By Jonathan Fildes
Science and technology reporter, BBC News
Nereus will switch between free-swimming and tethered configurations
A robotic submarine is undergoing final preparations to dive to the deepest-known part of the oceans.
If successful, Nereus will be the first autonomous vehicle to visit the 11,000m (36,089ft) Challenger Deep in the Pacific Ocean.
Only two other vehicles have ever visited the spot before, both of them human operated.
The $5m submarine will make the attempt in late May or early June after a series of increasingly deep dives.
From 6,500m to 11,000m metres Nereus has the field pretty much to itself
Ian Rouse National Oceanography Centre
"Instead of jumping directly into the deep end of the swimming pool with the vehicle, we'll probably dip our toe in first," said Andy Bowen of the Woods Hole Oceanographic Institution (WHOI) and one of the designers of Nereus.
"We'll work at 1,000m, 4,000m, 8,000m and then take a deep breath and see if we can get to 11,000m."
Ian Rouse, head of the deep platforms group at the National Oceanography Centre, Southampton, described the project as a "great technical challenge".
"[At less than] 6,500m deep (21,325ft) there are vehicles that can do a better job than Nereus due to its compromises in design," he told BBC News. "However from 6,500m to 11,000m Nereus has the field pretty much to itself."
Other teams, notably the British, French, Russian and Japanese will be watching the mission "with interest".
"The Nereus team [is] very experienced in designing and building other underwater vehicles, so I have no doubt they will succeed," said Mr Rouse.
The tests will take place on a research cruise between the 23 May and 6 June.
The Challenger Deep is the deepest-known part of the ocean, located in the Marianas Trench near the island of Guam in the west Pacific.
It is the deepest abyss on Earth at 11,000m-deep, more than 2km (1.2 miles) deeper than Mount Everest is high. At that depth, pressures reach 1,100 times the pressure at the surface.
Nereus aims to give researchers access to 100% of the seafloor
As a result, only two vehicles have ever made the trip to its crushing depths.
"From an engineering perspective, building something to operate in such an extreme environment is a huge challenge," said Mr Bowen.
In January 1960, Jacques Piccard and Don Walsh made the first and only manned voyage in a Swiss-built bathyscaphe known as the Trieste.
The vessel consisted of a 2m-diameter (6ft) steel sphere, containing the crew, suspended below a huge 15m-long (50ft) tank of petrol, designed to provide buoyancy.
During the nine-hour mission, the two men spent just 20 minutes on the ocean floor; enough time to measure the depth as 10,916 metres (35,813 ft).
No manned submersible has ever repeated the dive.
However, 35 years later, a Japanese remote-controlled vehicle called Kaiko returned, setting a depth record for unmanned exploration.
During its dive, the vehicle recorded a depth of 10,911m (35,797ft). It was also able to recover a sediment core and record pictures of life, including a sea cucumber, a worm and a shrimp.
THE NEREUS SUBMERSIBLE
Weight on land: 2,800kg
Payload capacity: 25kg
Maximum speed: 3 knots
Batteries: rechargeable lithium ion
Unlike Nereus, Kaiko had to rely on a cable connected to a ship at the surface for power and control.
The Japanese craft was lost in 2003 on an unrelated dive when a cable connecting it to its control ship snapped.
Currently, the deepest-rated vehicles are able to descend to 6,500m, allowing scientists access to 95% of the seafloor.
Nereus aims to change this to 100%, whilst also allowing scientists to survey a much larger area than vehicles like Kaiko.
It is able to do this by switching between two different configurations - free-swimming and tethered - depending on the type of mission.
In its intelligent, autonomous mode, Nereus can map large areas of the ocean floor (animation courtesy of Woods Hole Oceanographic Institution)
"The autonomous vehicle, as the name sounds, has autonomy from the human operators onboard the ship," explained Mr Bowen.
In this configuration, Nereus is able to fly pre-programmed missions, mapping vast swathes of the seafloor.
"It has sufficient onboard intelligence and batteries to find areas of particular interest through the use of chemical sensors, sonar and digital photography," said Mr Bowen.
Once complete, the submarine will automatically return to the ship where it can be converted to a remote-operated vehicle (Rov).
This involves adding a mechanical arm, which allows Nereus to gather samples and deploy instruments, as well as a single, 40km-long (25 mile) fibre optic cable that allows scientists to control the vessel from the ship.
Nereus can be transformed into a remote-controlled vehicle (animation courtesy of Woods Hole Oceanographic Institution)
Transforming the sub between the two different modes takes place on the ship's deck and takes around 12 hours.
Although hybrid vehicles have been designed before, Nereus will be the first to visit the entire ocean floor. It will also be the only vehicle currently in operation with this capability.
To do this, the engineering team has had to use new technologies and materials.
"We have had to throw away almost all of the extant technologies that we've been using for decades," Professor Chris German, also of WHOI, told the BBC.
For example, the team had to do away with conventional cables for the tethered vehicle.
"They typically had a steel casing, copper for conducting power down them and optical fibres for collecting data," he said.
"None of that works because we can't build cables that are strong enough to go down to 11,000m and not break at the top and yet be flexible enough to allow us to move around when we get there."
As a result of limitations like these, he said, the Japanese craft Kaiko had limited manoeuvrability and needed a custom-built ship for spooling out the reams of heavy cable required to attach it to the ship.
Nereus, on the other hand, uses rechargeable lithium-ion batteries, similar to those used in laptop computers, for power and a single hairs-width fibre optic cables - borrowed from Torpedoes - for control and telemetry.
Each charge allows the vehicle to remain submerged for 20 hours.
I think we are going to see all kinds of new life forms
Tim Shank Woods Hole Oceanographic Institution
In addition, the engineers have replaced traditional materials used to build similar craft, such as titanium and glass, with new, lightweight materials.
"We have very thin-walled and light ceramic material which has had to be made to withstand these huge pressures," explained Professor German.
However, the mission is about more than just proving new technology. Scientists are keen to get their hands on the new vehicle to explore the ocean depths.
"I think we are going to see all kinds of new life forms," said Tim Shank, a biologist at WHOI.
"There are going to be novel habitats, novel species and novel adaptations."
Previous discoveries at shallower depths have taught scientists to "expect the unexpected", he added.
"There are shrimps that are found at vents in the Atlantic that swarm by the millions that have lost their eyes."
Instead of eyes, the creatures have a U-shaped sensory structure on their back, which may be used to detect heat, Professor Shank explained.
Scientists plan to use Nereus to study destructive Earth processes
"Nobody would have anticipated that this one species would have evolved this kind of adaptation."
Geologists are also hoping to use the vehicle to directly study destructive Earth processes, such as where the oceanic crust is recycled into the planetary interior.
"We can study mid-ocean ridges where new ocean crust is produced, but we cannot study the other end in the same way," explained Professor German.
These subduction zones, as they are known, are commonly found along the edge of the Pacific, where oceanic crust plunges under buoyant continental crust.
The Marianas Trench - the location of the Challenger Deep - was formed when the Pacific Plate was thrust beneath the smaller Mariana Plate.
Professor German added: "Given that, among other things, it is in these subduction zones that the world's largest earthquakes ever recorded, [these areas] are potentially, really quite significant."
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