The Sunlight Zone (0 - 200 meters)#

This first layer is where things are most familiar to us.

• Light from the sun can still get through the water here.
• This is important because it allows photosynthesis to happen.
• Tiny single-celled plants and bacteria called phytoplankton (trillions and trillions of them!) form the very bottom of the ocean’s food chain.
• Bigger plankton eat the phytoplankton, and then other animals eat them.
• The seafloor in this area is super lively, often covered with things like coral reefs, algae, and other sea plants.
• These plants are home to tons of different sea animals.

We’ve spent most of our time and energy in this part of the ocean – it’s where we fish, swim, unfortunately pollute, and do most of our science. But there’s so much more below!

Leaving the Familiar Behind#

As we move away from the coast and into deeper, open water, we eventually hit the edge of the continental shelf. Then comes the continental slope, which is a long slide down towards the true deep sea.

• The deeper we go, the faster the light disappears.
• Without much light, there are hardly any plants down here.
• Looking at the steep continental slope can feel a bit like looking at the surface of the Moon.
• Beyond the slope, it’s just endless open water stretching out before us.

Let’s keep going down, past the slope, and enter a place known as the twilight zone. This is like the gateway to the deep sea.

The Twilight Zone (Roughly 200 - 1,000 meters)#

As we sink further down, the water pressure really starts to build up. It reaches levels that would be deadly for humans.

• The deepest a scuba diver has ever gone is 332 meters.
• At that depth, the pressure is intense – it’s like having 200 cars stacked on top of you.
• Even at this depth, we’ve only covered about 3% of the ocean’s total depth journey!

This zone might seem a bit grim, but surprisingly, many fish and other creatures actually spend at least half their lives here.

• During the day, it’s a good spot to hide from predators in the huge, dark waters and rest up.
• At night, they often travel back up into shallower, food-rich waters to feed more safely.

In this tricky area between dim light and total darkness, light itself becomes a super important tool.

• Over 90% of the species that live deep down use special chemicals to make their own light – this is called bioluminescence.
• They use this light for different reasons:
  • As camouflage to blend in with any faint sunlight coming from above.
  • To send signals to potential mates.
  • To confuse or scare away attackers.
  • To hunt for food.

Another clever way life survives down here is by working together.

• Around 700 meters deep, you might run into a colony of siphonophores.
• These are not a single animal, but a group working as one.
• They can be incredibly long, up to 50 meters, but only as wide as a broomstick!
• To catch prey, they make a surprisingly beautiful bright blue or red light.
• They also put out a curtain of tentacles loaded with toxic needles to catch anything that gets too close.

But for most creatures living this deep, they depend on something you might not expect: marine snow.

• This is white, flaky stuff that’s constantly drifting down from the surface.
• It’s made up of all sorts of things:
  • Dead pieces of plants or animals.
  • Fecal matter (yes, poop!).
  • Shells.
  • Sand.
  • Dust.
• While it doesn’t sound appetizing, this marine snow is absolutely vital. Without it, deep-sea life would literally starve.

This zone is also where some epic, unlikely battles happen.

• Sperm whales hunt and attack giant squid that can be the size of a house!
• Even though the squid fight back hard, they probably don’t win often.
• But, they do leave behind permanent marks (like scars) on the whales’ skin.

The Midnight Zone (1,000 - 4,000 meters)#

Reaching 1,000 meters is a big deal – that’s deeper than the tallest building humans have ever built. Down here, things get serious. This is the midnight zone.

• It’s a place of total darkness.
• It feels like a hardly explored, wet wasteland, just endless black water.
• Trying to swim here as a human is actually harder than walking in space because of the extreme pressure and conditions.

Finding food in the midnight zone is really tough, so life here has had to become super efficient with energy.

• Take the vampire squid, for example. It’s only about 30 centimeters long.
• It floats through the water without really moving.
• It extends long, thin “catching arms” covered in tiny stiff hairs.
• These hairs brush food from the water as it drifts, which uses way less energy than actively chasing food.

For meat-eating fish, it’s even harder because live prey isn’t common.

• Hunters have to be perfect on their first try to grab a victim, or it’ll get away into the dark.
• Many deep-sea predators have multiple rows of long, sharp, deadly teeth.
  • The viperfish uses its long fangs to trap even large prey and swallow it whole.
  • The frilled shark has an amazing set of 300 teeth that curve backward, like hooks, to keep victims from escaping once caught in its mouth.

The Abyssal Zone (4,000 - 6,000 meters)#

We keep sinking, past the 3,800-meter mark – that’s as deep as the famous wreck of the Titanic. We’re now in the abyssal depths.

• Life here moves incredibly slowly.
• Saving every last bit of energy is absolutely critical to survival.
• Most animals in this zone just hover without moving or swim in a slow, graceful way.
• The only time they move quickly is when they have to escape danger.
  • The Dumbo octopod paddles along using its ear-like fins.
  • The grenadiers fish uses slow, eel-like tail beats to get around.

At about 4,000 meters, we finally hit the ground again: the abyssal plain.

• It’s mostly covered in grey mud and rocks.
• This ground is dusted with the remains of the marine snow that drifted down.
• Animals like sea cucumbers, shrimp, sea urchins, and sea worms munch on these remains.

In some spots on the seafloor, you can see small, dark, round rocks called manganese nodules.

• Deep-sea corals and sponges use these nodules to anchor themselves to the bottom.

Even though life on the main abyssal plain is sparse, there are special spots where it thrives – like little oases.

• These are often found in rift valleys, where Earth’s tectonic plates are slowly pulling apart.
• Here, hot magma below heats up the seawater.
• This heated water shoots out of vents, carrying dark minerals, at temperatures up to 400℃.
• These vents build up into elaborate chimney-like structures.
• Special bacteria called extremophiles live here. They don’t need sunlight; they use the minerals from the vents to create food.
• These bacteria form the base of unique ecosystems found only around these vents.

We descend further. The deepest part of the general abyssal plain is around 6,000 meters. For most of the ocean floor, that’s as deep as it gets. But if we want to find the absolute deepest spot, we’re still only halfway there!

The Hadal Zone (6,000 meters and below)#

Let’s enter the hadal zone, sometimes called the “underworld” of the sea.

• This zone consists of long, narrow trenches.
• These trenches make up only a tiny portion, about 0.25%, of the entire ocean area.
• They are some of the most extreme places on Earth.
• Only extremophiles can survive here.
• One example is the amazing ethereal snailfish, which holds the record for the deepest-living fish ever seen, found around 8,000 meters deep.

As we sink past 10,000 meters, spiky and sharp black rocks whiz by outside.

Finally, we reach the very bottom slope of a trench within the larger Mariana Trench.

• This area has gently sloping sides forming a valley about 1.6 kilometers wide.
• This is it. The deepest point. The Challenger Deep.

The Challenger Deep (Around 11,000 meters)#

• We are now 11,000 meters below the surface!
• The water pressure here is unbelievable: 1,086 bar.
• To give you an idea, the pressure is like having to balance 1,800 elephants stacked on top of you.

But even here, life manages to exist!

• Besides sea cucumbers, you can see white and light pink amphipods wiggling through the water.
• Their size is astonishing. Their cousins in shallower waters are only a few centimeters long, but these deep-sea versions can grow up to 30 centimeters!

And sadly, other things were found floating here too.

• In 2018, scientists discovered plastic bags down here.
• It shows that even the most remote place on Earth isn’t untouched by human impact.

There’s nothing left to explore deeper, and our time is running out. So, it’s time to head back up.

(You might hear some calming deep-sea sounds as we go.)

Returning to the Surface#

After spending hours traveling through the dark, empty water, we finally start seeing a glimmer of light. We arrive back at the calm surface.

The oceans are incredibly deep, and there is just so much of them. We really owe it to ourselves and to the people who will come after us to take care of them as best we possibly can. There are still so many amazing, wondrous things down there waiting to be discovered.