to get caught, but those who do often find that professing an interest in urban archaeology or history can convince police or security guards that the escapade was harmless.

The explorers — or creepers are they are sometimes known — have the motto “We don’t take anything but photographs and leave nothing but footprints”. According to the unwritten rules, agents do not deface sites or steal mementoes but simply record what they see.

The excitement lies in the unknown. Another devotee, whose nickname is Milo, remembers creeping up a rotting wooden staircase in an abandoned mental asylum in London at the dead of night. “It was pitch dark and my torch was making weird shadows on the wall. I could feel the spirits of the poor people who were locked up in here. It was very creepy but it made me feel good to have been there and seen it for myself,” he said.

Urban exploration missions are planned in secret with the details and photographs only released after the explorers have got in and out safely. Many of the expeditions involve clambering around unsafe buildings, but the elements of danger and risk add to the adventure. Few explorers wear hard hats or other protective clothing. Getting wet and dirty is part of the experience.

Members communicate via the internet where there are dozens of websites dedicated to a cultural phenomenon which is kept hidden from view.

The infiltration movement began in the late 1970s in San Francisco with the San Francisco Suicide Club which carried out daredevil expeditions. It spread across the United States where there are now thousands of urban explorer clubs holding monthly meetings and publishing magazines about their exploits.

In Britain there are probably fewer than 900 devotees taking part in regular missions. Hotspots include London, Wiltshire and Kent, where Second World War underground bunkers and disused railway tunnels are a favourite haunt.

But in some city centres derelict buildings have become impenetrable fortresses. Battles, a retired urban explorer from Edinburgh, blames developers who have introduced increased security around old buildings, making it impossible for trespassers to sneak inside.

“Urban exploration in Edinburgh is, as far as I can tell, off the menu thanks to the developers who are munching their way through the city. It’s easy to understand that they would wish to protect their investment and that urban explorers are not their worry but the value of these properties means that it’s worth investing a fortune in security,” he said.

But for the hardcore explorers there is always a way. Evading CCTV or clambering over a barbed wire fence adds to the sense that they are taking part in a clandestine operation. Most urban explorers are frustrated spies.

“It’s about getting away with something that you’re not supposed to and going to places no one wants you to go. It’s liberating,” Laughing Boy said.

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