The ocean is, on average, four kilometers deep. Less than five percent of it has been mapped at any useful resolution. What follows is a brief and unhurried descent. Read with us.
I — Sunlight Zone0 to 200 meters
Sunlight ends here. Photosynthesis ends here. Every coral reef, every surfer, every whale you have ever seen photographed lives in this thin band — the top two percent of the column.
We talk about the ocean as if it were a surface, the way we talk about a country by its capital. But the surface is the smallest part. Below the wave-trough, the world goes on for another ten kilometers, almost entirely without us. The sunlight zone is what we mean when we say the sea. It is also where we stop.
II — Twilight Zone200 to 1,000 meters
Below 200 meters, the spectrum collapses. Red disappears first, then orange, then yellow. By 600 meters, what reaches you is a single hopeless wavelength of blue, and below that, nothing at all.
This is the dim country. The fish here are silvered, like coins, to mirror the failing light. Many of them carry their own lamps — photophores along their bellies, tuned to the exact blue still falling from above, so that when something looks up at them, no silhouette appears. They have erased their own shadows. We have invented submarines that try to do this, and ours are not as good.
“Most of the animals on Earth, by sheer count, live in this one band.” — Edith Widder, marine biologist
III — Midnight Zone1,000 to 4,000 meters
From here down, no surface light ever arrives, in any season, on any day. The animals that live in this dark have, in many cases, never encountered a photon they did not make themselves.
The cold is constant — about four degrees Celsius, the same as your refrigerator — and so is the dark. Pressure climbs. Bodies grow strange. The anglerfish dangles a lure of cultivated bacteria. The dumbo octopus flaps through it like a small grey ghost. The deep is mostly empty, and the things that live in it have learned to be patient.
IV — Abyssal Zone4,000 to 6,000 meters
More than half of the planet's surface, by area, is the abyssal plain. It is, on average, the most representative landscape on Earth, and we have seen less of it than we have of the surface of Mars.
What lives here lives slowly. The giant isopod can refuse food for five years and seem none the worse for it. The yeti crab grows its own bacterial farm on the bristles of its claws and waves them in the chemical-rich plumes of hydrothermal vents. The food, when it comes, comes from above — a slow rain of dead things from the sunlit world, called marine snow, drifting down for months.
V — Hadal Zone6,000 to 11,000 meters
The hadal zone is the trenches. There are about thirty-three of them worldwide, narrow gashes where one tectonic plate is folding under another. The deepest is the Mariana, just under eleven kilometers down.
Pressure here is over a thousand atmospheres. A styrofoam cup lowered to the bottom and brought back up returns the size of a thimble. And yet, against every intuition, things live here. The Mariana snailfish is small, translucent, and not particularly unhappy. Its bones are flexible. Its cells are stabilized by chemistry we did not know existed until we went and looked.
More humans have walked on the moon than have visited the bottom of the Mariana Trench. There is, almost certainly, a great deal still down here that we have not yet thought to name.
“We have better maps of Mars than we do of our own seafloor.” — Sylvia Earle, oceanographer