A remote-controlled robotic vehicle that once survived a trip to the ocean's deepest spot has been lost on a mission 6.2 miles below the water's surface off the coast of New Zealand.

Researchers believe the remotely operated vehicle called Nereus likely imploded due to deep-sea pressures while exploring the Kermadec Trench on Saturday, Live Science reported. During the trip, the ROV was exposed to pressures of 16,000 pounds per square inch, and debris from the robot has been recovered at the water's surface.

"It's an ever-present risk," Larry Madin, director of research at Woods Hole Oceanographic Institution, told The Boston Globe.

"The way things go down there, everything's operating normally and suddenly everything goes dark, and when it's 6 miles down in the ocean, you don't have a good way of knowing what's happened--you can only make inferences based on what you know of the system and the forces that could be acting on it."

The Kermadec Trench is 32,963 feet below the ocean's surface and comes in second only to the Mariana Trench, which is 36,201 feet down, according to Live Science.

Nereus, which launched in 2008 and was one of only four vehicles ever to have reached the Mariana Trench's Challenger Deep, had just started off on a three-year mission to explore the deep sea and its ecosystems.

"Without Nereus, we currently have nothing that will enable us to reach that part of the ocean," Madin told The Boston Globe.

Losing the ROV was a disappointing shock to the research team, which had planned to study deep ocean life as part of a federal program. The $8 million project may not be able to move forward without the help of Nereus.

On various earlier dives, the robot powered by more than 4,000 lithium-ion batteries had been using its mechanical arm to gather samples of sediments as well as biological specimens of deep-ocean creatures.

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