A robotic sub called Nereus has reached the deepest-known part of the ocean.

The dive took place on 31st May, at Challenger Deep in the Marianas Trench, located in the western Pacific Ocean.

At 10,902m below sea-level this makes Nereus the deepest-diving vehicle currently in service and the first vehicle to explore the Marianas Trench since 1998.

"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 named after the HMS Challenger which first sounded the depths on 23 March 1875.

The Challenger Deep is the deepest abyss on Earth at 11,000m-deep, more than 2km deeper than Mount Everest is high. At that depth, underwater pressures reach 1,100 times those at the surface. 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.

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. 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.