Chapter 1: Surface (Sunlight zone, 0 m to 5 m)
Seven tenths of our planet is ocean. On average, it's 3.7 kilometres deep.
We have better maps of the surface of Mars than of our own sea floor.
Almost everything alive that you have ever seen lives up here, in the thin, bright skin of the sea.
Take a breath. We're going all the way down.
Chapter 2: Colour (Sunlight zone, 5 m to 230 m)
Water eats light, one colour at a time. Red goes first, in the top ten metres.
By 40 m, orange is gone too. That's as deep as most scuba divers go. A cut down here bleeds green.
1%: of the sunlight that hit the surface is left at 100 m
At 200 m, plants give up. There isn't enough light left to live on. Only blue goes deeper.
At 214 m, Herbert Nitsch set the deepest freedive ever, on a single breath. No human body goes deeper unprotected.
Chapter 3: Twilight (Twilight zone, 230 m to 1,000 m)
Welcome to the twilight zone. Up there, the sun is now a faint blue rumour.
Animals here are small, silver and see-through, because being seen means being eaten.
The hatchetfish lights its own belly to match the glow from above, erasing its shadow. Living invisibility.
Every night, trillions of animals swim hundreds of metres up to feed, then sink before dawn. The biggest migration on Earth happens every single day.
The commonest animal with a backbone on the planet lives here: the bristlemouth. There are trillions. You've probably never heard of it.
And then the light runs out.
Chapter 4: Midnight (Midnight zone, 1,000 m to 1,500 m)
Below 1,000 m, sunlight has never reached. Not once in four billion years.
Your eyes would see nothing at all. And then, something blinks.
76%: of animals down here make their own light
Light is their language: to lure, to hide, to find a mate, and to scream.
When something bites the Atolla jellyfish, it spins a wheel of blue light: a burglar alarm that calls in a bigger predator to eat its attacker.
Chapter 5: Weight (Midnight zone, 1,500 m to 2,000 m)
Every 10 metres, the ocean piles another atmosphere of pressure on top of you.
×200: the air pressure at the surface. And we're not even a fifth of the way down.
Scientists tie styrofoam cups to deep-sea robots for fun. They come back shrunken, the air squeezed out of them.
So how does anything live here? Deep-sea animals have no air inside to crush. They're mostly water, and water barely squeezes.
Chapter 6: Hunters (Midnight zone, 2,000 m to 2,500 m)
Food is so rare down here that every meal is a gamble. So the hunters use bait.
The anglerfish dangles a glowing lure, full of glowing bacteria, right in front of its teeth.
The male is tiny. When he finds a female, he bites on and fuses to her body, for the rest of his life.
Remember red light? It never gets this deep, so nothing here can see it. That's why so many deep animals are red: it's invisibility.
The dragonfish cheats. It shines a red searchlight that only it can see, and hunts in a light its prey doesn't know exists.
Chapter 7: Giants (Midnight zone, 2,500 m to 2,992 m)
Some hunters come down from the surface, on a single breath.
A sperm whale can dive past 2,000 m and hunt for over an hour, seeing with sound.
Its prey: the giant squid, with the largest eyes of any animal, as wide as a dinner plate.
Their battles happen in total darkness. We know about them from the sucker scars on whale skin.
2,992 m: The deepest dive ever recorded by a mammal: a Cuvier's beaked whale, on one breath.
Chapter 8: Fire (Abyssal zone, 2,992 m to 3,600 m)
On the black sea floor, cold and lightless, you find fire.
Hydrothermal vents blast out water hotter than 400 °C. It can't boil: the pressure won't let it.
No sunlight, no plants. Here, bacteria eat chemicals instead, and everything else eats the bacteria.
Giant tube worms grow taller than you, with no mouth and no stomach. Bacteria inside them do all the eating.
Many scientists think life itself began at vents like these. Every creature on this page might trace back to one.
Chapter 9: The Floor (Abyssal zone, 3,600 m to 6,000 m)
This is the abyssal plain: cold, flat, endless mud. It covers more than half of our planet.
It snows here, forever. Marine snow: flakes of dead things drifting down from above. Some take weeks to land.
The Titanic lies at 3,800 m. Bacteria are slowly eating it, turning its steel into rust icicles.
Sometimes, dinner falls from the sky. A dead whale can feed a whole community down here for decades.
Sharks strip it, worms eat its bones, and bacteria feed on what's left. Nothing goes to waste.
Chapter 10: Hades (Hadal zone, 6,000 m to 8,900 m)
At 6,000 m the floor tears open. Trenches plunge into the hadal zone, named after Hades.
Snailfish live here: soft, pink, almost see-through. One was filmed at 8,336 m, the deepest fish ever seen.
Below that, scientists think fish can't survive at all. The pressure stops their proteins from working.
Drop Mount Everest into the deepest trench, and its peak would still be 2 kilometres underwater.
Chapter 11: The Bottom (Challenger Deep, 8,900 m to 10,935 m)
In 1960, two men sank here in the Trieste. They spent 20 minutes on the bottom and saw almost nothing but silt.
Then nobody came back for 52 years.
The water here presses with more than 8 tonnes on every square inch. Like a person holding up 50 jumbo jets.
And when explorers finally looked around, something was waiting for them: a plastic bag.
10,935 m: The deepest point in the ocean. You're here.
Chapter 12: Unknown (The whole ocean, 10,935 m to 10,935 m)
That's how far it goes. Everything you just passed, at true scale.
Only about a quarter of the sea floor has been mapped in detail.
9 in 10: ocean species may still be unknown to science
The deep isn't empty. It's unexplored. Most of the living space on Earth is down here in the dark, and almost no one has seen it.