07/09/2015 3:28 AM AEST | Updated 15/07/2016 12:50 PM AEST

Gunpowder Magazine Hidden Beneath Sydney Harbour Bridge

Video by Tom Compagnoni

More than 150,000 cars cross the Sydney Harbour Bridge each day but beneath its steel struts lays a shadowy secret.

Dwarfed by the granite pylons on the southern side is a small, rough-hewn structure emerging from the ground.

Crouching down, you can glimpse a darkened room, a scrawled name, some flaking paint.

Sydney Harbour Foreshore Authority archaeologist Dr Wayne Johnson leads us down a tight spiral of steps to a part of Australia’s history when paranoia ran rife about an imminent Spanish invasion and when America was the enemy - to a room so dangerous, it was omitted from all official blueprints.

This room can be glimpsed for the first time in 100 years.

We’re standing in a gunpowder magazine – built in the 1850s at Dawes Point Battery to store a secret stash of explosives to defend the city from enemy ships.

Now for the first time in more than 100 years, it’s being revealed to the public.

“We had no idea it existed,” Johnson said.

“Because it wasn’t included in any plans, the only clue something was down here was the staircase.

“The first time I scurried down, there was a cascade of debris – big sandstone blocks from the ceiling and rubble were strewn everywhere.”

That was back in 2000 and the Sydney Harbour Foreshore Authority has since embarked on a $2 million project to restore the underground series of rooms.

Still with its original coat of paint, the “essentially bomb proof” storage room is scrawled with the names of a handful of early graffiti artists.

“They could have been people who worked in the barracks or the children of families who lived in The Rocks at that time,” Johnson said.

Dr Wayne Johnson says Sydney Harbour was worth fighting for.

The bitumen floor still bears the imprints of gunpowder barrels.

The city had a good reason for defence in the 1800s – not only was Sydney a strategic point for whaling in the South Pacific, it was also rich with gold rush spoils and a key colony for Britain’s enemies to target.

“The whole place was in readiness for an attack,” Johnson says. “Initially it was thought the Spanish might attack because they had claims to the Pacific area which went right back to the 1400s.

“Later with the rise of Napoleon there was fear the French might attack, then at one stage between 1812 and 1814, Britain was at war with America.

"There was concern there might be an attack by Americans, who also had whaling interests in the Pacific. Then in the 1850s, a fear that Russia might attack for gold.”

Despite these extensive preparations, the cannons were never fired in anger.

At night, an eerie violet glow illuminates the buried structure. Johnson says it’s not just to build a sense of atmosphere.

“It’s ultra violet light that stops algae and other things from growing down there… but it is also quite eerie.”

You can peek into the gunpowder magazine from Dawes Point Battery and Johnson says there are plans to start public tours in 2016.

Original video by Tom Compagnoni.

This story was originally published on September 7, 2015.