Once more unto the deep (Image: Matthew Oldfield/Getty)

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Half a century after our first and only visit, humans are returning to the deepest point in the ocean to uncover its secrets

IT IS a sobering fact that more people have walked on the moon's surface than have visited Earth's lowest spot. During six Apollo missions between 1969 and 1972, 12 humans did the moonwalk. Just two have plumbed Earth's ultimate depth. Their names are Jacques Piccard and Don Walsh, and their expedition to the Challenger Deep - the deepest point of the Mariana trench, some 11,000 metres below the surface of the Pacific Ocean near the island of Guam - lies over 50 years back, in January 1960.

The immense pressures and extreme cold of the ocean deep make reaching it a technologically demanding business. But sending humans into space isn't exactly easy. The discrepancy is largely down to politics: while the space race was fuelled by the rivalries of the cold war, no such spur has ever existed to encourage exploration of our oceans.

With the will to fund such ventures from the public purse on the wane, at least in the US and Europe, various private initiatives claim to be poised to take over the baton in the space race. Meanwhile something is stirring in the oceans too. The US film-maker James Cameron plans to send a crewed submersible into the Mariana trench to film footage for his follow-up to the filmAvatar. And earlier this year, the British entrepreneur Richard Branson launched his one-person Virgin Oceanic submarine with the goal of visiting the deepest points of all of Earth's five oceans, the Challenger Deep included (see diagram). It is currently doing training runs with a view to performing the first full dives before the end of the year.

Branson is famous for his ability to attract publicity with well-funded, well-branded derring-do. "I didn't really understand why the oceans weren't being explored - it seems crazy when you think about it," he says. But this isn't just a Jules Verne-style yarn. A cadre of researchers have signed up to the project, and they have a long list of questions they want answered, from the ecology of deep-sea trenches to the role of trenches in Earth's geology. Is the light of science finally about to shine on our planet's deepest places?

The last time humanity made it down into the Mariana trench, it was ensconced in a contraption a little like a hot-air balloon in reverse. Piccard and Walsh's US navy submersible, Trieste, sank under its own weight into the trench and then discarded tonnes of iron shot to float back up. It spent 20 minutes at the bottom of the Challenger Deep, during which time the pair ate chocolate bars and looked out of the window. As they did so, they saw a shrimp-like creature float by in the inky blackness - a first proof that life could survive in a world with pressures well over 1000 times those at sea level.

In 1995, Japan's uncrewed Kaiko submersible provided further tantalising evidence of life down there in the shape of photographs of a worm, a sea cucumber and some shrimp. In May 2009, the uncrewed Nereus submersible, operated by researchers at the Woods Hole Oceanographic Institution in Massachusetts, collected liquid and rock samples from the bottom. And that's it - the sum total of humankind's interactions with the ocean's deepest depths.

The Mariana trench is a narrow scar in Earth's surface some 2500 kilometres long and averaging 70 kilometres wide. It began to form by the process of subduction some 50 million years ago, as the Pacific tectonic plate began to dive under the smaller Mariana plate to the west. The trench's depth is such that the currents that shuffle organisms around the rest of the ocean floor do not penetrate to its bottom. That makes the Mariana, and other trenches like it, evolutionarily isolated.

Charles Darwin showed 150 years ago how a similar isolation led the fauna of the Galapagos Islands to take bizarre forms - tortoises that grew to huge sizes, for example - and created the subtle diversity of Darwin's finches, each perfectly adapted to its island niche. But islands are mostly hospitable places, and can be reached by flying organisms, spores and pollen. The crushing pressures, cold temperatures and utter darkness of the ocean trenches, meanwhile, make them lethal to most creatures.

That means life found there was probably present as the trenches first started to form, allowing it to slowly adapt to the changing environment. "It really makes us wonder whether there are lost worlds of microbial organisms down there," says Katrina Edwards, a marine biologist at the University of Southern California in Los Angeles who is working with the Virgin Oceanic team.

But studying the ecology of the deep ocean floor is not as simple as dropping a probe on a cable, trapping something and hauling it back up. Organisms that live so deep tend not to survive the journey to the surface. "The pressure change just kills a lot of them," says Edwards. Instead, she and her colleagues rely on expensive automated landers, which use water pumps, filters and lures to collect principally microbial life and analyse it in situ.

Such kit is impressive, but limited. Visibility in trenches is poor and even with a live video feed the environment around the probe is generally unknown. "It is often hard to tell whether we have landed it on a cliff, in a ditch or near a vent," says Edwards. This matters: we know from terrestrial ecology that biological activity and diversity vary hugely according to local geology and chemistry. In subduction zones such as the Mariana trench, geological activity gives rise to hydrothermal vents and undersea volcanoes that create substantial differences in chemistry and temperature over small distances.

This is where Virgin Oceanic could prove invaluable. Edwards and her team will first drop a series of landers to the bottom of Challenger Deep, each equipped with analysis equipment including microscopes and DNA sequencers, as well as lures to attract and capture any creatures swimming nearby. Then, in a 2-hour swim along the bottom, the project's submersible will survey the areas around the landers, supplying sonar and video feeds of the topography around. "Our goal will be to visit each lander," says Chris Welsh, who will pilot the submersible. Mass spectrometers on board will also look for chemicals such as amino acids that are associated with life. "The hope is that continuously sampling the water flow will reveal active life areas that the vehicle has overflown," says Welsh.

It is not just biologists who have a keen interest in trench topography. Patricia Fryer, a geologist at the University of Hawaii in Honolulu, is hoping it will help us work out how Earth's continents first formed.

According to one theory, Earth's first dry land was created early on in our planet's history, when undersea subducting plates forced up overlying rock that eventually reached the sea surface as island arcs. Over geological time, further subduction along fault lines coupled with other tectonic movements caused these island arcs to migrate, collide and fuse, creating ever-larger land masses.

If this was the case, then patterns of elements and isotopes found in the middle of continents today, far away from fault lines, should also be present in active subduction zones. High levels of rubidium, strontium, barium, beryllium and light rare-earth elements found in continental interiors are thought to come from fluids driven off subducting tectonic plates and incorporated into the rocks of the overriding plate. Testing this idea means analysing sequences of rocks found in island arcs and down the deep fault scarps that lead out into the adjacent trenches.

Again, this is something we have so far done blind, dropping probes to the ocean floor with little knowledge of local factors that might skew the chemistry - a messy and expensive business. "A single experiment dropped in the wrong place can cost us millions of dollars," says Fryer. The plan for the Virgin Oceanic dive into the Mariana trench is to submerge near the island arc of Guam and travel slowly down the slope towards the Challenger Deep, taking high-definition video that should help identify the best locations for sampling stations. "It should prove a treasure trove for research," says Welsh.

One giant dive for humankind

The sudden release of forces built up along subduction zones as plates slip over and past one another also spawns great undersea earthquakes, often unleashing devastating tsunamis such as the one that hit the north-east coast of Japan earlier this year. Chemistry probably plays a significant part: as an ocean plate is scraped, pressurised and heated on its tortuous journey into Earth's interior, a patchwork of rocks of different strengths is created that could control when and where seismic activity is concentrated. "The more we know about these processes, the better we can understand where earthquakes are likely to occur," says Fryer.

The Mariana trench provides the perfect environment in which to do that, thanks to the mud volcanoes dotting its slopes. Mud volcanoes belch not fire, but fluids containing finely ground pieces of the subducting plate along with bits of the overlying rocks. Arranged at varying distances from the trench bottom, they provide an opportunity to tap material from as far as 20 kilometres down and so monitor the chemical processes occurring there.

On the Mariana trench dive, the Virgin Oceanic submersible is scheduled to visit a number of the mud volcanoes to record the coordinates of those that are active. That will not be without risk: the area is so geologically active that the explosive expulsion of material and hot water is a possibility, although Welsh anticipates that any serious activity will be picked up before the mission starts. The dangers posed by features such as overhangs and caves that could trap or damage the submarine are much more substantial, he thinks. Beyond some very sketchy sonar measurements taken from the surface, "these areas are not mapped at all", says Welsh.

Which leaves the obvious question, is it necessary for the Virgin Oceanic submersible to be crewed? Such a project to explore the ocean's deepest places could surely be carried out remotely, just like the two previous successful missions.

A crewed submersible might have some additional flexibility to respond in real time to unexpected points of interest on the ocean floor, but it is here, perhaps, where science meets showmanship. Far fewer people would have watched the moon landings had it not been an actual small step for a real representative of humankind. Branson is looking for a similar legacy. "I hope projects such as Virgin Oceanic will inspire generations who didn't witness the lunar landing to become great scientists of the future," he says. With an immense range of crucial answers to be fished out from the deep, a little pizzazz might go a long way.

Matt Kaplan is a freelance writer based in London and Los Angeles