The Ocean in Five Zones — From the Sunlit Surface to the Hadal Deep

Stand at the rail of a ship in the open ocean and look down, and you see a flat blue surface. What you cannot see is that the water beneath you is not one thing. It is a stack of worlds — five of them — and each is colder, darker, and under far more pressure than the one above. A stone dropped over the side falls through every one of them on its way to a seafloor that, in the deepest places, is as far below you as a passenger jet flies above the land.

This is what the descend view is built to show: pick a point, and fall straight down through those worlds to the real floor beneath it, reading the actual depth, the real measured water, and who lives at each level. Here is the map of what you are falling through.

Zone 1 — The Sunlight Zone (0–200 m)

The epipelagic zone is the ocean almost everyone has ever seen. It is the only layer with enough light for photosynthesis, so it is where the phytoplankton grow — and therefore where nearly all the ocean's food begins. Tuna, dolphins, most sharks, sea turtles, reefs and the great schools of fish all live in this thin sunlit skin. It is a thin skin: 200 m is about the length of two football pitches, a rounding error against an average ocean depth of nearly 3,700 m. Almost all of the sea is darker and deeper than anything in this zone.

Light falls off fast here. The reds and oranges of sunlight are absorbed in the first few metres; by 100 m only a dim blue remains, and by the bottom of this zone there is too little light to grow anything.

Zone 2 — The Twilight Zone (200–1,000 m)

The mesopelagic zone is where the light dies. Enough filters down for an animal's eye to register a faint glow, but not enough for plants. This is the realm of the lanternfish and the hatchetfish, of krill and small squid — and of one of the largest movements of life on the planet: every night, animals rise from the twilight toward the surface to feed in the dark, and every dawn they sink back down. This daily vertical migration moves billions of tonnes of life up and down the water column, and with it a great deal of the planet's carbon.

By the bottom of the twilight zone, around 1,000 m, the last trace of daylight is gone.

Zone 3 — The Midnight Zone (1,000–4,000 m)

In the bathypelagic zone there is no sunlight at all — not at noon, not at the equator, never. The only light is the light animals make themselves. A large share of deep-sea creatures are bioluminescent, glowing blue-green to lure prey, find mates, or vanish in a cloud of light. This is the home of the anglerfish with its glowing lure, the gulper eel, the vampire squid, and the giant squid. The water here is a few degrees above freezing and the pressure is already crushing — a hundred atmospheres or more.

The standard Argo float — the instrument that draws the measured temperature line on the descend view — profiles to about 2,000 m, in the upper part of this zone. For most points that is where honest measurement ends, and below it the descend view shows the floor's depth from GEBCO but leaves the water properties blank. The exception is the rare deep Argo float: a couple of hundred of them dive to 4,000–6,000 m, and when one is near the point you tapped, the measured line keeps going down into the midnight zone and the abyss below it. Where no float has read the water, we draw nothing. We will not draw a line into water no instrument has read.

Zone 4 — The Abyss (4,000–6,000 m)

The abyssopelagic zone — the abyss — covers the vast, flat abyssal plains that make up much of the actual seafloor of the planet. It is perpetually dark, close to freezing (around 2–3 °C), and under enormous pressure. Life is sparser here but real: sea cucumbers and brittle stars graze the sediment, grenadier fish and the pale, gelatinous snailfish move through the dark, and everything ultimately depends on the slow rain of dead matter — "marine snow" — drifting down from the sunlit world far above.

Most of the ocean floor lies in this zone. When you tap a random point in the open ocean on the descend view, the floor it lands on is very often an abyssal plain four to six kilometres down.

Zone 5 — The Hadal Zone (6,000 m and below)

Below 6,000 m the seafloor is no longer plain but trench — the deep gashes where one tectonic plate dives beneath another. This is the hadal zone, named for Hades, and it reaches its limit in the Mariana Trench at nearly 11,000 m: the Challenger Deep, the deepest known point on Earth. The pressure there is over a thousand atmospheres, more than a tonne on every square centimetre.

And yet it is not empty. Amphipods swarm bait dropped to the bottom; sea cucumbers and hadal snailfish — the deepest fish ever filmed, below 8,000 m — make a living in the cold and the crushing dark. The descend view caps its column at the real floor depth for the point you chose: at Challenger Deep it draws the full hadal plunge; over an abyssal plain it stops far shallower. The floor is always the real one, from GEBCO, dated to when it was read.

Where the data stops — and why that matters

The honest edge of this picture is the point of it. Three things are known with confidence at any tapped point: the depth of the floor (from GEBCO's bathymetry), the pressure at any depth (it follows directly from depth — one atmosphere per 10 m), and the light (it is gone by ~1,000 m everywhere). The temperature and salinity are known only as deep as the nearest real float reached — about 2,000 m at best for a standard float, or several kilometres deeper in the rare places a deep Argo float surfaced nearby and recently.

So on the descend view the measured line fades out partway down, the gauges keep reading exact pressure all the way to the floor, and nothing is drawn below the seafloor or below the float's reach. The deep ocean is the least-measured place on Earth. The most honest thing a map of it can do is show you clearly where knowledge ends and the dark begins.