Efforts to explore the deepest recesses of Earth’s oceans were dealt a heavy blow last weekend when one of history’s most accomplished deep-sea explorers imploded several kilometers beneath the Pacific and resurfaced in pieces. Fortunately, the ill-fated Nereus submersible was a robot and no one was injured when it succumbed to overwhelming subsurface pressure during a dive to the Kermadec Trench, a 10-kilometer rut in the Pacific Ocean floor where two tectonic plates meet northeast of New Zealand.

Researchers at Woods Hole Oceanographic Institution (WHOI) built the high-tech, remotely operated vehicle (ROV) to withstand pressure in excess of 1,125 kilograms per square centimeter. Yet it was roughly this amount of pressure that likely imploded a portion of the sub on May 10.

The six-year-old Nereus was a veteran of several expeditions to the deepest parts of the planet, having visited the Challenger Deep—a nearly 11-kilometer deep trench east of the Marianas Islands in the western Pacific—in May 2009. This time, WHOI outfitted its foremost explorer of the ocean’s hadal zone—below six kilometers—with technology designed specifically for filmmaker and aquanaut James Cameron’s historic solo dive to the Challenger Deep.

Cameron himself likens the loss of Nereus to that of losing a friend. "Nereus was an amazing, groundbreaking robot and the only currently active vehicle in the world that could reach the extreme depths of the ocean trenches," he wrote in a statement to WHOI after hearing about Nereus's final dive. "This is a tragic loss for deep science."

The filmmaker empathizes with the WHOI researchers, whom he has known for years, since he began studying marine engineering in preparation to make the 1989 movie The Abyss. "Andy Bowen, head of the team, and Tim Shank, chief scientist of the Kermadec expedition, are friends of mine, and my heart goes out to them," Cameron wrote. "They've not only lost a child, they've lost a great opportunity to explore one of the ocean's deep trenches—the last great frontier for exploration on our planet." Cameron noted that "the loss of a single vehicle now denies us access to an area the size of North America—which is what the combined area of the deep 'hadal' trenches equals. A dark day for many reasons."

After Cameron donated his DEEPSEA CHALLENGER to WHOI last year, the scientists used several components from that sub to upgrade Nereus’s flotation technology, cameras and lighting. The enhancements included about 1,500 hollow, softball-size ceramic spheres housed in upper parts of the vehicle to keep it upright and buoyant. Scientists installed LED lights under the noses of its twin hulls, just above the sub’s two high-definition cameras, which were able to pan and tilt as they took footage of the trench’s walls, floor and marine life.

Nereus—which cost about $8 million in (mostly) federal funding and weighed 2,800 kilograms (6,200 pounds)—also had a manipulator arm for collecting rock, sediment and marine life samples. More than 4,000 rechargeable lithium-ion batteries provided power for 20 hours of operation. A hair-thin optical fiber transmitted high-quality video to the research ship Thomas G. Thompson, which WHOI had dispatched to carry out the first of several missions to explore hadal ecosystems. Scientific American correspondent Mark Schrope wrote in detail about the WHOI excursion to the Kermadec in the May 2014 issue. WHOI writer Ken Kostel has been covering the team’s progress for Scientific American’s Expeditions blog since the Thomas G. Thompson left port on April 12. Kostel’s most recent entry describes the aftermath of losing Nereus.

Nereus’s fate lends a certain degree of gravitas to the risks that Cameron faced during his own mission in 2012, although he has downplayed the danger. The moviemaker stated in a Q&A with Scientific American last year:

“I doubt my pulse went up much. Maybe when we took away the floats [at the surface]. I was task focused. I had a multipage checklist that was organized around time and depth, metrics, things I had to do ... One of the biggest problems with the sub is the heat flux. The pilot’s sphere is very tiny, and it’s got a lot of equipment packed in there with me. If I turned all of the equipment on at the surface, I’d bake. I’d literally be like poached salmon. So I had this whole protocol for bringing things online as needed. The unique thing about the Challenger Deep dive is that I got through everything on my checklist that had taken me right down to the bottom of the New Britain Trench, then I had nothing to do. And I had 9,000 feet to go.”

Researchers on the Thompson lost contact with Nereus 30 days into a 40-day expedition, seven hours into a planned nine-hour dive at the deepest extent of the Kermadec trench. The team later identified pieces of debris floating on the sea surface as parts of Nereus.

Clarification: The headline of this post was changed to reflect the fact that an unmanned submersible, not an aquanaut, was lost.