Curious Canberra is a new series where you ask the questions and the ABC works with you to find an answer. In the search for secret tunnels, we found four underground spaces in Canberra.

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When Lisa Styles was 17, someone told her about a hidden tunnel that ran beneath Lake Burley Griffin to the Department of Defence.

"They told me they had been in it, in a little cart that ran along a train track," she said.

"They'd been blindfolded and taken from Old Parliament House to Defence, but I'm not sure I believe them."

While she had her doubts, the Belconnen teacher has wondered about the tunnel for over 30 years.

She asked me, through the Curious Canberra project, to find out whether the tunnel was real.

Lisa is not alone in her curiosity. I've fielded questions about secret tunnels ever since Curious Canberra began, and tunnels came out on top when we put three of your questions to a public vote.

What we uncovered was unexpected and at times muddy. There were hidden doors, spy-proof bunkers and tunnels too, some with strong ties to the urban legends that have been circulating for decades.

1. Tunnels, tubes and trenches at Old Parliament House

Historian Michael Evans shows Matthew Arnaudon an access hatch that leads to the tunnel at Old Parliament House. ( ABC News: Sonya Gee )

Of all the government buildings that were asked about in relation to secret tunnels, Old Parliament House was the most popular by far.

Michael Evans, head historian of the Museum of Australian Democracy at Old Parliament House, agreed to lead a rare tour of the building.

Joining me was Matthew Arnaudon, a student from Ngunnawal, who asked about secret tunnels and spaces under Canberra.

"Hopefully we find the truth," he said. "That's why I'm wearing my mind control tin foil hat."

There is a tunnel at Old Parliament House and it runs the length of the building.

Access hatches are scattered throughout the building but climbing into the tunnel is usually restricted to maintenance staff.

The tunnel beneath Old Parliament House is narrow and muddy. ( ABC News: Sonya Gee )

Michael made his first trip into the tunnel for this story. I followed him down a steep steel ladder.

"It looks like a secret tunnel," he said. "You can't see it from the outside, you're just not aware it exists. Under everything here's this passage that you can cross Old Parliament House underneath."

"It was really built to carry services, particularly the Lamson tubes, which were a pneumatic way of sending messages all around Canberra."

According to Michael, this could be where the rumour about a tunnel between Old Parliament House and Defence began.

"In the 1940s during the Second World War, they wanted to have secure means of transporting documents between Parliament House and the Defence department, which was in one of the buildings behind us. So they extended the Lamson tubes."

Pipes transported messages around Old Parliament House and to three off-site locations. ( ABC News: Sonya Gee )

The Lamson tubes used compressed air to move documents, tucked into sealed cylinders, to different points around the building.

The tubes also sent messages to three external locations - the General Post Office and Defence, then located in the East and West Block buildings behind Old Parliament House, and the Government Printing Office in Kingston.

Michael believes other facts have been distorted over time, fuelling the myth of the secret tunnel.

"During the Second World War, once Australia realised it could be bombed by aircraft, there was a lot of emphasis on building trenches," he said.

"Around Parliament House trenches were built and my guess is that they also included some roofed-in structures, like bunkers, where people would've gone if there was an air raid."

A trench is dug outside Old Parliament House. ( Courtesy of the Australian War Memorial )

Inside the tunnel, it's cramped and muddy, and where it does and doesn't lead has been the source of much speculation.

"It doesn't go anywhere," Michael confirmed, laughing. "You can't walk from one building to another under a tunnel."

This tunnel wasn't designed with pedestrians in mind, it's impossible to stand up in parts, ruling out the popular theory that it was once a Prime Ministerial escape route.

"Every Prime Minister's probably felt at some stage - 'If only I can get out of here'. There are other ways of doing that and a tunnel is probably one of the least effective ways of evacuating a building," Michael said.

2. Walking the tunnel beneath Defence

Inside one of three pedestrian tunnels that link buildings in the Defence precinct. ( ABC News: Sonya Gee )

Across Lake Burley Griffin, Matthew and I meet Rear Admiral Gregory Sammut, the Australian Defence Force Officer for the Russell precinct, to tour a tunnel on site.

Defence buildings are numbered, and the tunnel we enter connects buildings R1 and R2.

"The tunnel is one of the means by which a number of people walk to and from each building for the various meetings that are held throughout the day," Admiral Sammut said.

"There are no other facilities within the tunnel; it is purely a pedestrian walkway."

For staff, it's an easy way of moving between buildings, without having to go through security at either side. It's also a way of escaping Canberra's stifling heat or bitter cold.

Admiral Sammut estimated that the concrete tunnel is just over 70 metres long. There are two other tunnels at Russell, one that connects buildings R3 and R4, and another joining R2 and R3.

"There's always the element of intrigue that goes around what happens at various government departments, particularly perhaps with Defence," Admiral Sammut said.

"At the end of the day, we're going around business, like many other government departments do."

3. The hidden tunnel at University House

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In her book University House: as they experienced it: a history, 1954-2004, historian Jill Waterhouse wrote about a tunnel beneath the first building of the Australian National University (ANU).

She agreed to take me inside the tunnel and was quick to point out that it was a service tunnel, rather than a spy tunnel.

Like the service tunnel at Old Parliament House, this tunnel allowed maintenance staff to access the building's utilities.

According to Jill, the fact that it's large enough to walk around in, hints at another purpose.

The tunnel at ANU is high enough to stand in. ( Stuart Hay, ANU )

"University House was opened in 1954 but it took several years to build," she said.

"In those years the Korean War was still raging and both America and the Soviet Union had developed hydrogen bombs."

At the time of construction, bricks were in short supply, and yet work on the tunnel continued.

"The tunnel at University House was built, so it is said, with a capacity to withstand a nuclear attack," she said.

"If a bomb dropped straight on it - that would not be the case - but it was a soothing concept."

The tunnel doesn't go anywhere; it's a corridor that forms a square beneath the building.

"Nobody ever practiced bunkering down but I would say today, if there was a big bushfire sweeping down Black Mountain and you couldn't escape - one place to go might be the tunnel underneath University House," Jill said.

4. Inside the spy-proof bunker

Special efforts were made to ensure that the bunker was visually stimulating, as staff had no access to natural light. ( ABC News: Sonya Gee )

Large skylights in the carpark of the John Gorton Building give a glimpse into an underground workspace that was once a spy-proof bunker, enforced by steel and concrete.

Built in 1977 and operational from 1979, the bunker housed a "computerised system for all Government communications outside of Australia," according to the building's heritage management plan.

At the time, the John Gorton Building was occupied by the Department of Foreign Affairs, which didn't have room for the new technology, or sufficient security to prevent spying.

A site layout of the communications centre in Parkes. ( The Department of Finance and Deregulation )

The purpose built communications centre was adjacent to the building and beneath a carpark, and staffed around the clock.

It was used until 1996, and substantial building work began in 2001, to transform the bunker into a larger office space.

Courtyards and skylights were added to give incoming staff access to natural light.

Previously, in the absence of daylight, the bunker featured bright murals and artwork to ensure it was visually stimulating for staff.

Urban legend lives on

At the end of our tour of Old Parliament House, Curious Canberran Matthew Arnaudon seemed satisfied by what he had seen.

"I'm fascinated but I think the urban legend still lives on," he said.

"The human service tunnels - there's some kind of mystery still around there."

Historian Michael Evans is confident that stories of elaborate tunnels will persist.

"The more you disprove the legend, the more validity it gives some people," he said.

"As a historian, I go by fact but at the back of my mind, there's always this little thing that says 'maybe I don't know everything'."

Sewerage tunnel under Molonglo River at Commonwealth Avenue. ( National Archives of Australia )

Yet clues can be found in Canberra's sewerage tunnels.

Looking through a collection of historical photographs of Old Parliament House, I found a photo of a sewerage tunnel that appears to have a train track.

It reminded me of the story Lisa Styles was told as a teenager.

The city's sewerage network and its connection with secret tunnels had come up before, in a conversation with historian Jill Waterhouse.

"That is quite an interesting story in itself if you can get a thrill from learning about sewerage tunnels," she said.

The Lodge and Parliament House couldn't be accessed for this story and undoubtedly several locations remain unexplored.

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