Nereus can switch between free-swimming and tethered configurations
A robotic sub called Nereus has reached the deepest-known part of the ocean. The dive to 10,902m (6.8 miles) took place on 31 May, at the Challenger Deep in the Marianas Trench, located in the western Pacific Ocean. This makes Nereus the deepest-diving vehicle currently in service and the first vehicle to explore the Marianas Trench since 1998. The unmanned vehicle is remotely operated by pilots aboard a surface ship via a lightweight tether. Its thin, fibre-optic tether to the research vessel Kilo Moana allows the submersible to make deep dives and be highly manoeuvrable. Nereus can also be switched into a free-swimming, autonomous vehicle. "With a robot like Nereus, we can now explore virtually anywhere in the ocean," said Andy Bowen, project manager and principal developer of the sub at the Woods Hole Oceanographic Institution (WHOI). "The trenches are virtually unexplored, and I am absolutely certain Nereus will enable new discoveries. I believe it marks the start of a new era in ocean exploration." The Challenger Deep is the deepest-known part of the ocean, and part of 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 those at the surface. THE NEREUS SUBMERSIBLE Weight on land: 2,800kg Payload capacity: 25kg Maximum speed: 3 knots Batteries: rechargeable lithium ion
As a result, only two vehicles have ever made the trip to its crushing depths. 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 10,916m (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. 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.
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A Woods Hole Oceanographic Institution (WHOI) remotely operated vehicle touched down 10,902 meters (35,767 feet) in the Mariana Trench near the island of Guam.
Mariana Trench 

On Sunday, May 31, Nereus dove into the Pacific Ocean and began its descent into Challenger Deep the deepest area of the Mariana Trench.

"It's the deepest known part of the ocean. The trench is virtually unexplored, and I am absolutely certain Nereus will enable new discoveries," said Andy Bowen, project manager of WHOI, "Reaching such extreme depths is the pinnacle of technical challenges. The team is pleased that Nereus has been successful in reaching the very bottom of the ocean to return imagery and samples from such a hostile world. With a robot like Nereus we can now explore anywhere in the ocean. The trenches are virtually unexplored, and Nereus will enable new discoveries there. Nereus marks the start of a new era in ocean exploration."

However, this is not the first time that a mission was sent into the trench. In 1998, the Japanese robot Kaiko was launched. In 1960 a two man bathyscaphe vehicle, the Trieste descended 10,916 meters (35,813 feet) to the ocean floor.

Nereus can be operated remotely or it can run as an autonomous underwater vehicle AUV. It is currently attached to the mother ship via an optic tether the diameter of a human hair.

"Much of the ocean's depths remain unexplored. Ocean scientists now have a unique tool to gather images, data and samples from everywhere in the oceans, rather than those parts shallower than 6,500 meters (4 miles). With its innovative technology, Nereus allows us to study and understand previously inaccessible ocean regions," said Julie Morris, director of the National Science Foundation (NSF)'s Division of Ocean Sciences.  NSF assisted with the US$8 million project funding.

Nereus will send videos and collect samples on the ocean floor at the trench's subduction zone and area during its mission.

"The samples collected by the vehicle include sediment from the tectonic plates that meet at the trench and, for the first time, rocks from deep exposures of the Earth's crust close to mantle depths south of the Challenger Deep. We will know the full story once shore-based analyses are completed back in the laboratory this summer. We can integrate them with the new mapping data to tell a story of plate collision in greater detail than ever before accomplished in the world's oceans," said geologist Patty Fryer of the University of Hawaii.