The deep ocean is, still, less mapped than the surface of Mars. We have detailed topographic data for roughly 25 percent of the seafloor — a figure that has improved dramatically in the past decade, but that still leaves three-quarters of the ocean bottom in a resolution so low that a mountain range the size of the Rockies could hide in the gaps. A new fleet of autonomous underwater vehicles, deployed in a coordinated sweep of the central Pacific last year, has returned from eight months of continuous survey with data that has already forced the redrawing of maps.
What they found
The drones, operating at depths between 3,000 and 6,000 meters, used a combination of multibeam sonar, water-column sampling, and optical imaging to survey a swath of the Pacific roughly the size of Western Europe. The headline finding is geological: three previously unknown seamount chains, the largest of which rises 2,400 meters from the abyssal plain — taller than any mountain in the contiguous United States, completely unknown before last year.
The biological findings are stranger. At the base of one seamount, in water that receives no sunlight and experiences pressures that would crush most organisms, the drones documented a community of chemosynthetic organisms — bacteria, tube worms, and several species of crustacean — clustered around hydrothermal vents that were not on any map because the vents themselves had never been found.
The question of the plastic
At every depth sampled, the drones detected microplastic particles in the water column. This was expected; previous surveys had found plastic at comparable depths. What was not expected was the concentration at the greatest depths — levels two to three times higher than in the mid-water column above.
The explanation proposed in the preliminary report is hydrodynamic: the abyssal currents in this part of the Pacific act as a convergence zone, concentrating particles that settle from above. The deep ocean, in other words, is not just a place where plastic eventually arrives. In some areas, it may be a place where plastic accumulates.
The survey is ongoing. The fleet is being repositioned for a sweep of the Indian Ocean later this year.
