A remote-controlled research submarine that costs a whopping $8 million has gone missing off the coast of New Zealand.

The Hybrid Remotely Operated Vehicle Nereus was lost at 9,990 meters (6.2 miles) depth during its dive to the Kermadec Trench, according to the Woods Hole Oceanographic Institution (WHOI). Situated 75 miles off the coast of New Zealand, the Kermadec Trench is the fifth-deepest trench in the world.

The Hybrid Remotely Operated Vehicle Nereus was being operated as part of the Hadal Ecosystems Studies (HADES) Program funded by the U.S. National Science Foundation. Scientists believed a portion of unmanned research submarine likely collapsed under pressure as great as 16,000 pounds per square inch. It was working as part of a mission to explore the ocean's hadal region from 6,000 to nearly 11,000 meters deep.

The hadal zone is the name given to the deepest depths of the ocean, named after hades as the underworld god of Greek mythology.

Built in 2008 by the Deep Submergence Lab at the WHOI with primary funding from the U.S. National Science Foundation (NSF), Nereus can be operated either autonomously or controlled remotely from the surface. WHOI engineers incorporated some novel technologies into its design for use in remote operations, including an optical fiber tether for remote operations, ceramic flotation and lithium-ion batteries.

"Nereus helped us explore places we've never seen before and ask questions we never thought to ask," Timothy Shank, project's chief, said.

"It was a one-of-a-kind vehicle that even during it's 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."

Researchers lost contact with the vehicle seven hours into a planned nine-hour dive at the deepest extent of the trench. The team initiated a search near the dive site when standard emergency recovery protocols became unsuccessful.

The team later spotted pieces of debris floating on the sea surface, identified as parts of Nereus, indicating a catastrophic implosion of the vehicle.