The deep-sea abyssal zone is the coldest, darkest level of the ocean, where the pressure would splatter your head like a Gallagher act. Amazingly, life abounds here. Life from the howling pits of H.P. Lovecraft's nightmares.
Apparently what the abyss considers an "octopus."
Try to imagine being a small, hungry fish in the perpetual, freezing blackness of the abyssal zone. ABYSSAL ZONE. It's exactly as scary as it sounds. Without any plant life, nearly everything has to be a strict flesh-eater, and all you can usually see of other animals are whatever dimly glowing appendages they want you to see. Every time you bite down on what resembles a tasty shrimp, chances are good that it's the tentacle of a hungry monster ready to snap shut on your skull like a bear trap. If it happens to be smaller than you, it probably evolved to inflate like a balloon, unhinge its jaws and spend the following weeks digesting you alive. What did you expect from the ecosystem closest to Hades?
As usual, the fundamentals of Marine Biology are most elegantly expressed by Spongebob Squarepants.
In the 1800's, then-renowned naturalist Edward Forbes declared that life could not possibly exist deeper than 2,000 feet below the water's surface. Today, we know that more species thrive in the abyssal zone or "abyssopelagic" than any other environment on Earth, and the abyssopelagic technically begins at 13,000 feet. Suck it, Forbes. This is why most people reading this have never even heard of you. Every time we send a robotic camera, specimen trap or MANLY AS FUCK biologist to the primeval bowels of the sea, we end up with a host of new species that seem to do with evolution what Dali did with a paintbrush.
Those green things in its see-through forehead? Those are its eyes.
The majority of light in the deep comes not from our loving, life-giving sun, but from the cold biological luminescence of symbiotic bacteria cultured in the flesh of local fauna. Some produce this living light to communicate with members of their own kind. Others can scatter luminous slime or flare up like fireworks to confuse their attackers. More commonly, light is used to lure prey, such as in the famous "anglerfish" or "sea devils," also known for their outrageous reproductive habits (see: The 15 most bizarre animal mating rituals )
We swear to God this is a fish.
One reason the abyss supports such a wide array of organisms is, for lack of more poetic analogy, that it more or less functions as one giant oceanic septic tank. What scientists call "marine snow" is continually drifting down from above in tiny flakes, blanketing the abyssal floor in a thick, whiteish powder. This "snow" consists of about one part bacteria and one part organic waste; the final remnants of all the sea's excrement and dead bodies. Catch THAT on your tongue, Linus.
We would tell you what this is, but that would spoil all the fun.
Another fascinating (or terrifying, depending on your world view) quality of the deep is that it supports much larger invertebrates - A.K.A. "bugs" - than anywhere else on Earth, including worms you could hang yourself with, crabs that could feed a family for weeks and even single-celled amoeba the size of your big toe.
They're called woodlice, pillbugs or roly polies on the surface world. Your pitiful, laughable surface world.
As if the abyss wasn't brutal enough on its own, these depths are broken up by a form of secondary mini-environment: the deep-sea hydrothermal vent or "black smoker."
Hell is leaking.
These underwater geysers belch forth a steady stream of poisonous sulfides from even deeper in the Earth's crust at temperatures of up to 400 celsius, a boiling toxic oasis that actually supports life at up to 100,000 times the density of the non-boiling, non-toxic water only a few feet away. The species that inhabit these natural hellholes have adapted to live nowhere else on the Earth, by which we mean that any given vent can harbor species found exclusively at the base of that vent and that vent alone. Even smokers a few yards apart from one another can host entirely different, unique ecosystems, a possible glimpse into just how easily life could arise on even the harshest of alien worlds.