The average depth of the ocean is about 3,700 metres. Sunlight strong enough for a plant gives out at 200 metres, a twentieth of that, and the last glimmer at about 1,000. Everything below is dark, cold, under enormous pressure, and home to most of the living space on Earth. Two of Lunthra's series are named for the layers down there: Abyssal, for the animals, and Bathyal, for the machines people have built to follow them. Here is the map.
The ocean in layers
Oceanographers divide the open water by depth, and the names come up on every plate.
- Epipelagic, 0 to 200 m. The sunlit zone. Enough light for photosynthesis, so this is where the plankton grow and where nearly all the fishing happens.
- Mesopelagic, 200 to 1,000 m. The twilight zone. Faint blue light from above, too weak for plants, enough for an animal to see a silhouette. This is where an animal's own light starts to matter.
- Bathypelagic, 1,000 to 4,000 m. The midnight zone. No sunlight at all. Water a few degrees above freezing, and the only light is made by the animals themselves. Bathys is Greek for deep.
- Abyssopelagic, 4,000 to 6,000 m. The abyss, which covers the flat plains that make up most of the sea floor. Near-freezing, near-total darkness, pressure several hundred times that at the surface.
- Hadal, below 6,000 m. The trenches, named for Hades, down to nearly 11,000 m at the bottom of the Mariana Trench.
The pressure arithmetic is the useful thing to remember: every ten metres of seawater adds about one atmosphere. At 1,000 m the pressure is roughly a hundred times what you feel on land; at 4,000 m, four hundred. That single number explains most of what the animals look like and most of what the machines have to be.
The animals
A 2017 survey of seventeen years of submersible video off California found that about three-quarters of the animals observed in the water column between the surface and 4,000 m could make their own light. In the dark, bioluminescence is for everything: finding food, finding a mate, hiding, and startling whatever is about to eat you. The four Abyssal plates are four different answers to the same dark.
Lure: the anglerfish
The deep-sea anglerfishes, the ceratioids, carry a modified fin ray on the head that ends in a bulb called the esca. The esca glows, but not by itself: it is a pocket of symbiotic bacteria the fish cultivates, and the light is theirs. Prey swims toward the glow and the mouth below it, which is most of the fish, does the rest. The females are the familiar ones; the males of many species are tiny, and in some they attach to a female permanently. The Lure plate draws the fish at around 2,000 m, in the bathypelagic.
Cloak: the vampire squid
Vampyroteuthis infernalis, the vampire squid from hell, is neither a vampire nor a squid. It is the only living member of its own order, a relic line that split from the squids and octopuses long ago, and it lives in the oxygen-minimum zone between roughly 600 and 900 m, where almost nothing else can breathe. The webbing between its arms is a cloak it can pull over itself; its photophores let it switch its light on and off; and instead of ink it releases a cloud of glowing mucus. It eats falling detritus, not prey. The Cloak plate shows it with the arms spread.
Colony: the siphonophore
A siphonophore looks like one animal and is not. It is a colony of specialised individuals called zooids, each budded from the same founder, each doing one job: some swim, some feed, some reproduce, some are stinging tentacles, some light up. The Portuguese man o' war is the siphonophore everyone has seen; the deep-water ones are long, transparent chains, and one filmed off Western Australia in 2020 was estimated at around 45 metres, longer than any whale. The Colony plate draws the chain at around 1,000 m.
Fin: the dumbo octopus
Grimpoteuthis, the dumbo octopus, swims by flapping two ear-like fins above its eyes, which is where the name comes from. It is a cirrate octopod, the deep-living branch of the family. The plate puts it at 4,000 m; one has been filmed at nearly 7,000 metres in the Java Trench, deeper than any other octopus. Down there it has no need for ink, and has none. The Fin plate shows it mid-flap.
The machines
Getting a person, or a person's hands, to those depths is a hundred and fifty years of engineering against that one number. The four Bathyal plates are four stages of it.
Brass: the hard hat
The copper helmet with a brass breastplate, four viewports and twelve bolts was the standard diving dress from the 1840s until well into the twentieth century. Air came down a hose from a pump on the surface; the diver walked the bottom in weighted boots. The helmet in the Brass plate is the pattern from around 1916, about 25 kg of metal, and it took divers to a few tens of metres: salvage, harbour works, the first pipelines.
Bell: living under pressure
Below about 50 m the problem is not getting down but coming back. Gas dissolves into a diver's tissues under pressure and must be released slowly or it forms bubbles; a deep dive can need more hours of decompression than minutes of work. Saturation diving solves it by not coming back. The divers live for weeks in a pressurised chamber on the ship, and a bell, a steel sphere, carries them down to the job and back at working pressure. Decompression happens once, at the end of the month. The Bell plate is a two-diver bell rated to 300 m, about the ceiling of commercial saturation work; most North Sea jobs are shallower, between 100 and 180 m.
Atmos: the suit that refuses the pressure
An atmospheric diving suit is the opposite strategy. Instead of adapting the diver to the pressure, it keeps the pressure out: a hard shell with the diver inside at one atmosphere, and oil-filled rotary joints that let the arms and legs move under hundreds of tonnes of load. No decompression, no gas problems, a working depth of 610 m, and a dive that can last six hours or more. The Atmos plate draws one with its eighteen joints.
ROV: the hands stay on the surface
Most deep work today is done by nobody down there at all. A work-class remotely operated vehicle is a frame of thrusters with a flotation block on top and two manipulator arms in front, flown from a control room on the ship through a tether that carries its power and its video. The one in the ROV plate is rated to 3,000 m, with 150 horsepower and seven thrusters. It builds and repairs the pipelines the hard-hat divers started.
Why two series
The animals and the machines are the same story told from both ends: a place so hostile that life there had to invent light, and reaching it took people a century of invention of their own. Each plate carries its depth, its size and its defining number, so the two series read as one chart of the water column when they are side by side.
Everything in both series comes as a tee, a hoodie and a 16 × 32 inch desk mat. Start with the Abyssal series if the animals are the point, and Bathyal if the engineering is.
