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Nereus, WHOI’s one-of-a-kind vehicle that had plunged into some of the deepest parts of the ocean, was lost at sea last week.

Nereus, WHOI’s one-of-a-kind vehicle that had plunged into some of the deepest parts of the ocean, was lost at sea last week.

Photo courtesy of Advanced Imaging and Visualization Laboratory, Woods Hole Oceanographic Institution

Requiem for an ROV

“We lost Nereus today.”

Those were the sad, succinct words of Woods Hole Oceanographic Institution (WHOI) science writer Ken Kostel, in a 10 May blog post. Nereus, the institution’s $8 million intrepid, deep-sea diving, remotely operated vehicle (ROV) had plumbed the depths of Challenger Deep and explored hydrothermal vents off the Mid-Cayman Rise, the planet’s deepest midocean ridge. It was to be an integral part of a new, 3-year National Science Foundation-funded undertaking—officially called HADES, for Hadal Ecosystem Studies project—to explore and shed light on the mysterious ecosystems lurking in deep-ocean trenches at so-called hadal depths between 6000 and 11,000 meters.

The first expedition departed on 12 April for the Kermadec Trench off New Zealand, led by WHOI biological oceanographer Timothy Shank aboard the R/V Thomas G. Thompson. Thirty days into the 40-day cruise, disaster struck. Seven hours into a 10,000-meter dive into the trench, the camera feeds from Nereus went dark. Hours later, the team spotted debris on the water thought to belong to the missing ROV, suggesting a “catastrophic implosion” due to pressures as great as 110 megapascals at those water depths.

Condolences poured in. “We heard the news this morning and understand what a blow this is for the scientists and for WHOI, but also for all of us that understand the need to explore the oceans greatest depths,” wrote Margaret Leinen, the director of the Scripps Institution of Oceanography in San Diego, California. “I feel like I’ve lost a friend,” wrote Hollywood director, deep-sea explorer, and fellow Challenger Deep visitor James Cameron in a message posted to WHOI’s Facebook page.

ScienceInsider traded e-mails with Shank shortly after Nereus’s loss. (We’ve edited the exchange for brevity and clarity.)

Q: How was Nereus going to help you explore deep-ocean trenches? What does its loss mean for the future of hadal exploration?

T.S.: Nereus was a one-of-a-kind vehicle. With its loss goes our ability to fully conduct this research, including documenting and sampling completely unknown seafloor and life, including new known life forms—as Nereus did just days before she was lost. The loss for our current expedition is disheartening and difficult, but … for future hadal work [it] is devastating. It means that more than one-third of the ocean’s depth range is beyond our reach.

Hadal systems have been among the most poorly investigated habitats on Earth. ... The overarching goal is to examine the hadal zone (below 6000 meters) in trenches around the world in order to determine the composition and distribution of hadal species and ecosystems, the role of pressures (at its greatest, [110 megapascals], or the equivalent of 2 humvees on a thumbnail), food supply (specifically the distribution of carbon and biomass of trench organisms), and the role deep trenches play in shaping the diversity and evolution of life and diversity in our deep ocean.  

The ability … to obtain targeted imagery of trench animals and their habitats, conduct experiments at various depths within the trench, and collect samples of the sediments, microbes, and trench fauna is essential. The capabilities of Nereus—with a highly capable 7-function manipulator arm, high-definition cameras (including one ultra-high 4k HD camera)—[were] fundamental to the goals of our expedition.

Q: What other instruments can you use to continue your work in the trench?

T.S.: We can continue to conduct traditional trench research using free-falling camera landers and baited traps. These, however, give us only a snapshot (or short video clip) of what species are attracted to bait and does not provide us with the ability to examine the complex details of ecosystems or collect samples.

[The ability to] traverse over the seafloor, document and sample animals and their habitats, conduct the first systematic study of hadal ecosystems is, for now, lost. We can put landers on Mars (400 million kilometers away at its most distant from Earth) and have it explore the surface of that planet for years, but the best we could do, even with Nereus, was to study the seafloor 10 kilometers away for only 10 hours at a time. At best. We can no longer do even that.

Q: What was the Nereus doing on its last dive (its 14th of the expedition)? What had it collected?

T.S.: We had been working on the bottom of the Kermadec Trench at 9990 meters or 5.4 miles down (we think the deepest part of this trench is close to 10,063 meters) for over 6 hours. Nereus had collected cores of sediment and animals (two sea cucumbers and two polychaete worms) into chambers to measure their respiration rates (uptake of oxygen). We [had] started to move toward a free-falling lander called an “elevator” (a 6 foot by 6 foot platform) that we can release with floatation to bring items back to the surface ship for recovery.  

Q: So what happened on its final day?

T.S.: At 13:02 (GMT) on 9 May, all of our monitors (5 camera feeds) turned immediately black. While everyone in the control room gasped with disappointment, we knew this was not unusual, as this happens at the end of each dive when we purposefully cut the light fiber. Sometimes it happens by accident, when the fiber breaks.  

As is typical, following the gasp we said, “ok, let’s bring it up and reload for the next dive [in the] same location.” We then typically “speak” to the vehicle using acoustic communication and tell the vehicle to drop its weights and ascend back to the surface- this is the time when burn wires on Nereus will automatically “dissolve” and drop weights (as it has done for 75 dives).  

Except this time, dive #76, all communication was lost. We anticipated that the vehicle would drop its weights even without our communication -- another failsafe -- [so we] posted people as look outs around the ship. Nereus was meant to reach the surface at 07:30, and then again at 14:00 hours (local). Nothing was observed that morning, just ominously grey clouds and rain.  

It was around 13:45 when I saw white pieces of material—from golf ball sized to over a foot long—floating past the ship. We launched the ship’s small rescue boat and picked up pieces with nets, which we identified as structural components from inside the vehicle's hulls.

It was a horrible feeling—of denial and disbelief—at the thought of the set-back to hadal science that this represented.

Nereus was a vehicle that I had worked on since its inception 10 years ago, completing the final field testing in 2009 when we dove it to the Challenger Deep, in the Mariana Trench (10,899 meters). As many of the pieces as possible were recovered until increasing winds and choppy seas forced us to stop.

Q: Will another full-time science vehicle rated to 10,000 meters be built anytime soon?

T.S.: Yes, there are current plans for [WHOI] to build an upgraded Nereus, and there are rumors that others are also being planned by other countries. The Nereus concept of using a light fiberoptic tether (instead of a heavy steel tether that conventional ROVs use) has already been passed on to other purpose-built vehicles (e.g., one designed for under-ice use).  

Nereus was really a prototype that we, the science community, wanted to turn into a workhorse. As a result, we have been adding sampling capabilities to it for years. We had also worked hard to open hadal research to more of the science community and educate people about the importance of the hadal region and hadal research. [We] had been broadcasting live from the seafloor 6 miles down via a satellite “telepresence” connection on the final dive. There has been a tremendous upsurge of global interest to study these deep hadal regions, in part due to the desire to understand life’s adaptation to massive pressures, cellular function in the deep ocean, the evolution of life, and the unknown role these deep environs play in the global carbon cycle, which is so important now for understanding climate change.

We shouldn’t just build another Nereus, we should build a fleet of them. It is unconscionable that the loss of one vehicle prevents us from systematically exploring and studying the deepest part of what is an interconnected ocean. It’s as if we decided in the 1960s to only go half way to the moon. We can’t understand the ocean just by looking at the upper two-thirds.