The artwork shows the devil, a skull, flames and other evil-looking symbols.

Further along is a sign on the wall: "Public Air Raid Shelter. WARNING". Spindly tree roots straddle across the curved roof and drop to the ground. We see all this in the beam of our torches as we wade through water halfway up our wellingtons. Otherwise there would be pitch darkness.

"You are standing in the back end of the air raid shelter," says Tony Eid, executive director of Sydney Trains. "They were purposely designed during World War Two for the folks of Sydney in the event of an air raid or some sort of invasion where people could be safe."

We are 30 metres beneath Hyde Park in the disused railway tunnels beneath St James Station. Above, Sydney goes about its daily business.

The tunnels were constructed in the 1920s as stubs for proposed eastern and western suburbs lines. The idea was that operation of St James would not be disrupted if future work were needed.

But the plans were postponed with the advent of the Great Depression and World War ll. No track was ever laid and the tunnels never fulfilled their intended purpose.

This is the gold ticket for Sydney Open on November 5-6 and it's easy to see why. For one weekend of the year the doors are unlocked to the city's most important, inspiring and intriguing buildings and spaces. Purchase a ticket by October 18 and you go into a ballot and a select few can, for an hour or so, become tunnel dwellers.

In the shadowy, dank tunnels it feels as if we are on the set of a horror movie. In fact we are. The Tunnel was filmed here in 2011. A film crew goes underground to investigate why homeless folk are going missing. The plot involves an emaciated humanoid, a monster in the lake and plenty of blood. The budget was $36,000 but it only grossed $1348. A box office bomb.

"There is no fresh air coming through from anywhere," says Mr Eid. "It is moist and rather hot, about 24-26 degress. As we get deeper into the tunnel that becomes really apparent."

Happily the need for the air raid shelters never arose but there are plenty of messages on the wall from the time, many written by soldiers in 1942.

Also during the war the Women's Auxilliary Australian Airforce kept secret information in the tunnels until the musty air started to affect their health.

Between 1933-34 the tunnels were used as an experimental mushroom farm. The fungi, at least, was happy in the dank air.

Sophie Lieberman, head of programs at Sydney Living Museums – which runs Sydney Open – said interest in visiting the tunnels was "overwhelming".

"We wish we could have them open all the time, it is a really special experience," she said.

"It's an opportunity to do something really unusual and to see a hidden part of your city from a different perspective."

smh.com.au