Filmmaker James Cameron makes a dangerous one-man journey to the bottom of the ocean Few men are fortunate enough to have pockets as deep as their ambitions, but yesterday the film director James Cameron embarked on a dive to the lowest point on Earth. He travelled alone into the 6.8-mile-deep Mariana Trench, nine hours each way, hunched in a tiny submersible built to withstand 1,000 times the pressure at sea level. In doing so, the multimillionaire behind Titanic and Avatar became only the third person to have visited Challenger Deep, the lowest point in this Pacific abyss. The Deepsea Challenger submersible, and its lone, rich, curious passenger, capture the imagination on several levels. It is science fiction brought dramatically to life, with one man voyaging to the bottom of the sea (the previous visit was a two-man effort in 1960). It is also hubris made good, the triumph of one human ego for the greater benefit of all. For not only did Captain Cameron have a director's eye - and a film rig's worth of lighting - trained on the murky depths, so that he could bring images of the deepest ocean to the big screen, but he also demonstrated the unbridled passion of the amateur scientist. Alongside the cameras there was a robotic arm, collecting samples of sediment and water and, of course, sea life. Cameron was also the courier of a lander designed by the Scripps Institution of Oceanography, one of the world's leading organisations for deep-sea science. The lander has 3-D cameras that will photograph passing fauna, attracted by bait. "The deep trenches are the last unexplored frontier on our planet, with scientific riches enough to fill 100 years of exploration," explained Cameron, before he set off on the most dangerous journey of his life. The director intuitively understands what many people don't: that the ocean depths are barely charted and unimaginably alien. They cover not just a vast geographical area but average a depth of around two and a half miles. The deep sea - definitions vary, but let's say below around 200 metres - is by far the world's largest habitat, providing more than 80 per cent of the space for life on Earth to exist. At depths down to around three miles, researchers have documented creatures that look like the stuff of nightmares. There are glowing octopuses, ghostly squid, creatures that look like flying mouths, and worms that resemble grasping tentacles. The waters contain an abundance of ugliness that is beyond even Pixar's imagination. We can only guess at the life forms that occupy the lower depths, floating silently in the intense pressures, low temperatures and permanent darkness, or lurking among sediment grains on the ocean floor. Last year, Scripps researchers released landers equipped with cameras - so-called Dropcams - into the Mariana Trench and were rewarded with pictures of enormous amoebas. These xenophyophores are the largest single-celled organisms known to science; some cells are bigger than teapot lids. Dropcams have also spotted translucent jellyfish down there. They are all examples of extremophiles: organisms that are supremely suited to the kinds of extreme environments that would kill most other organisms, including us. There is barely a corner of the Earth that does not host these exotic, baffling creatures: you will find extremophiles buried inside the pristine, icy frozen lakes of Antarctica and teeming inside the superheated mouths of underwater volcanoes. Extremophiles matter because they have barely changed since the birth of the planet - these "living fossils" give scientists an insight into how life might have started. In contrast to the oceans, space seems well-trodden (12 men have walked on the Moon) and ultimately barren: there, it is the journey rather than the destination that thrills, with the expensive rockets and the heroic endurance of the astronauts catapulted to the International Space Station. I have never envied them their tedious whirligig around the Earth, possibly embittered at being caught between two epoch-making generations: their predecessors, who first strode on the Moon, and their descendants, who will perhaps colonise Mars in the centuries to come. Unmanned space missions remind us repeatedly that the Solar System is devoid of life: if it is out there, it will be microbial rather than monstrous. The news that Ashton Kutcher, the Hollywood actor, has reportedly booked a $200,000 seat on Virgin Galactic - a bit like Virgin Holidays, but minus the beaches, cocktails or gravity - surely strips space travel of its last vestiges of glamour and mystery. Closer to home, Antarctica is no longer the virgin continent: armed with a doctor's certificate, some thermals and about £33,000, you can book a trip to the South Pole. That is why the oceans harbour such appeal: they truly are a last frontier, with Challenger Deep the ultimate destination. You have to be clever, determined, rich and brave to pull it off; no wonder that Sir Richard Branson and Google's ex-CEO Eric Schmidt were competing against Cameron with their own submersibles. Russia, too, covets the same destination and has engaged three teams of designers to submit submarine designs. There are thought to be at least a million undiscovered species in the deep sea - surely a more alluring trophy than a fistful of lunar dust.