Living in Longlea, 15 kilometres to the east of Bendigo, there was no skate park or shopping centre at which to meet with friends. 

Instead, he would entertain himself on the rivers’ banks, eyes peeled for water scorpions, mudeyes and other insects, the names of which he has long since forgotten. 

“I'd put them in tanks and watch them for days on end,” Mr Gaskell remembered. 

“Then I realised there's an ocean that's much bigger.” 

Mr Gaskell’s fascination for what lurked below the water’s edge would prove to be the catalyst for a career spent at sea. 

Now 36 years old, the one-time central Victorian calls the Whitsundays home, working as a marine biologist on Daydream Island.

“The diversity of animals you didn't think existed is just mind-blowing,” he said when asked what fueled his passion for the ocean. 

“You never have a snorkel that is exactly the same as the last.”

When he is not running information sessions for the island’s visitors, Mr Gaskell is searching Google Maps for new spots to explore off shore.

It was on one of these online reconnaissance missions he struck waterborne gold: a pristine, deep blue hole. 

The circular abyss, with a diameter about 150 metres wide, dropped at least 20 metres towards the ocean floor.

From aboe, it reminded Mr Gaskell of the Great Blue Hole in Belize, the largest known formation of its kind. 

Even though it sat directly in the path of Cyclone Debbie, which tore through northern Queensland six months before, the deep lagoon and the coral it contained was perfectly preserved. 

“They were completely healthy, these staghorn corals with huge, branch-like extensions, longer than I've ever seen anywhere,” he said.

Unsurprising, Mr Gaskell’s voyage into the blue has caught the eye of scientists and nature lovers alike.

And it is for that reason the hole’s location remains a secret.

A fleet of just a few boats could be enough to harm the precious ecosystem, he said.

Sunscreens could also damage corals, as could the kicking feet of swimmers.  

“They're so delicate that it would only be a matter of time until they were smashed.”

Mr Gaskell and his colleagues are perhaps among only a few people to ever visit the void – and could well be the last.

The marine biologist is well acquainted with humans’ impact on the ocean. While he stressed much of the reef remained in tact, living and working on Australia’s north coast gave him a front-row seat to the ravages of global warming. 

But he said a lot of people still did not believe water temperatures were on the rise.

“People go for a swim in Melbourne and think it’s as cold as its ever been,” he said, with a bemused laugh. 

“It only takes about a one-degree increase, two (degrees) definitely, to bleach some corals and cause a slow death of the corals. 

“We might be left with a Great Barrier Reef that has one or two species of coral, or we end up with nothing.” 

The pockets of Australians would also feel the pinch of the reef’s deterioration, he said. Deloitte has previously estimated the economic contribution of the natural wonder at $6.4 billion per year. 

But the greatest loss would be the sights and sounds of the ocean.

Asked for a moment underwater he would never forget, Mr Gaskell recalls the day a loggerhead turtle joined him on a snorkel off the Ningaloo coast. 

It was huge, he said, its head bigger than his own. 

“Most turtles are pretty afraid of people, but every now and again you'll get one (like this),” he said.  

“I don't know if it couldn't see and it thought I was another turtle, but it just came up to me. 

“It could’ve been nearly 70 years old, and it was just hanging out, swimming next to me.” 

Mr Gaskell also loved the relative quietness of life underwater, with worldly noises like the hum of traffic replaced by only the clicking of snapping shrimp.  

Budding Bendigo biologists need not to travel to Queensland for a chance to check out sea life, Mr Gaskell said.

He recommended those wanting an up-close encounter with waterborne creatures to snorkel around the piers and jetties in Port Phillip Bay. 

“All up and down them you'll get your algal gardens, seahorses, stingrays, even little harmless sharks,” he said.

“Just jump in and have a look.”

While the ocean was a cause for consternation among many land-dwelling humans – and the fodder of the horror movies they watched – he said more exposure to the world underwater would help their fears abate.

Even Mr Gaskell started off nervous about surfing, saying his knowledge about sharks kept him closer to the shore. 

“But you eventually realise that the fear was based on an extremely unlikely scenario, on something you've seen or read,” he said.  

Anyway, there is too many treasures under the water to stay on land for long.

Follow @johnny_gaskell on Instagram.