There was none of the usual Hollywood fanfare as James Cameron pulled up anchor and headed out to sea with his cast of adventurers last Monday on the final, spectacular leg of what will arguably be the greatest production he has ever worked on.

The departure of the Mermaid Sapphire and Barakuda expedition vessels from Apra Harbor Guam, set the famously thrill-seeking film director on course to dive in a 12-ton submersible to the bottom of the world, a place of perpetual cold and darkness, life-threatening depth, and abiding mystery.

Inclement weather has kept Cameron from his goal over the last few days, prolonging the suspense and forcing him and his team of more than 60 support workers, technical advisers, scientists and family to wait it out for most of the week on the tiny Pacific atoll of Ulithi.

But with a possible window in the weather Saturday, the big moment could now be just hours away. Cameron, 57, and his Deepsea Challenger are ready to roll, he announced through National Geographic, his expedition partners, early this morning.

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The journey will, if all goes well, take him seven miles below the surface of the Pacific Ocean to Challenger Deep, the lowest known point on Earth, located in the Mariana Trench south-west of Guam.

If he descends safely, he will be the first person to complete the journey alone to what he describes as “a place less understood, more forbidding and perhaps less forgiving than the farthest reaches of space.”

Among his advisers currently aboard the Mermaid Sapphire mother-ship is a man who might be considered the ultimate authority on Challenger Deep, retired US Navy Captain Don Walsh, 80, who took the plunge aboard the steel bathyscaphe Trieste in 1960 with Swiss oceanographer Jacques Piccard.

No one had made the 36,070-foot trip before and no one has done it in the 52 years since.

What can Cameron expect? “For us, it was rather like a foggy day in your car, where you turn the lights on and the fog comes back in your face,” says Capt. Walsh, whose Trieste crewmate died four years ago.

“As you descend, you lose your natural light fast and you’re in the abyss pretty quick – no sunlight penetration at all. But if you turn off the lights, there’s this host of bio-luminescence generated by creatures of the abyss. It’s not that bright, like little snowflakes with light reflecting off them. As you pass down through them, the illusion is that they’re streaming up from below.”

Despite measuring 24 feet in total, Deepsea Challenger affords its 6 foot, 2 inch pilot a space just 43 inches in diameter; the bigger one builds the pressurized compartment, the thicker its walls need to be and, therefore, the heavier the sub will become. Smaller equals lighter, equals slicker.

The water pressure will be so great – eight tons per square inch – that the sub’s dimensions will shrink by around 2.5 inches (6.3 centimeters) on the way down.

Cameron has been practicing yoga to help him fold his frame into the cramped, spherical space and maintain his position for nine hours – 90 minutes to two hours for the descent, six hours exploring the sea floor, and an hour back up to the surface.

The sub is rigged with high-tech gadgetry and controls, life support systems, powerful lights to illuminate the underworld and externally-mounted sampling instruments, including a curiously-named “slurp gun” to suction up biological and geological specimens.

It will descend like a bullet, dragged vertically at 500-700 feet a minute by 1,000 pounds (450 kilograms) of steel weights fixed onto it by electro-magnets. Once on the ocean floor, Cameron will maneuver using joysticks to command 12 thrusters. When he’s ready to resurface, the weights will be jettisoned at the flick of a switch to allow the sub to rise back up.

Naturally for an Oscar-winning movie-maker with a passion for extreme ocean exploration, there are also eight cameras to record his every move and, perhaps, images of hitherto unknown creatures of the deep.

Other members of the team, including Captain Walsh – who has dived to the wrecks of the Titanic and the German battleship Bismarck with Cameron – were made to sign non-disclosure agreements to keep plans and technological details under wraps as the inventor, environmentalist, explorer, and director of films including The Abyss, Titanic, and Avatar labored for eight years on the project.

Whereas Sir Richard Branson declared last year that he intended being the first to send a manned craft to Challenger Deep, Cameron and Australian co-designer Ron Allum have long led the field.

“Unlike the other guys, Branson, who said ‘We’ll get to the deepest point in the ocean by the end of 2011,’ Jim’s not made a big show of things. This has never been a race,” said Capt. Walsh.

Nor is Cameron’s expedition just about bragging rights. He will explore both the seabed and the sheer cliffs that lead down to it, gather samples of sediment, and collect sea life specimens with a grappling claw and suction gun.

Scientists from Scripps Oceanographic Institution and NASA, among others, will be waiting topside to take charge of his haul, hoping that it will bring them the kind of thrills that his five-mile test-dive in the New Britain Trench off Papua New Guinea yielded earlier this month; giant shrimp-like creatures measuring seven inches (17 centimeters) long were brought to the surface with him and others twice the size were caught on camera.

Whatever life lurks in the Mariana Trench, scientists believe that it will open up a whole new appreciation of the oceans’ past, present, and future.

“One of the issues on everyone’s mind is how much we’ve changed the ocean since Don and Jacques made their epic descent into the Mariana Trench half a century ago,” wrote Dr Joe MacInnis, a Canadian physician, scientist and deep-sea explorer who is on the expedition. “For the past 52 years we’ve been engaged in an all-out war against the creatures that inhabit its depths. Our weapons of mass destruction include nets, dredges, long-lines, harpoons, and toxic chemicals…We’re in the midst of a Darwinian death spiral affecting all life on Earth and every member of the human family,”

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“For all of us, Jim’s dive is more than a 14-mile round-trip into the deep end of the ocean. It’s a journey into the history, geography, and future of the planet’s major life-support system. He’s diving into a place that inspires the imagination and wounds the heart.”

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