Near a busy railway platform at a station in the centre of Sydney, a small door opens into a hidden world that few commuters ever see. The door is typically locked and public access is forbidden.
But Sydney Trains agreed to open it and allow this reporter a rare glimpse of the network of abandoned tunnels that lie beneath the hustle and bustle of the city centre.
So, as commuters were making their way home one recent Friday afternoon, a rail official opened the small door and led me into the dark labyrinth of tunnels that extend out from St James station, which is at the end of Hyde Park. Now used by about 13,000 people a day, St James was opened in December 1926 and was one of the first underground stations in Australia.
Inside, a walkway took us to two large "ghost platforms" that have never been used. The platforms have decorative tiles and spaces for advertising billboards. All was quiet, aside from the sounds of nearby trains rushing through.
At the end of these platforms, a narrow doorway led farther into the tunnels, which were completely shrouded in darkness.
Wearing boots, we trod carefully through the knee-high water that filled some of the tunnels, apparently due to condensation.
The ensuing web of tunnels, illuminated by torchlight, revealed a history of the city, told from the underground. There was a set of caverns used as a secret bunker and planning post during World War II, including facilities for monitoring radar stations, weather signals and airport traffic. It is still possible to see graffiti from soldiers posted there during the 1940s.
In some tunnels, roots of the trees above us in Hyde Park had broken through the ceiling. In others, there were patches of mushrooms, said to be remnants of an attempt to cultivate an underground mushroom farm.
The St James tunnels - like others across the city - were intended to form part of Sydney's railway network but were never completed.
The tunnels fell victim to a mix of poor planning decisions, changed transport needs, a lack of funding, and interruptions in the 1930s and 1940s caused by the economic depression and World War II, and were abandoned. Now they form a sometimes eerie network of underground caverns.
According to a description in Australia For Everyone, an online travel guide, Sydney has almost as many abandoned railway tunnels as those that are actually in use.
"This has occurred because of the growth Sydney has experienced since the first railway tunnel was cut in 1855... with new tunnels being built on new lines and existing lines as the system was upgraded and expanded," the website says.
A Sydney Trains spokesman said sections at St James have been used as air raid shelters and sets for The Matrix film series. Elsewhere, an underground freshwater lake has formed in an unfinished tunnel. Nicknamed "St James Lake", it is said to be 7m wide, up to 5m deep and about a kilometre in length.
"It's rumoured that during World War II, US general Douglas MacArthur used the southern air raid shelter area as his headquarters for Pacific operations," the spokesman said.
In recent years, the tunnels have attracted much interest, and there have been occasional public tours.
Records compiled by the NSWrail.net website indicate that New South Wales state has about 29 tunnels that were used but have been closed. There are also stretches of tunnels that were never completed.
The question for the city authorities is what to do with them. The public tours have proven popular and there have been calls to convert them into permanent attractions.
An Australian tourism expert, Dr David Beirman of the University of Technology Sydney, said the tunnels could be turned into a "niche" attraction or as venues to screen horror films. Australia's fast-growing tourism market means such niche offerings could still be commercially viable, he said.
"To make it a commercial success, you would need to customise tours of 'underground Sydney'," he told The Straits Times.
"There is a category of tourist who wants to do something no one else does. Our tourism market is so big that there are always enough people to make a viable enterprise out of something."
The state government has increasingly attempted to publicise the tunnels and ensure their historical value is recognised.
"You cannot go anywhere else in Sydney and experience something like this," Sydney Trains official Tony Eid told ABC News last October. "It's something... quite unique to Sydney and we need to preserve it for many generations to come."
But Dr Beirman pointed out a potential drawback for turning the tunnels into large-scale tourist spots. "A lot of people feel a bit claustrophobic if they end up down in a hole in the ground," he said.