The New South Wales Hunter region is an area steeped in military history.

From the iconic Fort Scratchley, which proudly sits guarding Newcastle's port, to the Williamtown Royal Australian Air Force base, with its high-tech roaring jets moving at breakneck speed, the region has regularly housed the nation's armed forces.

But it is further north, at Port Stephens, where a mission to crack a World War II mystery is unfolding.

What started with listening to his mother-in-law recount tales of life in Port Stephens during the war has propelled Charles Tennyson on a 20-year journey to uncover what he believes is a covert WWII United States military bunker at Fly Point, Nelson Bay.

Having had the project approved by the local council, Mr Tennyson will soon begin excavating part of the parkland where he believes the three-storey bunker, which was filled in and deserted by troops at war's end, lies hidden beneath the surface.

Adding to the urgency is the fear that if there are contaminants inside, they could be impacting a protected marine park that is within metres of the site.

Charles Tennyson has spent 20 years investigating the reported bunker. ( ABC Newcastle: Robert Virtue )

During WWII, the Australian and United States militaries established bases at Port Stephens.

It was seen as a strategically important port, with towering heads that acted as a gatehouse to protect one of the country's biggest steel-producing facilities in Newcastle.

Mr Tennyson said his mother-in-law, Cecily Haddock, grew up at Little Beach and used to recount what she saw during the war.

"She remembered one day, all of a sudden, the US Navy were in the harbour," Mr Tennyson said.

"[It] scared the family next door. They took off quite quickly, but Grandma and her family stayed.

"They were given a password weekly that kept changing to go down this patrolled road, Victoria Parade."

It was the information about the password-protected road, bounded by the ocean on one side and scrubby bush on the other, and the reported presence of a tunnel, which spurred Mr Tennyson into action.

"Knowing that, she said she was able to view two trucks going and coming out of this tunnel at the same time — that's a big site," he said.

Scarce records make task more difficult

The Fly Point site is in close proximity to the heads of Port Stephens. ( Supplied: Google Earth )

Mr Tennyson is originally from Illinois in the US, and when he decided to pursue the challenge of finding the bunker, he joined with his project partner Mark Rawson.

Researching the site has been a challenge due to scant official records documenting the precinct.

"There's nothing on record regarding this site and all the other sites out there that we know about," Mr Tennyson said.

"This is a black-ops site. Even though there's a secret diary that says 'depot', you cannot presume this is a depot.

"We believe Grandma's story. Once we did test drilling. Everywhere we went down 10 feet, we were hitting concrete. We actually broke the drill trying to get through the concrete.

"There's something there."

Blurred facts and speculation

Bushland now covers the hill where the bunker is believed to be located. ( ABC Newcastle: Robert Virtue )

From the anecdotal information they have collected, and from what official information they have found, Mr Tennyson believes the site could contain an array of military paraphernalia left by the US military.

"Hopefully [there's] nothing, but, on the other end of the scale, presume anything that was used 70 years ago during the war," Mr Tennyson said.

"[There could be] trucks, jeeps, there could be boats, there could be drums and drums of fuel, oil and other kinds of nasties.

"We have the inventory list of everything that did come to Australia. We have no records that we have found where they actually took this out to sea and dumped them.

"At the end of that war, they just wanted to get home."

Mr Tennyson said the fact the site bordered a marine park was also a concern, especially if there proved to be contaminants in the area.

"One of the very sensitive parts of this marine park is actually right outside the door of the tunnel where the sponge beds are, so it's quite sensitive," he said.

"On top of that, it's in a park."

A mystery under a hill

Archaeologist Darrell Rigby is helping scan the site for clues. ( ABC Newcastle: Robert Virtue )

Mr Tennyson hired archaeologist Darrell Rigby to help assess the site and conduct ground penetrating radar scans.

"It's a highly interesting project," Mr Rigby said.

"Everyone loves a mystery, and there's a huge mystery here on this hill.

"Whether it's there or not, we don't know, but it's a really great thing to try and work out."

Mr Rigby has also been researching the site's history.

"Really what we're looking for is a needle in a haystack type thing," he said.

"It's 70 years ago, it was never talked about at the time. It hasn't been talked about much since.

"Part of the process is trying to work out what's true, what's hearsay, and can we actually detail what it is physically and practically. At the end of the day, that's what archaeology is all about."

Mr Rigby is realistic the search could prove fruitless.

"Potentially nothing is here. There's a whole bunch of loose ends which we bring together, and we try and make sense of those in a logical process," he said.

"Then what we do is put the jigsaw together.

"At the moment we've got a lot of pieces, and we've got part of the jigsaw, but we don't have the whole jigsaw."

Countdown on for digging to start

Mr Tennyson and Mr Rigby are not certain what could be below ground. ( ABC Newcastle: Robert Virtue )

Tens of thousands of dollars have been poured into the excavation project, along with a stream of bureaucratic paperwork.

Mr Tennyson said he had had no push-back from the US or Australian governments, and final approvals had largely been given.

"What we understand is the US used to keep an eye on these [sites]. The Australian government may know about these," Mr Tennyson said.

"The old guard has changed. If there are any records, they're gone, they're buried, they're dusty, they're forgotten about.

"These things are not on their radar anymore … They're on our radar. These things need to be investigated."

Excavation of the site will begin once a final check by Crown Lands has been approved, hopefully in the coming weeks.

"We're just going to make a big enough mark to get in there," Mr Tennyson said.

"Our job is to just dig, get through the block, enter this site, photograph, film, inventory, and report back to both governments and all agencies involved [about] what we find.

"This is just to investigate."

There was a hive of military activity in Port Stephens during WWII. ( ABC Newcastle: Robert Virtue )

Mr Tennyson said he was excited at the prospect of solving the mystery once and for all.

"It's a boy's own story," he said.

"If we can get some historical facts out of this for the benefit of Nelson Bay, that's a beautiful bonus.

"It'll be fun just to get that one tick done, to get Grandma's story proven correct. But there's more [sites], there's a whole lot more.

"I'm excited to get to those too."