Australian city dwellers seem to have an unending fascination with underground tunnels. And with rumours abounding of wartime ingenuity, sneaking spies and underground roads wide enough to drive a car down, Darwin's subterranean secrets may be some of the best.

Wendy Scott, a teacher, had been living in the Northern Territory for close to a decade.

She took a keen interest in local history, brushing shoulders with historical groups and taking local tours. But eventually, she had begun to feel like she'd seen it all.

"From my point of view, I'd seen everything there is to see in Darwin," she said.

Or had she?

Curious Darwin is our story series where you ask us the questions, vote for your favourite, and we investigate. You can submit your questions on any topic at all, or vote on our next investigation.

Ms Scott recalled taking a tour of Darwin's WWII oil storage tunnels, where a simple equation prompted a mystery.

She learned that only two tunnels were open, despite an additional four being built. She had heard early plans showed designs for up to 11 tunnels.

"When I went down to do this walk through the two tunnels that are open, I asked the fellow about it and he just looked at me and went, 'well, you don't quite know'," she said.

The rumour mill started churning. Colleagues would repeat talk that the tunnels were still in employ — perhaps for government business, perhaps by ASIO.

"I was just curious because I'd heard a lot of things, and you can always say hearsay is one thing — but what's the real truth?" Ms Scott said.

"What happened to the other tunnels? Are they being used? Are they still there? Or is it, say, they they're flooded out and have got too many problems to be used for anything?"

These tunnels were used to store oil. Today, they mostly just store water. ( ABC Radio Darwin: Jesse Thompson )

We were unsurprised to receive Ms Scott's question about tunnels beneath Darwin; after all, there are underground tunnels in Sydney and Canberra. More than 20 people wanted to know about tunnels beneath Adelaide.

In Darwin, history buffs the ABC spoke to said tales of underground tunnel networks are largely hearsay.

So, we ventured underground to separate rumour from subterranean reality.

Inside the storage tunnels

This investigation began in the same place as Wendy's curiosity: running several hundred metres under the city, the WWII oil storage tunnels are hot, large and cavernous.

They are wet.

The sheer quantity of water trickling down every surface is so distracting I forget to ask why the water is here. It hasn't rained in at least five months.

For the dry season, there sure is a lot of water in the oil storage tunnels. ( ABC Radio Darwin: Jesse Thompson )

"This is all natural water," explained the tour guide, Robert Marchant.

"They tried to stop the water when they were building this, but the way water works, they were not going to be able to stop that quickly."

Mr Marchant operates tours through the tunnels and has a deep interest in military history. He spends a lot of time underground.

As he explained it, these tunnels were failures, to some extent.

This tunnel ran close to Parliament House and is filled with cement. ( ABC Radio Darwin: Jesse Thompson )

As the northern city was bombed in WWII, the Government moved to protect oil that had been stored there in the 1920s. The idea — which Mr Marchant described as a major effort — was to store it in tunnels.

"Once they got inside, shale light rock interspersed with quartz outcrops collapsed it on their heads, so they had to use hammer and tap," he said.

"So instead of taking a year to eight months to build eight tunnels, six were completed, eight were started, and today there's only five left here in Darwin."

They were eventually abandoned, but some are still visible.

"There's one underneath Parliament House, but the footings come within three metres of the footings of Parliament House, so they filled it in with concrete," Mr Marchant said.

Casually, he then offered this smoking gun: "The other two are down the road, about three hundred metres, and they're blind. They're the largest of the tunnels at 210 metres long."

Rob Marchant runs tours through the WWII oil storage tunnels. He has spent a lot of time underground. ( ABC Radio Darwin: Jesse Thompson )

Tunnels hidden by rainforest

Sure enough, these closed tunnels can be found further down the escarpment, hidden by rainforest to all but those who know where to look.

Mr Marchant has been inside.

"It's like walking back into 1970," he said.

"It's quite good: very hot and humid, lots of water and a tunnel and tank, as we'll call it, with oil in the bottom — probably about an inch and a half of water."

These days, just two tunnels remain open to the public and are visited by temporary exhibitions, the odd concert, and tourists willing to put up with the moderate humidity in the dry season and extreme stifling heat in the wet.

The entrances to tunnels 10 and 11 are somewhat well-hidden. ( ABC Radio Darwin: Jesse Thompson )

Working in the tunnels has turned up all kind of secrets; some recent cleaning revealed a 'no smoking' sign on a wall near where gallons of oil were once stored.

But one thing they have not turned up is evidence of more tunnels, despite the flood of visitors with theories that secret tunnels connect council chambers, parliament, and even suburbs.

"There's no-one escaping from Parliament House, there's no-one escaping from council, and there's no-one in a hospital underground," Mr Marchant said.

"I've actually worked here for the last 20 years and I've never been able to find one. So that's a lot of idle chatter."

Rob, who runs tours through the tunnels, is the guide. ( ABC Radio Darwin: Jesse Thompson )

Were there tunnels at East Point?

Even Mr Marchant, otherwise sceptical, believed there were subterranean tunnels connecting military memorabilia at East Point, an historic military area near the CBD.

According to Norman Cramp, the director of the nearby Darwin Military Museum, the city's history means many of these myths have marinated in elusive wartime stories.

"There's also the secrecy about what was going on out here during the war and the military hush-hush," he said when I took Mr Marchant's theory to him.

"I came up here in 1974, just before Cyclone Tracy; I heard about it then. And then certainly the myth never got blown away with Cyclone Tracy.

"I think people are just fascinated with tunnels. We like a good wartime mystery."

Norman Cramp has scientific evidence that there are no tunnels beneath the ground at East Point. ( ABC Radio Darwin: Jesse Thompson )

Sadly, Mr Cramp can say with scientific certainty that there are no tunnels at East Point.

"We have had two scientific surveys conducted out here utilising ground-penetrating radars to try and find if there was any type of structure," he said.

"We were looking to see if there were tunnels or a tunnel that connected gunning placement number one with the command post right here."

Aside from some disturbed ground, both searches turned up nil.

But a photo recently delivered to the museum by a stranger could suggest why these theories persist. Believed to have been taken in about 1943, the photograph shows two soldiers standing in a deep trench near East Point.

A stranger gave Norman Cramp this photo of trenches in East Point. ( ABC Radio Darwin: Jesse Thompson )

The photo came with this note written on the back. You can click to enlarge it. ( ABC Radio Darwin: Jesse Thompson )

Mr Cramp believed it was a communications trench.

"We found no traces of any concrete stretches that would then define that trench as a tunnel," he said.

"The scientific surveys say there are no tunnels at East Point. I'm sorry!"

'There's tunnels right across the CBD'

One of the most useful clues in researching this story came from a tip-off from a newsroom journalist.

A few years ago, he'd been asked to cover the opening of a new Power and Water substation in the city.

Also in attendance was former deputy chief minister, Peter Chandler, who held portfolios in police, fire and emergency services, business, transport and more across four of his eight years as an MLA.

At the time, he was also the essential services minister.

"We were down there for the opening of that, and while we were down there we had the opportunity to visit the bowels of the building," Mr Chandler recalled.

"We noticed that there were these tunnels that ran off in different sections: at least two that I could see from the substation alone.

"I put a couple of questions to the adviser and the staff at Power and Water. They were the ones that said to me, 'oh yes, there's tunnels right across the CBD where all the essential services run'."

By his recollection, the tunnels coursed beneath several major streets. With the services equipment removed, he believed you could drive a small car down them.

"It made me think, 'what a wonderful excursion'," Mr Chandler said.

"Sadly I wasn't in government long enough to pursue my wish, and that was to visit these tunnels."

The network of tunnels course beneath major streets in Darwin's city. ( ABC Radio Darwin: Jesse Thompson )

The mystery remains

According to an NT Government spokesperson, the tunnels were constructed in the 1970s to house power cables as the city rapidly grew in tandem with Darwin's population.

The network is 4.6 kilometres of tunnel spanning in several directions beneath major city streets.

When Cyclone Tracey struck in 1974, the tunnels were the reason power was quickly restored to some areas: there was less damage to the cables because they were stored underground.

Construction finished the following year, but streets had to be closed and hard rock blasted away during the long installation.

These days, an asset management plan shows they are subject to environmental challenges such as cyclones, high temperatures, humidity and a large volume of annual rain.

"This unique environment results in a more rapid rate of asset deterioration," the report states.

This voltage feeder tunnel is one of the main entries into the tunnel network from the substation. ( Supplied: NT Government )

So, there are indeed tunnels beneath Darwin CBD, although they aren't entirely secret, but they can be credited with keeping the city's power on when Cyclone Marcus hit earlier this year.

"Obviously ASIO is not down there at all, hey," Ms Scott said when I relayed this information back to her.

An attempt to access these tunnels was, sadly, declined.

"Unfortunately you will not be able to see the tunnels, as safety is the biggest issue with all the cables underground," a Government spokesperson said.

For now, we can only imagine.