These are the relics from decades past that have fallen into disuse but which have gained a new lease of life as unlikely tourist destinations.

Interest in the likes of Vaucluse House or Rouse Hill House and Farm is easy to comprehend. These historical sites provide an insight into how we used to live – how we cooked in primitive kitchens on open fireplaces or farmed with weird agricultural machinery.

But why the interest in walking through a disused tunnel in wellingtons, ankle deep in water with a lamp strapped to your head at risk of tripping over a submerged girder?

This isn't the fad of a few. Try booking to explore the redundant tunnels under St James Station or the ghost tunnels that were never used under Central Railway Station. The Open Sydney annual event is invariably vastly oversubscribed. Your name will go into a ballot for a place. You may be lucky.

A good example of a redundant engineering project reborn is a 400-metre tunnel bored through sandstone in 1907 as part of the Newnes railway line. It served the kerosene oil shale mines operating during the early 20th century. The railway closed in 1932 and the rails were pulled out of the tunnel. With departure of the trains a population of glow worms decided to take up residence.

The tunnel curves through almost 180 degrees and is consequently very dark. In normal weather a small creek flows through it. These conditions are apparently ideal for glow worms which inhabit the walls and roof of the tunnel.

Staying with the railways, move to the Maldon to Dombarton rail bridge. In 1983 the Wran state government announced the line would be constructed with completion expected in 1986. That was 30 years ago. The project, and the bridge, have been left up in the air. It stands as an embarrassing monument to a financial blowout and, thereafter, indecision. The public love it.

Some understand why these relics are so captivating. Laila Ellmoos, a historian at the City of Sydney, says people are curious to discover hidden places.

"There is a nostalgia for that lost heritage," she says. "When a factory is in full swing and you have got all the pollution, you can't really romanticise about it. But when it is a bit of a relic you can appreciate the architectural quality of the build and you then respond to it differently.

"The bridges and tunnels are like abandoned hope. People love the glow worm tunnel because it is a natural wonder. A fairly noxious industry has created that refuge."

Caroline Butler-Bowdon, the director of curatorial and public engagement at Sydney Living Museums, says the fascination with abandoned architecture is about leaving the contemporary city behind.

"There is an interest in underground and disused places across the globe," she says. "They are not what people would normally think of as heritage. People are seeking an experience. It is part of being an adventure-seeker. They like the bizarre and the off-limits.

"We don't want cities that have stood still or become historic theme parks. But we do want to value cultural heritage of different eras and integrate it into the way we see ourselves."

Modern technology may also have triggered a renewed interest in historical sites, adds Butler-Bowdon.

"It's also about the Instagram moment," she says. These spaces make for fantastic photography. People want to be the first to see these spaces and share them. We find they are the most 'Instagrammable' spaces."

There is, of course, another element that is certain to guarantee tourist interest. Throw in talk of a ghost or two, as is the case with the unused tunnels under Central Station, and entry to a dank space suddenly becomes a hot ticket.

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