In fact, the most mysterious of these dark places is right here on Earth.

A place with no sunlight, crushing pressure, and near-freezing temperatures is not the desolate wasteland that one might expect it to be.

It is instead populated with a huge variety of life.

Life that takes shape in ways that our terrestrial minds can barely comprehend.

Some creatures of the deep have enormous, piercing eyes.

Others have no eyes at all.

And others still have eyes inside their transparent, domed heads.

Others glow for defense or glow for predation.

And at these depths, where photosynthesis cannot occur, the food chain becomes a little unpredictable.

Some creatures typically classified as filter feeders have even evolved to become unlikely, terrifying carnivores.

Others become such effective predators that their mouths are bigger than the rest of their entire body or have teeth that pierce the darkness in every direction.

From the ethereal to the spooky, the dark sea holds a world of adaptations that jars the imagination.

And the deeper you go, the creepier it seems to get.

What is it exactly about the deep, dark sea that creates some of the wildest creatures ever discovered?

And is there a reason we can't help but find them to be bone-chillingly creepy?

The uppermost layer of the deep sea is the mesopelagic, aka the twilight zone, and it extends from 200 meters to 1,000 meters.

A little bit of light still penetrates at these depths, but not much.

The very first color to disappear is red, because it has the longest wavelength and the least energy of the visible spectrum.

And a lot of creatures that spend time in the twilight zone use this to their advantage, evolving to be shades of red that render them practically invisible at these depths.

Enter the vampire squid.

Catching a glimpse of it in the lights of a submersible, you might understand why it got such a ghastly name.

On top of its red skin, it has enormous eyes that seem to glow, and spine-like protrusions from its tentacles all give it a far more fearsome look than the cephalopods of shallower waters.

But just as its red color seems eerie to us, but has a vital evolutionary purpose, so too do its other sinister-looking features.

Despite its spooky name, vampire squid are not actually bloodthirsty hunters.

Unlike almost all other cephalopods, this creature is a detrivore.

It gets almost all of its calories from marine snow.

Marine snow is particulate matter that trickles down from higher up in the ocean and is made of decaying crustaceans, zooplankton, larvacean houses, diatoms, and fecal matter.

The vampire squid has four arm pairs with finger-like tendrils called ciri, plus two long retractable filaments.

It sits practically motionless around depths of 600 to 900 meters, slowly collecting marine snow on its mucus-laden filaments and slurping it down.

Scientists were surprised to discover that marine snow has a pretty high nutritional value, but the vampire squid still doesn't have a whole lot of energy to expend.

They get by thanks to having a suppressed metabolism much, much lower than any other cephalopod of a comparable size.

But as you descend deeper, the cephalopods become even more terrifying.

The genus Magnapina, or the bigfin squid, are thought to be the deepest-occurring squid genus, with sightings as deep as 6,212 meters.

This makes the bigfin squid the only known squid to occupy the hadal zone, the deepest region of the ocean.

Their arms and tentacles can be up to 8 meters long and are usually held at disconcerting right angles, the squid giving itself the appearance of having elbows.

And its translucent, shimmering mantle makes the creature seem completely otherworldly.

Little is known about the feeding behavior of these squids.

Scientists have speculated that they might use their long arms by dragging them along the seafloor and grabbing whatever edible organisms that they can.

Alternatively, the arms might passively grab food such as zooplankton or bits of marine snow.

But in an environment without any plant life to form the base of the food chain, being a detrivore is far from the only strategy.

In fact, a number of organisms that are herbivores or filter feeders in shallower parts of the ocean become carnivores in the deep sea.

That's the case with the predatory tunicate.

It belongs to the ascidian family who are also known as sea squirts, a class of colorful stationary filter feeders that generally live in shallow water.

But the predatory tunicate lives much deeper, anywhere from 600 to 1,100 meters and perhaps even deeper.

These tunicates are perhaps less terrifying than the bigfin squid, but just as strange.

They look almost like an eyeless sock puppet with its mouth open.

But this odd design has an important purpose, to help them catch prey.

As small invertebrates and crustaceans swim by, they sometimes drift into the predatory tunicate's open mouth.

That mouth quickly snaps shut and the prey is trapped, facing a slow death by digestion.

This is also the case with a whole family of organisms known as carnivorous sponges.

They generally start appearing at 200 meters, become fairly common at 400 to 500 meters, and remain dominant all the way down to the hadal zone, below 6,000 meters.

Shallow water sponge species are filter feeders like sea squirts, living on bacteria and phytoplankton.

They're even harvested as shower loofahs, their porous bodies being a good mechanism to hold and form soap bubbles.

But you most definitely do not want to wash yourself with a carnivorous sponge.

And that's because they're covered in tiny spears and hooks, used to passively trap small swimming creatures.

The sponges don't have to do much work.

Whether it's the harp sponge with its long rows of spines, or the ping pong tree sponge with its bulbous spheres, these stationary organisms are highly effective predators.

They just wait for the ocean currents to sweep creatures into their spines, and then they feast.

Despite this abundance of passive carnivores, there are plenty of hunters in the deep sea as well.

One such startling specimen is the gulper eel, found at depths over 1,800 meters in the midnight zone.

Its mouth is larger than the rest of its entire body, and is loosely hinged so that it can swallow animals much larger than itself.

Its proportions may be off-putting, but in an environment with limited food, being able to gorge on a massive meal when you have the chance is crucial.

Its massive mouth can also be used to make itself look intimidating by ballooning up to a hilarious degree.

But most infamous among the deep sea hunters might be the anglerfish, most of which live below 300 meters.

But some can be found down to the abyssopelagic zone, all the way at 6,000 meters.

There are hundreds of species that fall under the catch-all term of anglerfish, which comes from the way they generally hunt, with a lure attached to their heads to attract prey.

This is comprised of an elysium, the modified dorsal spine that acts as a fishing rod, and an eska, the bulbous bioluminescent fishing lure which gets its glow from symbiotic bacteria.

And by using these lures, anglerfish attract a variety of prey species to their gaping mouths, and they've been known to consume absurd quantities of food all at once.

Researchers once captured a black sea devil that weighed 8.8 grams, but its stomach held 12.3 grams worth of eels.

In other words, its stomach alone managed to hold one and a half times its body weight in food.

To be able to consume whatever they come across, anglerfish have highly flexible jaws and generally have teeth that point inward, meaning that whenever they trap their prey, there's little chance of the creature escaping.

And just like the gulper eel, their strategy of gorging themselves whenever they can get a meal is a huge advantage in an environment where food is scarce.

While some creatures use bioluminescence for predation, others use it to disguise themselves, a strategy called counter-illumination.

The lights on their bellies helps obscure their outline against the very faint patches of light coming from above.

But there are some predators who have found ways to see their prey camouflaging in this way.

And perhaps the most incredible of these is Macropinna microstoma, or the Pacific barreleye fish, arguably the craziest looking fish in the entire ocean.

These fish have enormous tubular eyes that are directed upward, allowing them to see prey swimming overhead.

And these eyes are encased inside their head, closed in by a transparent shield.

Their vision is so good that they can not only pick out the silhouettes of their prey, even if it's using bioluminescence, but also gauge the depth they're swimming at and therefore their distance from them.

But initially, scientists were confused about how it could effectively hunt when its tiny mouth points in a different direction than its eyes.

They wouldn't be able to keep their eyes on the prey while moving their mouth to eat it.

But the scientists were shocked to discover that the eyes can roll forward and see through the front of their heads as well as the top, for a maximum arc of 75 degrees.

For comparison, humans can only move our eyes about 45 degrees at most.

For us to do what barreleye fish do, we would essentially be able to look directly up at our brains.

This strange adaptation allows barreleye fish to keep their prey in view at all times, even as it snaps its body vertically to snatch the food swimming above it.

But barreleye fish live in the twilight zone, where there's still a tiny bit of light available for them to make use of.

What about fish that are even deeper, down in the bathypelagic or even abyssopelagic, all the way down to 6,000 meters?

With no sunlight penetrating these depths and with little need to detect bioluminescence, some deep-sea creatures forgo vision altogether, like the tripod fish.

Instead of it relying on sight, they balance on the seafloor like little tripods, waiting for other creatures to swim by.

Their elongated pelvic fins stretch down to the ground, holding them motionless and upright, while they use their pectoral fins like hands reaching forward.

These fins are filled with enlarged spinal nerves, which helps them sense the water currents and whatever objects might float their way, from small crustaceans to fish.

In this way, they're essentially able to sit tight and wait for food to come to them, even though they can't see it.

And while these fish are certainly deep, they are by no means the deepest ones.

Recently, scientists exploring a marine trench near Japan were stunned to find a fish living 8,336 meters below the surface, the deepest fish anyone has ever found and is likely to ever find.

What they found is a type of hadal snailfish, a translucent, scaleless, tadpole-shaped creature.

Little is known about this ghostly, ethereal fish that thrives in a place named after Hades itself.

Is this deepest of fish the creepiest?

Maybe not, though it's certainly not winning any beauty contests.

But from the beginning of the ocean depths, just as sunlight disappears, down to the crushing depths of the deepest trenches, there's no denying that the creatures who live here seem like characters out of the darkest corners of our imagination.

And yet, every freaky adaptation is something crafted by millions of years of evolution to live in such an unusual environment.

It's not hard to see why deep-sea creatures invoke creepy feelings in us.

The creatures are nothing like the ones we see in our daily lives, and without the advent of robotics, we would never be able to see them in their natural habitat.

Their habitat is so far removed from us that we liken it to the underworld.

And for whatever reason, humanity subscribes to the archetype that the sky holds possibilities, euphoric cloud nines, and heaven itself, whereas the depths hold fears, secrets, and eternal death.

Perhaps this is no surprise as intelligent, air-breathing mammals that what is bright and known is safe to us, and the unfathomable sea, mysterious, dark, and otherworldly is not.

But our ancestral consciousness perhaps never grasped that the world is all connected, that the bountiful rivers and shores that we rely on are directly linked to the deepest parts of the sea.

That what seems alien is in fact a vital part of our world, and deserves our consideration, care, and protection like any other part of our planet.

The evolution of ocean creatures is undeniably fascinating, for all the freaky and foreign adaptations that excite our imaginations.

But some of the most astonishing adaptations on Earth happen right here on land, and in our own species.

Human eyesight is one of the most complex biological processes we know of.

But what's even more amazing is what individuals who have lost their sight are still able to see.

Human sonar allows people without sight to see with sound.

And other individuals who lost their sight due to damage to their visual cortex can still complete obstacle courses with no knowledge that they even did so.

These phenomena are the subject of the next Real Science video, which you can watch now, two weeks early, on Nebula.

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