The hadal zone begins 6,000 metres down and reaches 11,000 metres, inside ocean trenches where the pressure is more than 1,000 times what it is at the surface. Woods Hole Oceanographic Institution and NASA’s Jet Propulsion Laboratory have built a small robot submarine to work there, called Orpheus.
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Orpheus can thread the rocky narrows of a trench and work alone or in a swarm to map a trench and study what lives there. WHOI singles out one ability: the vehicle can rework its own mission mid-dive when something unexpected turns up.
What makes a trench hadal
The name comes from Hades, the Greek god of the underworld. Most of the ocean floor lies 4,000 to 6,000 metres down, so it takes a trench to reach hadal depths. The water there hovers just above freezing, and no sunlight arrives. Any light comes from animals that glow to signal each other.
The sources disagree on how much of it there is. WHOI puts the combined trench area at about the size of Australia. Tim Shank, science lead for the Orpheus programme, writing for NOAA Ocean Exploration, puts it at about half that.
Why so little is known
Until 2014, much of what scientists knew about hadal life came from two 1950s voyages, Denmark’s Galathea and the Soviet Vitjaz. Shank writes that neither sampled at comparable depths or often enough to compare one trench with another.
Robots have had a hard time too. WHOI first tested a vehicle able to reach full ocean depth, the hybrid remotely operated vehicle Nereus, in 2009. Nereus was lost in 2014 nearly 10,000 metres down in the Kermadec Trench, north of New Zealand, during a survey of life nobody had seen before.
Crewed dives picked up after that. Victor Vescovo took his submersible Limiting Factor to the deepest points in all five major ocean basins. Shank notes that systematic, wide-ranging surveys have still mostly stalled, and Orpheus is meant to restart them.
Why anyone wants to go
Some trench animals live on chemical energy from seeps in the sea floor, and others on carbon that sinks from the waters above. WHOI suggests that carbon piling up at the base of trenches may matter for the carbon cycle and climate. Shank points to hadal animals’ physiology as a possible lead for Alzheimer’s treatments.
There is a reason beyond Earth as well. WHOI says hadal research will inform the exploration of oceans on the moons of Jupiter and Saturn.
Where the unexplored sea floor is
Challenger Deep, in the Mariana Trench, is both the deepest point known on Earth and the most familiar patch of hadal sea floor. Most hadal sea floor lies elsewhere, though. A NOAA map of the Pacific shows most of it sits mid-basin, away from the trenches along plate edges, and barely explored. WHOI says the same of hadal sea floor away from trenches.
