The ocean is mysterious. We know more about the surface of Mars than we do about our own oceans. The really deep, dark parts of the ocean are even more mysterious. When depicted in fiction, the ocean floors and trenches may be populated by unrelenting horrors the likes of which no human can comprehend: Alluring Anglerfish, Giant Squids, gulper eels, horrific leviathans — any briny monster you can name, and plenty more that you can't.
Until recent technological developments, no one had a chance at observing the deep sea. No ray of light can reach that depth, and Dark Is Evil. Oceanographers had almost no information to work with, and Nothing Is Scarier. Imagination ran wild. Some surmised that the ocean depths were a sort of Lost World where monsters that had ages ago vanished from the surface could still be alive and hiding. Great strange beasts that have not set foot on the shore since prehistory might be silently stomping across the silty seafloor. Creatures spawned before The Great Flood might be swimming down there, torpid and undisturbed — not disturbed yet, at least.
Submersibles and ROVs have allowed closer study of the ocean depths, and oceanographers don't expect to find Cthulhu. Nevertheless, this is still Truth in Television for different reasons: the environmental conditions (low or no light, high pressure, low temperature, low oxygen, and very little food or prey compared to shallow waters) in the midnight,note abyssalnote and hadalnote zones of the ocean have led organisms to evolve traits we land-dwellers tend to consider horrific. For example, many creatures do not have eyes or have evolved special eyes due to the lack of light, and are usually either very dark, transparent, or bright rednote in color. This biome is utterly inhospitable to humans, and the logistical difficulties of conducting research in it only adds to these regions' mystery.
Not to be confused with Time Abyss. May overlap with Eldritch Location, although the work can portray the deep ocean as relatively "normal" but just home to creepy things. Also compare Space Is an Ocean, as stories about exploring the deep sea may share genre conventions with space stories and portray sea life in a manner similar to aliens, and Ocean Madness, the feeling of insanity after being out (or in this case, down) at sea for too long.
- Dungeon Keeper Ami: A very toothy, ugly deep sea fish is described:
A tiny deep sea fish investigated the probe from up close, startling Ami by filling the screen with a set of fangs that even Rabixtrel would have envied.
- In Finding Nemo, Marlin and Dory accidentally swim into deep waters and encounter an anglerfish, which almost eats them.
- In The Little Mermaid (1989), Ursula's lair is in a deeper, darker part of the ocean (as she was exiled from Atlantica for being, well, a witch) and inside a creepy-looking prehistoric animal skeleton. The whole atmosphere should tell you she's Obviously Evil.
- In The SpongeBob SquarePants Movie, SpongeBob and Patrick make their way into a deep ocean trench with horrific monsters. Subverted when the monsters take a liking to them after they show off their impressive dance skills.
- The Abyss by James Cameron, which features a United States SEAL team trying to salvage a destroyed nuclear submarine in the deep ocean, and encountering strange creatures dwelling there.
- The Always Chaotic Evil creatures who reside in the Trenches in Aquaman are all horrific monsters. They actually evolved from original Atlanteans into what they are because of the natural environment of trenches. Going even deeper, we eventually find a tropical Lost World with Living Dinosaurs.
- DeepStar Six has an underwater military outpost attacked by a big worm-like Sea Monster.
- The Evil Below is about a cursed shipwreck, guarded by a Sea Monster.
- Godzilla: King of the Monsters (2019) has a sequence in the Underwater Ruins of an ancient, Godzilla-worshiping city deep within the Hollow Earth. The ruins are infused with radiation from the Earth's core, making it Godzilla's preferred place to rest and heal after a fight.
- It Came from Beneath the Sea opens with a submarine exploring a deep ocean trench, where they have their first encounter with the giant octopus. Unfortunately, it is not content to stay down there.
- Leviathan is about an underwater mining crew 15,000 feet below the surface that slowly dwindles due to attacks by a mutant creature.
- Lords Of The Deep is about a deep sea research station crew encountering mysterious psychic creatures.
- Life of Pi: The dream sequence where the camera goes ever deeper in the ocean features a sperm whale attacked by a giant squid then exploding into zoo animals, a hideous anglerfish/squid hybrid, and the sunken cargo.
- In The Meg, scientists accidentally release a megalodon sharknote from the Marianas Trench. According to the original novel, megalodons are explained to have evolved into an abyssal species, but are no less horrifying or threatening for it.
- Pacific Rim: The Breach from which the monstrous kaiju come from is located at the bottom of the Pacific Ocean, and there is an extended underwater fight scene as the PPDC tries to get closer and close it.
- The Phantom Menace: The core of the planet Naboo holds vast subterranean oceans, which are inhabited by titanic dinosaur and eel-like creatures. Jar Jar believes a trip through the Core would be a Suicide Mission, but the main characters survive with only minor incident.
- In the horror-survivor film Sweetheart, the waters immediately surrounding the deserted island are sparkling, clear and shallow ...except for an impenetrably dark, perfectly round, and seemingly-bottomless black pit from which a monstrous, shark-like predator of unknown origin emerges each night.
- The Rift is about a submarine crew on a doomed rescue mission to an underwater rift full of monsters. Unlike the above films, it came out in March of 1990, just a few months late to the party.
- Underwater follows a deep-sea drilling team who dig too deep and end up unleashing Cthulhu. Yes, the Cthulhu.
- 20,000 Leagues Under the Sea is an early instance of this trope. Mostly, it emphasizes the idea of the ocean as a place of wonderful nature, but in the deeper parts, the Nautilus crew encounter some very strange things, including a Giant Squid and the Underwater Ruins of Atlantis.
- H. P. Lovecraft, creator of the Cthulhu Mythos, had an irrational fear of the sea, leading to many of his Eldritch Abominations bearing traits of aquatic creatures, and many of his threats coming from the ocean's depths.
- The Shadow Over Innsmouth. The ocean floors are inhabited by the Deep Ones: Ageless, cultish Fish People who worship Eldritch Abominations from beyond the stars and receive strange powers in return. That said, it's the Deep Ones who come to land for mates who are the most troublesome...
- The Call of Cthulhu. The "nightmare corpse-city" of R'lyeh lies at the floor of the remote South Pacific. In it, the Great Old One Cthulhu lies in deathless slumber, waiting for the Old Gods' return.
- The Temple. The crew of a German U-Boat during World War One succumb to what is either Ocean Madness or the influence of whatever lurks in the Underwater Ruins they find at the bottom of the Atlantic Ocean.
- Deeplight: The Undersea, a sub-ocean created by human fear and populated by Eldritch Abominations. The water is breathable and the crushing pressure that ought to exist in the depths is absent. You can swim around with no equipment, but the environment messes with your mind and may leave you with "Marks" (small mutations, a tentacle here, a patch of shell there)...
- The Gorunna Trench, mentioned a number of times in the series, is the deepest part of the Disc's seas and home to horrific things — according to some, horrors from the Dungeons Dimensions still lurk within it. The Colour of Magic notes that even krakens only go through it in pairs, and deep-sea fish keep their lights doused to avoid attracting attention to themselves.
- Going Postal proposes the notion of areas deep enough that the pressure surpasses the density of sunken objects, causing ghost ships that float on seas-under-seas.
- Into the Drowning Deep features killer mermaids which inhabit the darkest and deepest parts of the Marianas Trench. These mermaids kill and feast upon anything that enters this territory and are armed with sharp teeth and fangs. They also despise light hence their affinity for the darkness of the trench.
- In From The Deep Of The Dark, the invading sea-bishops originally emerged from a portal at the bottom of a deep ocean trench, and are conspiring to re-open it so their countless fellows can join them.
- In Alfred, Lord Tennyson's poem "The Kraken", the monster in question lurks in such a location. This poem was, unsurprisingly, a big influence on The Call of Cthulhu, listed above.
Below the thunders of the upper deep
Far, far beneath in the abysmal sea
- In The Voyage of the Dawn Treader, when Lucy peers over the gunwales and views deep-sea merfolk settlements beneath the crystal clear water, she notes that the towns and castles are all on sea mounts and other high peaks. She surmises that deeper, darker, colder waters must be considered undesirable or even scary places by the sea-dwellers, in an inversion of how steep, icy mountaintops aren't much favored by humans.
- Barotrauma is an indie game involving a submarine navigating the abyssal depths of Europa's frozen oceans, which contain nightmarish deep-sea predators as well as the remnants of a mysterious alien civilization.
- One late-game chapter of Beyond: Two Souls is set in and around a pseudo-Chinese deep sea base where the Chinese military is conducting experiments on a nearby Infraworld portal. Since precious little in poor Jodi's life ever goes right, things spiral out of control quickly until the portal starts unleashing swarms of hostile Entities that proceed to lay waste to the facility and everyone in it, thus truly turning the place into an eldritch horror scenario.
- The Persephone prison in Bioshock 2 is suspended over a huge ocean trench, with glass walkways showing it. Sofia Lamb had set up explosives intended to drop the entire structure into this trench, but whoever survives is determined by your actions in the game.
- Played With in Bloodborne. In the Back Story, the Byrgenwerth scholars had originally discovered evidence of the Great Ones' existence in the old underground labyrinth of Pthumeru, but their first actual encounter with a Great One in the flesh took place in the Innsmouth-like Fishing Hamlet where a Great One named Kos washed up ashore. From this one discovery, the scholars have jumped to the false conclusion that all Great Ones are linked to great bodies of water, and subsequently focused much attention on the ocean. Lady Maria, in particular, after cutting her ties to Byrgenwerth and joining Laurence's newfound Healing Church, spearheaded the research into human ascension by having Research Hall subjects imbibe sea water until their heads turned into fleshy blobs, then extracting "brain fluid" from said heads, and feeding it back to them until only the head blobs remained. Needless to say, this was a dead end, and Maria committed suicide after realizing it. It wasn't until after the Choir was formed to take over her research that the Church realized that while Kos dwelt in the ocean, most Great Ones actually reside in the cosmos above their heads.
- Both Endless Ocean games have levels themed around diving into deep ocean trenches and meeting animals such as goblin sharks and sperm whales.
- Final Fantasy V has the Great Sea Trench. Although the dungeon is located underwater, the characters can breathe in it. Likewise, the enemies are not aquatic, but still grotesque-looking, such as brain-like blobs, large worms, and lumps of bones attached to the ceiling, all of them known as "Unknown".
- Implied in Guild Wars 2, though the deepest sea is never shown in game. The depths of the Unending Ocean are occupied by the Deep Sea Dragon, an Eldritch Abomination who's sent the other residents packing. Such residents include the Krait, an Always Chaotic Evil race of snake-people, and the Largos, a mysterious race of creepy (though not necessarily evil) humanoids with Vader Breath.
- Kingdom of Loathing: There are a number of decidedly Lovecraftian encounters that can be had when searching for two of the Sea Monkees on the Sea Floor.
- In the mer-kin deepcity, you can come face to face with — and also kill — the two gods of the mer-kin: Shub-Jigguwatt, Elder God of Violence, and Yog-Urt, Elder Goddess of Hatred. The father Sea Monkee is found at the end, hooked up to a machine that opens a portal to... somewhere that might be outer space but is likely something far more alien.
- The Caliginous Abyss is themed entirely around being a horrific, alien place in the deepest part of the ocean. When the player heads there to look for the mother Sea Monkee, they face several surreal non-combat encounters — such as fish with twisted human faces, incongruous mother-hen messages carved into rock walls or written out of glowing sea life, areas of the seafloor carved by the passage of hundreds and hundreds of tentacled somethings — and combat encounters against things such as enormous eyes staring out of the darkness, slithering armored things, a twisted parody of Mr. Peanut and a group of carnivorous things referred to simply as a "school of many".
Within the darkness there are fish, and things that are not fish, and things that are not anything a land-dweller was ever meant to see. This is a school of one such creature, and it doesn't look friendly. It looks mostly like teeth and hiveminded appetite.
- League of Legends has this theme to the Forgotten Depths skin line, which recasts Fizz, Kassadin, Kog'Maw, Malphite, Nami, Nautilus, Syndra, and Thresh in a Lovecraftian The Shadow over Innsmouth-esque light.
- The appropriately named Mass Effect 3 DLC Leviathan. Shepard investigates some mysterious orbs with Mind Rape capabilities, and ends up descending into the depths of the ocean inside a specially-built mech. At the bottom are the creators of the Reapers. Shepard ends up browbeating them into helping with the war effort.
- Implied to be the case in Salt and Sanctuary. The monsters of the world, referred to as Kraekan, all believed to originate from the sea, and many even boast aquatic features even though they appear to be terrestrial. The ending further implies that the island, itself an Eldritch Location, exists within the depths of the ocean.
- In SOMA, while most of the game takes place in the Pathos-II Underwater Base situated on a huge plateau, the last third of the game involves taking a 4000-meter elevator ride down into the Abyss. To reach sites Tau and Phi, you have to follow a path of barely visible lights while enduring a powerful ocean current and rabid sea creatures mutated by Structure Gel, including a Giant Squid and an enormous angler fish.
- Numerous biomes in Subnautica count as this, but the Inactive Lava Zone, Active Lava Zone, and Lost River, and the Crater Edge biomes fit this trope the best — being inhabited by enormous monsters such as the Sea Dragon and Ghost Leviathans, and littered with the fossils of even more massive creatures.
- Subnautica: Below Zero continues this trend with the Crystal Cavern and Fabricator Cavern, the two deepest biomes in the game. Generally with both games, the deeper you go, the more twisted the landscape becomes and more dangerous creatures you encounter.
- The eponymous Sunless Sea is this, naturally - the entire game involves exploring the "Unterzee", a colossal, ever-shifting, underground expanse of water full of dangers and oddities. Late-game dialogue crosses this with Space Is an Ocean, serving as a Sequel Hook for Sunless Skies.
"One day, you'll sail another ship, on another sea more sunless. One day."
- A Commodore 64 game Terror of the Deep (one figures there are only so many possible combinations of "deep" and "terror") has you descend in a steampunkish bathysphere into Loch Ness — meaning, not an ocean, but Tropes Are Flexible — in order to defeat the monsters living in it. The oxygen slowly runs out, they may come from any direction, and you can only look through one window at a time.
- Indy game Water Womb World involves a Christian scientist hunting for the supposed true origin point of man in Gaspar's Zone, a remote location in the middle of the ocean. Outside his sub in pitch darkness he finds ancient species of fish, mysterious coral, and the actual being man evolved from, then becomes one himself.
- Water Womb World is a short indie game involving a scientist exploring a deep part of the ocean to find the real place in which God created the first humans. What they find may deviate a bit from how the Bible is normally understood.
- We Need To Go Deeper is an indie game involving a submarine crew navigating a labyrinth of tunnels while fending off giant sharks and whales. Later updates to the game, however, have added increasingly Lovecraftian monsters.
- In World of Warcraft the Old God N'zoth was chained at the bottom of an oceanic trench by the Titans. Since then he has corrupted the surrounding sea life, creating the zoatroids and mutated fish through which he can communicate. The Naga city of Nazjatar and Azshara's Eternal Palace lead to the abyss where players inadvertently free N'zoth.
- Before N'zoth, there's the underwater super zone of Vashj'ir filled with multiple demigods that take the form of crustaceans the size of cities, faceless ones, and several elites that will oneshot any player foolish enough to attack them.
- As the title implies, X-COM: Terror from the Deep is about an independent military force funded by the world's governments that battles ancient alien invaders originating from deep within the Earth's oceans. The game borrows heavily from Lovecraftian influences, going so far as the Big Bad being a sleeping ancient evil that the X-Com forces must keep the aliens from awakening.
- Subverted in 8-Bit Theater: Despite Black Mage's impressive narration about the horrors of the deeps, they don't actually encounter anything particularly mind-breaking.
Black Mage: We have always lived in the ocean. There is no before. There will be no after. There is no place for time here among the dark, among the alien things that crawl and swim in a sea without light. We are one of them now. We have always been one of them.
Red Mage: Black Mage? You're narrating again.
Black Mage: My companions succumbed to sea madness weeks ago.
Thief: Dude, it's been two and a half hours.
Black Mage: I alone maintain a gimmer of humanity. I fear it is fading fast.
Red Mage: What kind of dark wizard in league with nameless forces of primordial evil are you that you can't even make a successful sanity check versus boredom?
Black Mage: I ignored the pitiable babbling that issued from their misshapen lips. Their wet gurgles, a noise that had once been words, came to an end when one of the things outside assaulted our craft. I welcome death.
- Unsounded: Scripture teaches that the ocean floor — which is cut off from the Background Magic Field and therefore from the afterlife as they understand it — is where the Gods built Hell, for the souls of the damned to be trapped in the frigid, crushing dark for all eternity.
- This picture of the contents of the ocean,◊ which gets more terrifying the lower you scroll, with the absolute bottom of the ocean containing Spongebob Squarepants.
- Gemini Home Entertainment: "The Deep Blue" portrays the "Demisia Tunnel", a fictional offshoot of the Marianas Trench, as a cave of unknown depth that is inhospitable to all life. Not including the glowing-eyed, tentacled Eldritch Abomination seen at the end.
- The animated short The Deep features a dark deep-sea ocean full of fish and sea creatures made from various mechanical parts, accompanied by eerie ambient music.
- In the "Rock Bottom" episode of SpongeBob SquarePants, the town of Rock Bottom is populated by numerous deformed-looking fish that terrify SpongeBob and Patrick.
- Played with on the Wild Kratts octopus episode, when what seems like a gargantuan deep-sea horror to the explorers is actually a normal octopus turned Mix-and-Match Critter by a Power Suit malfunction. Averted in the double-length hydrothermal vent episode, in which the weirdness of the local sea-bottom life forms merely evokes the team's wonder and admiration.