One of the most capable deep-sea research subs in the world has gone missing,

At the time, the robotic vehicle Nereus was exploring the Kermadec Trench, one of the ocean's deepest spots, which lies north east of New Zealand.

Nereus was a flagship ocean explorer for the US science community, according to BBC News.

"Nereus helped us explore places we've never seen before and ask questions we never thought to ask," said Timothy Shank, from the Woods Hole Oceanographic Institution (WHOI), which managed the sub's activities, in a statement.

"It was a one-of-a-kind vehicle that even during its brief life brought us amazing insights into the unexplored deep ocean, addressing some of the most fundamental scientific problems of our time about life on Earth," Shank added.

The vehicle was lost on May 9, according to WHOI.

The $8 million robot was manufactured back in 2008 and could operate in autonomous mode or be controlled remotely, according to BBC News.

Nereus used a lot of innovative technologies that allowed it to go places that were off-limits to other research subs.

These technologies included rechargeable lithium-ion batteries, which provides extended power, similar to those found in laptop computers.

"To obtain some kinds of knowledge - particularly when physical samples are required for analysis - there is no alternative to sending equipment into the deep ocean, because the ocean's watery veil masks its depths from many forms of 'remote sensing," said British oceanographer Jonathan Copley, from the University of Southampton, in a blog post this weekend.

The project was funded by the U.S. National Science Foundation.

"And although we have learned a lot from a century or so of largely 'blind sampling' by equipment such as trawls and seabed corers (which are still fine for answering some questions in some areas), we now often require more detailed sampling and surveying, using deep-sea vehicles, to answer further questions," said Copley.

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