Media playback is unsupported on your device Media caption Before he set off for the Mariana trench, James Cameron told the BBC's Rebecca Morelle why he was risking it all to make the dive

Hollywood director James Cameron may be close to making a dive to the deepest place on Earth.

In a one-man submarine, he plans to dive 11km (seven miles) down beneath the waves to the bottom of the Mariana Trench, in the western Pacific.

There has only ever been one manned dive there, and that was half a century ago.

We are there to do science, but we are also there to take the average person who only imagines these things and show them what it is really like James Cameron

Mr Cameron and his team have set sail to reach the trench and are now waiting for calm weather to begin the dive.

The BBC met up with the director in Guam, just before he set out for the high seas. This tiny tropical island is the nearest major landmass to the Mariana Trench - the focus of Mr Cameron's ambition.

In the balmy heat, the team was making last-minute preparations for this journey to the deepest depth in the seas.

The Abyss and Titanic director has had a long-standing obsession with the oceans, but now he has created for himself the ultimate part.

In a prototype submarine, called the Deepsea Challenger, that fits just one person, he plans to make the first manned mission to the bottom of the trench for 50 years.

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He says he came up with the idea while he was using submersibles to film a documentary on the wreck of the Bismarck, a German battleship that lies 4,800m (17,500ft) underwater.

"I started to think about what would it take to go deeper, what would it take to go to full ocean depth - that was kind of the holy grail from an engineering standpoint," he told BBC News.

"So you start 'noodling' up designs, and thinking how it would be possible and what would it take. And then there is suddenly this moment that seems to transpire with no transition where you are suddenly doing it."

He adds: "I seem to have that curse that once I imagine something being built, I have to build it."

Mr Cameron and his team have spent the last few days in Guam, docked at the port, re-supplying the ship that has been their home for the last few months.

They arrived straight from Papua New Guinea, where the filmmaker performed a successful 8,200m test-dive. But the ultimate challenge will be to see whether their vessel can plumb the ocean's deepest point.

They will need calm seas to launch and recover the sub - but while in the harbour, a strong wind has blown in, white caps topping the waves.

Media playback is unsupported on your device Media caption Engineer David Wotherspoon shows off the Deepsea Challenger

The team are doing everything they can to make the sub as safe as they possibly can - but the weather is out of their control.

The craft, which is housed on the ship's deck in a large, air-conditioned hangar, is bright green, weighs 11 tonnes and is more than 7m (23ft) long.

Once in the ocean, it flips on its end, and descends vertically through the water column.

Image caption The emphasis is on total safety

The compartment in which Mr Cameron will spend his nine-hour dive is tiny: a thick, metal sphere with an internal diameter of just 109cm (43in). He will be curled up inside, unable to stretch his arms or legs.

The rest of the sub is made from specially designed syntactic foam, similar to the sort of thing a surf board is made from.

It counterbalances the weight of the pilot's compartment, which will have to keep the filmmaker safe from 1,000 atmospheres of pressure.

The submarine has been built and designed by an Australian team of engineers - many of whom have worked on James Cameron's films. And a lot of elements of the craft come straight out of the movie industry.

Image caption Like many on the team, Dianna Bisset has brought her theatrical skills to the project

Australian engineer Ron Allum, who co-designed the sub with Mr Cameron, is an expert at creating rigs that attach hi-tech cameras to submersibles.

But the Deepsea Challenger is the first submersible that he has built from scratch.

It is packed full of 3D cameras and huge lighting systems so that the director can capture the excitement of the voyage all the way to the bottom.

The straps that keep the pilot's sphere attached to the foam have been created by Dianna Bisset, who designs harnesses used in films and stage productions that help actors and dancers to "fly".

Even the life-support system has a theatrical link: its project manager John Garvin spent years playing the lead in the hit musical Buddy.

Image caption Don Walsh (left) will be very pleased to see his feat matched by the movie director

Mr Cameron fondly describes the 30-strong group as an "extremely talented, rag-bag bunch".

He says: "They have to be the kind of people who don't accept the normal limitations.

"They all have to be a bit 'whacked' to think it is possible to do something that is normally the province of governments or scientific institutions."

The director plans to release a documentary with footage captured on his dives - but he also has science in mind.

The sub, which has been financed by Mr Cameron along with National Geographic and Rolex, is fitted with robotic arms that can bring sediment, rocks and samples of deep-sea flora and fauna back up to the surface.

When he attempts the Mariana dive, a science team, headed up by Doug Bartlett from the Scripps Institute, will be dropping a lander - also kitted out with 3D cameras - which is baited to attract any passing life.

What lurks in the deep? Ocean trench: Scroll 11,000m down

Mr Cameron says: "Every single dive, I'm going to see something no-one's ever seen before.

"I'm going to do my best to image it, light it properly, bring it back in 3D - grab samples if I can, grab rocks if I can.

"We are there to do science, but we are also there to take the average person who only imagines these things and show them what it is really like."

Don Walsh, who made the first and the only manned mission to the Mariana Trench with Swiss oceanographer Jacques Piccard in 1960, has joined the team at sea for the dive.

The former US Navy captain has been bombarded with questions by the crew about his vessel, the bathyscaphe Trieste.

He's happy that 50 years after his dive, someone could finally be about to go back.

Image caption The sub will first make an unmanned trip to the bottom of the trench

He says: "This sub looks a little bit different from the way ours did, but I can see our fingerprints all over, 50 years later here.

"Our legacy of manned exploration was all of the technologies and innovations that we begat."

As the team set off from Guam, friends, family and colleagues wave and cheer until their ship fades into the distance.

If they can get a few days of good weather on their side, the team hopes to first make an unmanned 11km dive with the sub, to check it works properly at this deepest of ocean depths - and then it will be time for James Cameron to enter the craft and take the reins.

He tells the BBC: "I think the apprehension is before you are in the sub. Once I'm in the sub, I'm in the office, open for business.

"Who knows what we might find - every dive will be a revelation. That I can say for sure."

The dive will be a colossal test for this sub.

But given the risks involved, it will also be a huge gamble for Mr Cameron himself.