When a seemingly insurmountable challenge arises, it is often referred to as a "personal Everest" - a mammoth undertaking, looming ominous, shadowy and intimidating.

There was, literally, a whole new measurement needed for what Bulli diving expert John Garvin and a ragtag crew of engineers chaired by blockbuster filmmaker James Cameron accomplished in reaching the bottom of the Mariana Trench, the deepest part of the world's ocean - a hole so deep, Mount Everest would fit inside it with two kilometres to spare.

John Garvin resides in the foothills of Bulli, half a world away from the tiny coastal village in Cornwall, England, where he was born. Spending childhood weekends snorkelling off the United Kingdom's southern coast was the spark for a lifelong love affair with the ocean.

"Diving lets me relax. No matter how stressful life is, you can submerge into this incredible other world where everything else is left on the surface," Garvin says.

"I come out of the water very relaxed, my shoulders have dropped a few inches. Maybe it's because there's no mobile phones under water."

Garvin today works as a screenwriter and was previously a successful theatre actor. He once starred in The Buddy Holly Story, playing the lead role of the '50s entertainer, and would find his later life continually and surprisingly interweaving his two passions of diving and drama.

Garvin's early career was spent teaching diving in the Turks and Caicos Islands, a small Caribbean island chain east of Cuba. His speciality was the use of oxygen recycling rebreathing systems.

The apparatus would be a lynchpin in the success of the Deepsea Challenger submersible, the tiny vessel which, in 2012, became just the second manned vehicle to plunge 11,000 metres to reach Challenger Deep, the lowest point of the Mariana Trench off Guam.

"A rebreather system is not like scuba. It recycles a diver's breath to let them stay down much longer," Garvin says.

"It has been around since before scuba, but only since the end of the 1990s did it become affordable enough, and the technology advanced enough, to be used widely. Most underwater nature or feature films are shot by cameramen using these systems."

Garvin kept his pen busy writing screenplays while working as a diving instructor and cave diver. With brother-in-law Andrew Wight, he wrote box office smash Sanctum, starring Richard Roxburgh. The story of a cave diver in Papua New Guinea, Garvin was brought to the project by the film's executive producer James Cameron, director of Titanic, Avatar and Terminator.

"James wanted a diver who could also write," Garvin says.

"A lot of people have many different careers in their life and this was an example of how two very different careers collided, and gave me the skills to get involved with Deepsea Challenge."

Throughout our conversation, Garvin continually equates the deep-sea mission with space exploration, the Apollo missions to the moon and the space race, and for good reason; scientists say we know more about the surface of the moon than we do about the plunging depths of the ocean.

The mission statement of the DSV (deep submergence vehicle) Deepsea Challenger was simple - reach the bottom of the Mariana Trench, an astonishingly deep section of ocean known as Challenger Deep. Its deepest point is almost 11,000 metres below the surface.

As a comparison, the peak of Mount Everest stands at just 8848 metres.

"Imagine the cruising altitude of a jet. Next time you're on a plane, look down at the ground - that's how deep the trench is," Garvin says.

"It's difficult to comprehend the water pressure at the bottom. It's the equivalent of two armoured vehicles balanced on your fingertip."

The film Deepsea Challenge stands as testament to the sheer monstrosity of the task and the ingenuity and drive of the crew that built and guided the craft to the ocean floor.

The submersible was built in Sydney, in nine months in 2011.

In addition to directing some of the most popular and commercially successful films of all time, James Cameron is also an accomplished and respected deep-sea adventurer, his passion for the deep sparked by earlier films The Abyss and Titanic.

Cameron, the pilot of the vessel, had been chasing the dream since at least 2005; the dream of being the first solo mission, and just the second ever vessel behind the 1960 Italian voyage Trieste to reach the depths of Challenger Deep.

"Jim does these big blockbuster films to fund his true passion, exploration," Garvin says.

Garvin joined the team in 2011. At that stage, the submersible did not exist - the preceding six years had been spent creating a pilot's viewing sphere that could withstand 11 kilometres of water pressure and creating a system that would allow the vessel to float back to the surface.

"When I joined, we just had a metal ball and a recipe for syntactic foam," Garvin says, referencing the complex foam lining the Deepsea Challenger that would give it sufficient buoyancy to reach the surface again.

Such a craft would normally be made over years, in a huge factory by a massive company or even a national navy. Deepsea Challenger was built in secret, in a matter of months, in a sleepy factory sandwiched between a plumbing warehouse and a plant making packing crates, by a team of builders and engineers who had never built a submersible before.

"We came from a range of backgrounds, from diving to the aviation industry and yacht building, but we all had this ability to think outside the box and question the designs already out there," Garvin says.

"It was my dream job, but I told them I had no experience with subs. 'Neither do we,' they told me."

While piloting a metal box created by self-confessed submersible novices may sound like a deathwish, Garvin says the unconventional approach was actually the bedrock of the mission's success.

"A traditional submersible company would go far over budget, it wouldn't be flexible enough to get the design we needed, and would get hogtied with red tape," he says.

What the team of 40 builders and crew came up with was an unconventional design for unconventional goals.

Seven metres long, with steel walls almost seven centimetres thick to withstand the 16,000 PSI of pressure at the bottom - pressure more than 1000 times stronger than at the surface. Weighing 12 tonnes, the vehicle was armed with 3D cameras and other scientific equipment to sample the environment at the ocean depths.

It had to be heavy enough to reach the bottom, but light enough to rise to the surface again; able to withstand searing 40-degree temperatures on the surface, and freezing temperatures on the ocean's floor. Despite the size of the vessel, James Cameron - standing at over 1.8 metres tall - had to squeeze into a pilot's space no bigger than a refrigerator.

"This ragtag team was able to come up with some elegant solutions to the endless problems that came up," Garvin says.

In the film's trailer, Garvin outlines the expedition faced and overcame "a hundred horrible ways to die" at the depths of the sea.

As the mission's manager for submersible internals and life support, he says that statement was far from hyperbole.

"The biggest danger was the pressure of the water outside the viewing sphere, if the sphere would crack and fail. At that depth, a jet of water would slice the pilot in two - they would be a meat cloud in less than a second," he says. "The oxygen delivery system could fail. Deep vein thrombosis was another, with the pilot sitting in a foetal position for 10 hours."

However, he says the biggest danger was an unexpected one for a vessel that would be under water.

"Jim's biggest fear was fire. There was a tragic accident in the space race when astronauts were consumed in an oxygen fire, in such a confined space with all these electronics," Garvin says.

As the vessel's safety officer, Garvin spent hundreds of hours inside a dive simulator testing electronics, safety systems and failsafes. He racked up over 500 hours, including a marathon 18-hour session, cramped inside a simulator barely wide enough for him to fit.

Deepsea Challenger was tested several times before the big dive into the Pacific Ocean - but not as many times as would be expected. A blowout in construction time and a rapidly dwindling window of good weather necessary to launch and retrieve the sub in the remote waters off Guam meant the vessel was tested just a handful of times.

"The first dive was in Sydney harbour, to a depth of one metre. Most of the systems failed," Garvin says.

Deepsea Challenger was then tested in Jervis Bay. Just as kinks began being worked out, tragedy struck - Andrew Wight, Garvin's brother-in-law and the film's producer and director, and American cinematographer Mike deGruy were killed in a helicopter crash near Jasper's Brush in February 2012 as they prepared to film an on-water test.

Aside from the emotional toll it took on Garvin and the crew, the tragedy proved a turning point for a mission already on the ropes - could the expedition be salvaged, or should it be abandoned?

"We stalled, but after much soul-searching we regrouped and went on in honour of their memory. They were both very experienced explorers, every part of their make-up was driven to explore and overcome," Garvin says.

"Obviously they are featured in the story. When I first watched the film, to see them on screen again was very confronting. I maintain they would be proud of this film, and it's a fitting tribute to them. I'm very proud we managed to regroup and go on to accomplish the extraordinary."

In short, the mission was a success. On March 26, 2012, James Cameron reached the depths of Challenger Deep, spending nearly three hours at the bottom.

"When he called and said he was at the bottom, our ship exploded in celebration," Garvin says. "It felt like the moon landing, like 'the eagle has landed'. After all the doubt, the naysayers, it was an incredibly touching moment."

There are plans to return the sub to its birthplace and display it in a Sydney museum, but the legacy of Deepsea Challenge is set to extend beyond the initial mission itself.

"I'm writing the pilot manual for the submersible, documenting the technology we developed. We're confident our designs can be used in a whole new generation of submersibles," Garvin says.

While the mission itself was a success, Garvin says he hoped the true impacts of the expedition would be felt far beyond sea exploration. "It's an inspiring story to teach kids it is OK to dream big. Hopefully it's a clear message to younger generations that we have to explore, we can't stagnate," he says.