In May, the remotely operated underwater vehicle Nereus descended 10,000m to the bottom of the Kermadec Trench, one of the ocean’s deepest, and never came back. It’s believed that Nereus—a hybrid remotely operated vehicle, or ROV, that could also operate autonomously—likely imploded. The pressure at such depths can be as great as 16,000 pounds per square inch.
What’s weird is that Nereus was *designed* to withstand such pressure. That’s what made it unique. Unlike most other ROVs, which get their buoyancy from a material called syntactic foam, the Woods Hole Oceanographic Institute (WHOI), which designed and built Nereus, opted for a radical new design involving hundreds of ceramic spheres instead.
While we still don’t really know how or why Nereus failed–it completed numerous previous dives, some to deeper depths, without issue–there’s no denying that its novel design allowed Nereus to dive deeper, be built lighter, and stay underwater longer than probably any other ROV in existence. So, implosion aside, why aren’t we yet building more ROVs like Nereus—even the ones that aren’t destined for places as deep or pressures as intense as those of the Kermadec Trench?
Putting anything underwater requires a delicate balance between buoyancy and weight, explains Andy Bowen, director of the WHOI’s National Deep Submergence Facility, and maintaining that balance becomes more difficult the deeper you go down.
“You want the vehicle to be slightly positively buoyant, or at least neutrally buoyant. So all the stuff that weighs something has to be offset by something that doesn’t weigh as much–or, in fact provides, a buoyancy offset,” Bowen says. “You can broadly divide these things into parts that float or parts that don’t.”
Obviously, batteries, cameras, lights and motors are the things that don’t, and it’s the job of people like Bowen to make them float. Traditionally, manufacturers have used a material known as syntactic foam, which is composite material filled hollow microscopic glass bubbles. These bubbles lower the material’s density, making it buoyant. It’s flexible, well-understood, and has been in use for decades. When you look at a photo of a typical ROV, it's the brightly colored material mounted to the top of the robot's frame. "You can make syntactic foam to go just about anywhere you want it to go,” says Bowen, “but with a price.”