As director James Cameron plans to film Avatar sequel seven miles below the sea's surface, we go into the deadly deep with the only two men who've been there



Five thousand fathoms under the waves, a deafening clang rang out through the cramped, freezing submarine, causing the whole ­vessel to shake like a leaf.



Squinting through their tiny Plexiglas window into the abyss, the two explorers’ hearts missed

a beat.



‘It was a pretty hairy experience,’ they said afterwards with some understatement. The outer

layer of their porthole had cracked under the ­unimaginable weight of six miles of seawater — and they still had more than a mile to descend.



Fortunately, their so-called ‘bathyscaphe’ ­submarine, an extraordinary piece of Swiss-­Italian-­German engineering, sustained no further damage, and the explorers — Jacques Piccard and Don Walsh — lived to tell the extraordinary tale of this unique descent.



Twelve men have walked on the surface of the Moon and maybe 500 have travelled into space, but only Piccard and Walsh have visited the very deepest point of the ocean, which they reached on January 23, 1960.



The Challenger Deep dive was one of the most extraordinary — and surprisingly little known — feats of human exploration in history, the voyage in a submarine to a place even more extreme than the surface of most planets.



Now it has been announced that the multi-Oscar-winning film director James Cameron plans to add his name to the very exclusive club of those who have travelled to the ­bottom of the Challenger Deep, part of the Mariana Trench in the western Pacific, and the deepest known point in the world’s oceans.



Cameron — who, after all, made a fortune with Titanic — plans a follow-up to his billion-dollar 3D blockbuster Avatar, this time set in the teeming oceans of the film’s fictional alien planet of Pandora.



And last weekend it was reported that he has commissioned a bespoke submarine, built of high-tech, man-made composite materials and powered by electric motors, which will be capable of surviving the tremendous pressures at a depth of seven miles, from which he will shoot 3D footage that may be incorporated in Avatar’s sequel.



It seems bizarre that no one has repeated the feat of Piccard and Walsh in more than half a century (two unmanned submersible robots have made the trip since). But then no one has to date built a working replacement for their vessel, the Trieste.



Explorers Andy Rechnitzer, left, and Jacques Picard reached the deepest point of the ocean in January 1960

Designed by Challenger Deep pilot Jacques Piccard’s father, the Swiss scientist Auguste Piccard, and mostly built in Italy, the Trieste, which was bought by the U.S. Navy in 1958, is a truly extraordinary vessel.



Most deep-diving craft up to that point (and, indeed, up to today) were tethered vessels, linked to their ‘motherships’ on the surface by steel cables and umbilical cords to ­aid breathing.



The 50ft-long Trieste was, in ­contrast, a wholly self-contained submarine, free-diving and with its own life-support systems. It was not attached to the surface in any way during its extraordinary five-hour descent to the ocean floor.



The Trieste in some ways resembled an underwater airship. It consisted of two parts: a huge cigar-shaped ­‘balloon’ filled with 22,500 gallons of petrol to provide buoyancy (petrol is lighter than water).



Attached underneath this balloon was a tiny steel sphere, manufactured by Krupp of West Germany, just 7ft across, into which the pilots were crammed.



Effectively, it worked like a hot air balloon underwater, since the petrol in the balloon was incompressible, unlike air. So even at great pressure, the ­petrol balloon kept its shape and the craft remained buoyant.



But if the petrol in the balloon was lighter than water, how did the submarine descend? Nine tons of iron ­pellets were attached to the craft to make it sink — and when the pilots wanted to ascend again, they were ­jettisoned on to the ocean floor.



During the dive, temperatures in the dank, unheated pressure sphere fell to a few degrees above zero, and the ­shivering pilots ate chocolate bars to conserve their strength.



Oscar-winning film director James Cameron has reportedly commissioned a bespoke submarine to travel to the bottom of the Challenger Deep

At 30,000ft below the ocean surface, the outermost layer of their small Plexiglas porthole cracked, sending shockwaves reverberating through the submarine. Fortunately, the thick, cone-shaped block of transparent plastic in the window held.



After nearly five hours, descending at a rate of less than two knots, the Trieste settled a few inches above the floor of the lowest point on the Earth’s surface, a depth of 10,916m (35,814ft), where the crew spent 20 anxious minutes.



Conditions at the bottom of the Challenger Deep are almost ­unimaginable. Here, the seawater is more than a mile deeper than Everest is high, generating pressures of more than eight tonnes (the weight of a double-decker bus) per square inch.



The total force on the Trieste’s sealed capsule thus amounted to more than 177,000 metric tons. Even the strongest, titanium-hulled military submarines, built by the USSR, can dive no deeper than 3,000ft, sustaining hull pressures of ‘only’ 1,800lb per square inch.



The reason the Trieste could withstand the pressure was not only that its petrol balloon was incompressible, but also that the reinforced sphere in which its pilots sat was so tiny.



Even at the surface of the planet Venus, considered one of the most hostile environments in the solar system, ambient pressures are a mere sixteenth of those at the ­bottom of the Challenger Deep.



At the very bottom of the Pacific, it is pitch black; not a single photon of sunlight can penetrate to these depths. And it is cold, too. On the abyssal floor, water temperatures hover at a constant zero degrees.



No unprotected diver could possibly survive such extreme conditions. At these pressures the body’s many air-filled cavities would implode.



Despite this there is, amazingly, life. Piccard and Walsh, peering through their tiny porthole

and playing the Trieste’s external electric lamps onto the seabed through the crystal-clear water, saw several creatures, including a flounder-like flatfish and some shrimps. Oddly, the fish had eyes, even though there was no light with which to see.



The presence of clearly healthy marine animals shows that at these depths some oxygen must be present in the water — something thought unlikely before the expedition.



Blockbuster: Cameron's proposed film will be a follow-up to his billion dollar 3D blockbuster Avatar, this time set in the teeming oceans of the film's fictional alien planet Pandora

Piccard later said that ‘by far the most interesting find was the fish that came floating by our porthole. We were astounded to find higher marine life forms down there at all.’ The seabed itself down there ­consists of a thick layer of ooze, formed by the skeletons of trillions of microscopic sea creatures. At these depths, there are few currents and the water is very nearly still.



The Challenger Deep is at the southern end of the Mariana Trench, a 1,600-mile-long, arc-shaped, undersea chasm to the east of the Philippines. The trench is five to seven miles deep and 43 miles across, and is formed as a result of one vast slab of the Earth’s crust — the Pacific ­tectonic plate — being thrust westwards at a rate of a few inches a year underneath another, the smaller Mariana Plate.



This caused a gigantic geological fault called a subduction zone.



The Pacific is ringed by huge, active, grinding faults such as this, which give rise to the earthquakes and volcanoes that make life around the edge of this ocean so perilous.



So how is the film-maker and part-time scientist Cameron planning to follow in Piccard and Walsh’s footsteps? Not in the Trieste, which is on display at the Navy Museum in Washington DC.



According to the reports, Cameron has commissioned a team of ­Australian engineers to design and build a submersible capable of ­taking him to the floor of the ­Challenger Deep, and capable of filming in 3D at these depths.



The precise design of this submersible is unclear, but it is likely that it will resemble the $4million Deep Flight Challenger commissioned by the American aviator and explorer Steve Fossett in 2007.

Fossett was killed in a plane crash that same year, just before his ­Challenger was due to start sea ­trials. Fossett’s estate owns the submarine, which has never been used, but the design is the property of San Francisco-based firm Hawkes Ocean Technologies, founded by British engineer Graham Hawkes.



Intrepid: Walsh and Piccard viewed fascinating fish through the porthole of their craft

What is known is that it was ­constructed from Kevlar — used in body armour — and carbon fibre, and had a transparent dome made from pressure-resistant resin.



Hawkes Ocean Technologies have told me that they are not building Cameron’s submarine, although they have worked with him in the past, ­supplying submersibles used in the 2005 documentary movie he directed called Aliens Of The Deep.



The Hawkes design uses a ­completely different way of reaching the ocean floor to that used by the Trieste. Rather than passively sinking, the 17ft-long Challenger actively ‘flies’ downwards, using hydroplanes and electrically powered thrusters to descend. And unlike the Trieste, the Challenger can be manoeuvred with ease at depth.



If Cameron succeeds in his voyage to the bottom of the sea, what will he find? In all likelihood nothing more than the etiolated crustacea and fish spotted by Piccard and Walsh. But the abyssal floor at the bottom of this trench remains by far the least explored environment on Earth. Indeed, we have better maps of the surface of Mars than we do of the bottom of the Pacific.



And this means there are bound to be surprises. Many scientists ­suspect that some large animals, giant squid and perhaps even whales may occasionally plunge to extreme depths and survive, despite the cold, the pressure, the lack of light and absence of food.



And a few years ago a loud underwater noise nicknamed the ‘bloop’ was picked up by U.S. Navy sonars. It appeared to be coming from deep in the Pacific ocean. To this day no one knows its origin, though theories abound — ranging from a top-secret Russian submarine to some sort of gigantic sea monster new to science.



The fantastical colours of the big screen planet of Pandora, the ­fictional setting of Avatar, are unlikely to be present in this inky world of greys and browns.

