A robotic submarine is undergoing final preparations to dive to the deepest-known part of the oceans.

一台潜水机器人正在为潜入已知的世界上最深的海洋做准备。

Nereus will switch between free-swimming and tethered configurations¹

If successful, Nereus will be the first autonomous²（自主的） 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.

"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".

"Below 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, notably³ 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（巡航） between the 23 May and 6 June.

Nereus aims to give researchers access to 100% of the seafloor

Deep records

The Challenger Deep is the deepest-known part of the ocean, located in the Marianas Trench⁴ 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.

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（深海潜水器） known as the Trieste.

The vessel⁵ consisted of a 2m-diameter (6ft) steel sphere（球体的）, containing the crew, suspended below a huge 15m-long (50ft) tank of petrol, designed to provide buoyancy（浮力）.

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 sediment⁶（沉淀物） core and record pictures of life, including a sea cucumber（海参）, a worm and a shrimp⁷.

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.