James Cameron dives deep for Avatar

Only once before has anyone made the seven-mile descent into the Pacific's Mariana trench, the deepest point on earth. Now film-maker James Cameron wants to repeat that incredible journey for his Avatar sequel

In the grim-grey light before sunrise on 23 January 1960 a stick of TNT exploded somewhere beneath the waves in the western Pacific and shot a plume of white water high into the sky with a shuddering ker-ump.

The crew aboard the USS Lewis, a Navy destroyer, had spent the past few days lobbing more than 800 charges overboard and were using the blasts to sound the ocean depths. The echo from the latest detonation took a full 14 seconds to bounce off the seabed and back to the ship. The vessel was 200 miles off the coast of Guam and directly above the deepest chasm in the world's deepest ocean.

As daylight broke, American sailors tossed buoyant flares into the water to mark the spot where Captain Don Walsh, a US Navy submariner, and Jacques Piccard, a Swiss engineer, would embark on their descent into "Challenger Deep", the name of the deepest fissure in the Pacific's Mariana trench.

Inside their steel submersible, the Trieste, the two men, sat on two little stools, descended for five hours into the gloom, unsure of what they would find and uncertain of whether they would return. A small bulb inside cast enough light to read the depth gauge, a thermometer and dials that monitored the water currents, but the powerful mercury vapour lamps on the submarine's hull were switched off most of the time. Any marine life that happened by might have seen a gentle glow in the darkness as the dimly lit hulk plunged deeper into the abyss.

As a tale of naval derring-do, Walsh and Piccard's journey to the deepest spot in the world's deepest ocean is all the more nerve-racking for the half-century old technology they relied on. Despite great leaps in underwater exploration, so far no one has attempted to repeat the descent. But that is about to change.

Film director James Cameron – the man behind Avatar, Aliens, and aptly, The Abyss – has gathered a team of engineers and given them the job of building a submersible capable of returning to the Mariana trench. Cameron, who has filmed on the wreck of the Titanic, has said he plans to use his new submersible to gather footage for a sequel to Avatar. The vessel is being assembled in Australia and tests on the hull are already completed. Insiders say a trial dive could be on the cards later this year.

The prospect of a return to the Mariana trench comes as scientists are just beginning to understand the importance of the deepest realms of the oceans. These are habitats with extraordinary and unique lifeforms; places that behave like deepwater stores for the carbon locked up in marine life when that life comes to an end and gravity drags them down.

Cameron's engineers will have pored over the details of Walsh and Piccard's descent in the hope of avoiding the kinds of glitches and heart-stopping moments that put the mission somewhere on a line between bravery and bravado. Less than an hour into the descent, at a depth of 4,200ft, a dribble of water appeared and meandered down the wall of the vessel. It soon stopped. At 18,000ft the vessel sprang another leak but it too sealed itself again. At 32,400ft, which is deeper than Mount Everest is high, there was a dull crack and the Trieste's cabin shook hard. For a few minutes, Walsh and Piccard stopped everything on board that made a noise. Tiny cracking sounds came from all around, but the submersible seemed OK. The echo sounder used to locate the bottom of the ocean revealed nothing but more water beneath. "Let's go on," said Piccard.

At 12.56pm, the echo sounder quivered and scrawled its first trace as the ocean floor came within reach. Walsh and Piccard switched on the Trieste's lamps and peered into what turned out to be crystal-clear water. The traces on the echo sounder got stronger and stronger, until at 13.06, the Trieste landed on the bottom, kicking up a cloud of white ooze. The depth was recorded as 35,800ft, just shy of seven miles and as deep as a transatlantic passenger plane flies high. The pressure of all that water on the cabin was around 200,000 tonnes.

Walsh and Piccard spent only 20 minutes on the bottom of the ocean – enough time to eat a Herschey's chocolate bar and test their experimental acoustic telephone system – but their descent into the Mariana trench set a record for man's deepest dive. Jacques Piccard died in November 2008; but Walsh, now 79 and living in Oregon, advises a company called Deep Ocean Expeditions, which arranges trips in submersibles to iconic wrecks including the Titanic. He recalls why he signed up for the Trieste mission with what turns out to be characteristic understatement. "I had to get away from my desk job," he says. "And it seemed like interesting work."

Walsh and Piccard's ascent back to the surface went without a hitch. The crack that made the vessel shake on the way down was a Plexiglas window fracturing, not from the pressure but the cold. The cracks sealed themselves as the Trieste rose into warmer waters and eventually broke through the waves, in time for two navy planes to zoom overhead and tip their wings in salute. Getting out was a relief. "By the time all our kit was inside, there wasn't much room for us," says Walsh. "It was about as much room as you get inside a large household refrigerator. And about the same temperature."

When Walsh and Piccard talked about the record dive later that day they agreed that someone would be back in those waters, doing the same descent, within a year or two. But the Trieste's mission to explore the deepest ocean on Earth turned out like Nasa's Apollo project to the moon. Once it was done, no one went back.

The Mariana trench is not a small crack in the ocean floor. Research vessels have mapped the trench at close to 2,500km long and around 70km wide. The huge scar is a testament to the violence of plate tectonics. This is where the vast Pacific plate bends steeply beneath the Philippine plate, leaving a huge groove in the Earth's crust. The deepest part of the trench, named Challenger Deep after the Royal Navy's HMS Challenger II vessel that surveyed the region in 1951, may be a tear in the Pacific plate. To oceanographers, Challenger Deep is beyond the what they call the Abyssal zone. Go deeper than 6km and you enter the Hadal zone, named after Hades, the God of the Greek underworld.

Deep ocean trenches are not teeming with the diversity of life seen in shallower waters. Sunshine cannot penetrate this deep, though many organisms generate their own meagre light, through a process known as bioluminescence. There is little or nothing in the way of seaweed or kelp. The towering walls of the trench are bare and rocky in places, but overwhelmingly they are covered with sediment that builds up as dead plankton, the glass skeletons from algae called diatoms, and the excrement from all the marine life swimming above, descend and settle. Some of the sediments are so soft that oceanographers are careful to make sure that any equipment they send down does not trigger vast underwater avalanches.

Though humans have not descended into the Mariana trench since Walsh and Piccard, the technology used to explore the ocean depths has transformed. The Trieste was designed by Jacque Piccard's father, Auguste, a physicist, inventor and hot air balloon enthusiast. In the 1930s, Piccard Sr flew a series of air balloons with pressurised gondolas capable of soaring to record altitudes. Before the decade was out, he realised that, using the same principle, he could build a machine to drift through the ocean depths. The Trieste was a natural progression of this thinking: instead of a basket held aloft by a balloon of hot air, the Trieste was a steel cabin kept buoyant by 32,000 gallons of lighter-than-water gasoline. The Trieste sank with a ballast of tonnes of iron pellets. To ascend, Piccard simply jettisoned the pellets by cutting the power to an electromagnet that held them in place.

Walsh and Piccard came back without any pictures of the bottom of the Mariana trench. The cloud they kicked up as they touched down obscured their view from the port holes and made photography impossible. "It was like being in a bowl of milk," says Walsh. But shortly before reaching the bottom, Piccard saw what he believed to be a flat fish, resembling a sole, about a foot long and half as wide, on the seafloor. The claim was contested almost immediately by a Danish biologist, Torben Wolff, who thought Piccard had probably seen a sea cucumber. Today, many marine biologists are sceptical that a flatfish would venture so deep.

In the 50 years that have passed, oceanographers have developed sophisticated underwater robots that can plumb the depths of deep trenches such as the Mariana and bring back stills and video. Robotic "landers" operated by Aberdeen University have filmed fish at a record-breaking depth of nearly 8km. Hordes of ghostly white snailfish, which resemble foot-long tadpoles with suckers on their bellies, appeared when one lander released bait in front of its onboard camera.

"The startling thing is that we see so many of them," says Monty Priede, director of Aberdeen's Oceanlab. "These fishes are deep-sea specialists. In each trench we explore, there seems to be a different species of snailfish." For scientists, this poses a tough question. How did the deepest parts of the oceans, which are effectively isolated from one another, become home to such subtly different versions of these peculiar creatures?

Shrimps abound at extreme depths. The steep walls of undersea trenches funnel dead marine life and other organic matter down into the deepest crevices, gathering the food so the shrimps don't have to. "This is the end of the world. This is where all the rubbish from above is going to end up, and all these shrimps are waiting for it," says Priede.

A few days before Christmas, Ronnie Glud, who holds academic positions at the University of Southern Denmark and the Scottish Association for Marine Science, returned from an expedition to send a robotic lander to the bottom of the Mariana trench as part of a broader project to map carbon in the world's seabeds. The lander confirmed Glud's suspicion that deep ocean trenches store a disproportionate amount of carbon, making them an unrecognised, but major component in the planet's carbon cycle. "We need to know how much is being stored down there. The more carbon that is captured in the seafloor, the more oxygen there is in our atmosphere," Glud says. Expeditions to other deep-sea trenches are in the pipeline.

There is a fallacy that life cannot survive the crushing pressures at the bottom of the deepest ocean trenches. To humans this is profoundly inhospitable territory, but fish, microbes, sea cucumbers and small crustaceans, including shrimps, have adapted. One oceanographer described how as follows: a human is like a party balloon – taken down to great depths it will be crushed to nothing; but a deep-sea fish is like an untied balloon – take that down to the bottom of the ocean and the pressure has little effect. Perhaps the greatest difficulty is that intense pressure causes nerves to stop working, a problem that deep-sea life has side-stepped by evolving a more robust physiology at molecular level.

Last year, researchers at Woods Hole Oceanographic Institute in Massachusetts steered their Nereus robotic vehicle 10,902m down into Challenger Deep. The robot spent more than 10 hours on the seafloor, beaming video back to the surface via a hair-thin optic fibre. The expedition collected sediments and rock samples, but was intended as a trial run ahead of a comprehensive scientific mission to explore the trench sometime in the next two years.

"Every time people look they make new discoveries about these deep regions of the oceans," says Andrew Bowen, director of the National Deep Submergence Facility at Woods Hole. "Think of the Earth as a puzzle. Only through scientific understanding and exploration can we begin to understand the shapes of the pieces and fit them together into a coherent view of the world."

Cameron's plans to descend into the Mariana trench are more about the technical challenges of building the submersible and filming at depth than a route to scientific discovery. But Bowen believes that repeating Walsh and Piccard's record-breaking dive can only be a good thing. "Exploring such an inhospitable and extreme realm of the deep ocean is hugely valuable, because it creates a chance for people to engage their imagination. There is nothing quite like being there," he says.

Back in Oregon, Walsh says he wasn't anxious about his descent in January 1960, and adds that going so deep that day meant just "a longer day at the office". He is more than happy about Cameron's plans to repeat the dive. "I wish him luck," Walsh says. "I'll be cheering him all the way down."