Brainstorming Thread: Undersea Horror

What elements make up a good "undersea horror" game?

>Unsettling marine life?

>Graveyards of wrecks?

>Dark, crushing depths?

What scenes and setpieces are must-haves for such games? What encounters or enemies are best suited to them? How do you really put fear of the watery abyss into your players?

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youtube.com/watch?v=H4QFhfQCc5w
pastebin.com/wxkJQtwc
exhentai.org/g/775255/97810c12ff/
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daddy cthulhu's tentacles

Check out Diluvion. It's got all that shit you need.

youtube.com/watch?v=H4QFhfQCc5w

I like what I'm perceiving here to be a contrast between comfort and danger. The subs and stations are rustbuckets, but warm and familiar ones, while the undersea is wondrous and mysterious but also terrifying and unpredictable.

Bingo. You want players to feel scared? You need to give them places to feel safe. If they feel scared all the time, scary becomes the new normal and it stops being scary. Have shallow regions of sea that are familiar, well-traveled, and relatively safe. There's still dangers there, of course- pirates, sudden and violent currents, etc.

When they're trying to return to these safe areas, provide obstacles, add delays. Limit how long the sub can go without docking somewhere to restock air, fuel, etc- this adds urgency when they can see this vital resource just tick tick ticking away.

The greatest source of unease is going to be that they're fucking blind in the dark depths of the ocean. Sonar is only so effective, and spotlights only go so far. What's worse?

Both are going to attract the attention of other things in the deep. Not just submarines, but creatures. If you're trying to shake a kraken, for example, the second to last thing you want to do is kick on your floodlights to make sure you aren't going to bump into anything. The absolutely fucking last thing you want to do is send out a sonar ping, because then that nasty sea beasty is going to know exactly where you are, and so will every other nasty sea beasty for miles and miles.

The players should have to decide if they want to use sonar or not. Sure, it reveals vital information about their surroundings, but it broadcasts their position to everyone. Have the players navigate a minefield while being pursued by pirates or assassins. They have to send out sonar pings to try plotting courses, but each one they put out brings their pursuers closer to plugging a torpedo/scrap cannon shot up their ass.

That puts me in mind of some location, a stretch of the abyss where a sub crew must maintain silence lest they attract the attention of something dark and terrible. No sonar. No radio. No talking or making any loud noises inside the sub at all. Test how well the PCs can use improvised hand signals and note-writing and maintain their cool when surrounded by danger they can barely perceive.

Tell me about the setting you have in mind, user. Why are the players underwater at all in the first place?

If you don't really have any in mind, I cannot recommend you play Diluvion enough. It has such a unique and colorful setting that would be perfect for a TTRPG.

In all honestly, I hadn't begun the thread with a particular game in the works. I enjoy the creative process and seeing what wonders and horrors Veeky Forums can come up with together. I thought it might be fun and inspirational to others wanting to run undersea horror games.

The first thing I would do is make a systematic chart of the sea and a nautical map.

Another thing I would do is have the locations of all sunken islands or flooded areas that have been undersea for longer than 1000 years.

Dark crushing depths are a must, but remember, despite the enormous pressure, quite a few things are huge as fuck in the deepest darkest areas, due to abyssal gigantism.

As for unsettling marine life, Dunkleosteous is a good example of that, so is the Pelican Eel and the Goblin Shark. The Greenland Shark is unsettling too, but seems to be mostly a scavenger and has small weak teeth. Anomolocaris is another freakish creature, and Hallucigenia. Note that I'm not 100% I'm spelling these latin species names right, but they're all extinct, I doubt they would get triggered by it.

But, as long as we're brainstorming, there are a few ways you could get a group of PCs underwater. Either you create a setting where mankind in general live there:

>Fantastical underwater world where there is no topside to return to.

>Post-apocalyptic waterworld wracked by storms on the surface.

>Sci-fi waterworld where live can only exist under the alien oceans.

Or you create a situation that sends the PCs there:

>The PCs crew a sub sent out to investigate strange radio signals.

>The PCs' ship capsizes and sinks, forcing them to escape somehow.

>The PCs are treasure hunters that find much more than they bargained for.

Is this undersea horror thing you're discussing set to take place in some specific year? I have some stuff that could help you, but its from a game that took place in the evolution-of-life period, long before humans or technology existed yet, so its mostly prehistoric creatures.

For me just having water nearby is scary enough for my players. They hate all the penalties for fighting underwater, needing air, etc.

But the things I do, even when I can't send them to the bottom of the ocean is try and show people able to survive off the bounty of the ocean in spite of the terrors out there.

Oh sure, they have an amazing pearl diving area just off the coast in a beautiful tropical reef, but only the bravest and fastest ever return alive. The bones of the foolish and slow are scattered amongst the clams they tried to harvest. And some people tell tales of the massive, sentient mollusks that climb up out of the surf, bury themselves in the sand close to docks and homes to ambush weary fishermen and townsfolk.

I'd had vague notions of a 1970s-1990s era adventure, but your "evolution-of-life period" ideas sound interesting.

In the game this was originally for, you were playing as the species and struggling to move towards sentience and total domination of the world, so its listed as 'advantages and disadvantages' rather than hard and fast stats.

I'll put together a pastebin of all the underwater beings the game used and post it here.

...

Forgot my picture. This was a fun fight. Basically a Mimic, but from the sea. One of my players kept the tongue blade and forged it into a weapon for himself.

Thanks!

Violent mollusks and crustaceans or squid that use shipwrecks for shells would be good.

Set to expire in two weeks.

pastebin.com/wxkJQtwc

Shipwrecks offer a ton of opportunities. Basically any sort of 'encounter' you might find at the bottom of the sea at any depth is good stuff.

>shipwrecks and what is left inside, or now using it as a home.
>dead creatures of unusual size and what is feeding/growing on them.
>Brine Pools. (google it)
>Reefs
>sudden drop offs
>undersea volcanoes
>vast tracts of endless nothing (abyssal deserts).
>unnaturally deep caverns where two bodies of water intersect such as a bay or delta, or the bottom of a waterfall.

>pastebin.com/wxkJQtwc
Very nice! I'll have myself a browse.

The game used a d100 rolling attempt system, so where you see +80s and such it simply meant that attempts of that action got a +80 out of 100 possible (so pretty much never failed).

Like Belize's Blue Hole, but in the middle of the wide ocean and with no bottom yet discovered.

Could be a gate to the elemental plane of water, or the spot where the gods opened the seabed when it was ready to receive water, the water coming out of the blue hole.

Don't forget the shipwreck could also be a flying vehicle or an outsider's trans-dimensional vehicle.

I personally like using shipwrecks to be a 'show, don't tell' way of explaining some piece of my world's history.

Is there an enemy that's been forgotten for an age, just waiting to return? Find the preserved corpse of one amidst the skeletons of a crew of pirates. Maybe a buried journal that tells the tale of them chasing a beast out to sea or taking the last of a species out to the trench to drop it in so no one would ever find it. The wreckage lets you know they were unsuccessful.

Also, need to add flavor to your boring low level monsters? Give them a swim speed and the ability to breathe underwater and suddenly your goblins become much scarier (pic related).

Agreed, one way I do something similar is that I usually use Archeology sites as 'show don't tells'. Ancient ruins whether above or below water can perform this function admirably as well as shipwrecks.

I find that its easier to put together a history of tidbits and then put them together like words in scrabble to make whichever world history path is needed.

In the sea however, it should always be possible for PCs to encounter things that are unknown eldritch who even knows wtf they are creatures. First ed D&D did this with its 'Sea monster' entry in the monstrous manual. It was more a wide-range of creatures with enough room for imagination, than 1 specific sea monster with hard and fast stats.

2 words for you.

>Zombie
>Whales

Barotrauma had an atmosphere of tension, though not necessarily horror. It was set upon Jupiter;s Moon, Titan, where research complexes and mining outposts are scattered across the surface with you serving as research assistants and couriers of supplies, depending on what jobs you accept.

In a singleplayer game, the only conflict came from managing the sub and the creatures outside which could vary in size from "small enough to fit inside your crew compartments" to "Bigger than your submarine".

In multiplayer, additional tension came from one of the crewmembers potentially being a traitor whose job was to make sure one or more of the crew never make it back ashore, or from someone being attacked by a specific enemy that lays its eggs in its victims that can eventually takeover and puppet parasite them.

How would a modern ship or submarine crew respond to finding a real bona-fide cursed you're all going to die treasure?

To make it more horror-y, have it so they end up underwater in a normal way, like a submarine, but then they can't leave. If they try to go up, they go up and up and up... but all their instruments and measuring tools show that they're still down. They climb for miles and miles, and never get any higher.

Have a few fun horrid nasties for your water and near water encounters.

Sluicemouth gatefish - grows ~18 feet long, its jaw is covered in 3 inch razor sharp teeth and is 8 feet long when fully extended. It gets its name because it can open it's mouth, and bite, in around half 1 turn.

Walrus Cucumber - Grows until it can't grow further, though it only grows about 1/4th of an inch per year, its lifespan is extremely long. It gets its name because it has two huge teeth in its otherwise sucking/rasping mouth. Specimens over 60 feet long have been found.

Ripplestriped Strangler - A color-changing (continuously changing colors) octopus who signals aggression by suddenly color-changing to become nearly invisible, the ripplestriped strangler may only be 2 feet wide with eight 6 foot limbs, but it is an innate telepath and of beyond genius intelligence. Approach with extreme care.

Lockbiter Piratefish - So called because it's 'black and white' coloration resembles the jolly roger. The lockbiter's bite force is enough that it gets its name by biting completely through sea-wreck treasure chest locks to reside in the treasure chests. It's favored method of attack is to spring out of a container and bite the living HELL out of you. It isn't especially big and it isn't poisonous, but it's bite is capable of sheering through a steel lock.

Gnomeheaded swallower fish - So called because it's head looks like a big pair of gnomish-make sock-hats, the gnomeheaded swallower fish's long trailing head tails have dozens of tiny eyes on their tips, and it's bite is like being skewered by a pike, as a single large tooth shoots out to impale. It is unknown exactly how big they get but specimens up to 50 feet long have been seen.

Ghoul Shark - So called because it wallows in corpses as a means of dislodging meat to eat it. Ghoul Sharks are extremely toxic, as they carry a version of leprosy that doesn't harm them (only you), and they grow to be about 16 feet long.

Have a concept I once shared here.

>Hermit crab that reproduces via fear
-So these guys live in a specific place on the continental shelf
-They have a scary appearance, the reason of which I'll explain in a sec
-If a person or animal with enough brain got constantly scared in their territories, a small hermit crab begins to grow in their brain
-Once a fear threshold has been reached,the crab undergoes accelerated growth, crushing the infected's cranium outwards
-Then they go on to scare more critters
-They share their habitat with a similar creature, which are sea spiders

And some more horrid nasty fish things for you.

Abyssal Abalone - So called because it's shell is completely black, it sweats out a light blue sludge that glows very softly, and it's foot muscle branches into five flexible tentacles, rather than simply one big foot. It is capable of growing to the size of a galleon ship, but this takes over 1000 years. It is thought to be one of few naturally immortal animals in the world. Fully grown Abyssal Abalones can drown whales by gripping them until they suffocate.

Slurry Batfish - So called because it sweats a light red grease from itself continuously and its large flexible fins resemble a bat's wing. The light red grease has many flamability related uses, though the meat needs to dry out for 2 days before it can be eaten. ~4 feet.

Cadaver Jellyfish - A 4 foot wide jellyfish with 6 foot tentacles, it is 'infected plague victim' colors and it's touch is very toxic, as it causes bubonic sores to form, further symptoms include; difficulty breathing, shivering, eyes watering, the delusion that you are already dead, bleeding profusely, a single big seizure, usually followed by death.

Scimitar Whale - A beaked whale that is around 15 feet long, its beak is a pair of mono-teeth, the top 1 big tooth and the bottom likewise, that are bent in such a way that they resemble a crossed pair of scimitars, getting bitten by it almost always results in amputation.

Skeleton Crab - So called because of its coloration, a cadaver eating crab with six 8 foot long legs and 12 stick thin 6 foot long arms ending in nipper-claws.

Shaggy-Headed Butcher-fish - So called because of its vicious array of sawtooth and razor teeth and the fact that it lives in polar seas so cold it actually has a small amount of outwardly visible hair. A vile tempered brute who will bite anything that gets anyplace close to it, though it is only 2 feet long.

It’d Be a minor detail, but in certain under water caves, the salt water and fresh water have settled so it looks like there is air where there isn’t, and could be really jarring.

It sounds crazy but I think that's actually chemically possible. I can see how a 'bubble' of fresh water might settle over its source in such a way that it appears to be an air-chamber surrounded by salt water.

I’ve seen footage of it in a documentary: it’s pretty trippy. Doubly so if you are underwater spelunking, think you’ve found an air bubble, only to swim into it, and have to wait ten minutes until your brain can figure out what just happened. It’s a completely mundane thing the uninitiated could mistake for magic.

Now I just have to come up with some insanely brutal sea creature that uses this as an aid to predation.

>There is a seemingly bottomless hole in the ocean, into which many maritime explorers have vanished over the years.

>Those that have returned tell tales of unnatural sealife, eldritch ruins and the ghosts of the seamen that never returned.

What assort of curse are we talking about?

...

I've thought of a few, here they are.

Forceps Lobster - Ranging in color from light purple to solid brown, Forceps Lobsters have very long, slender claws, which are razor sharp and clench with sufficient force to cut through copper. A forceps lobster adult weighs around 7 to 10 pounds.

Deadly Reaperfish - So called because its lower jaw curls around like a scythe curve, and is coated in a highly malignant poison. The fish is a mix of black and white shades and has long, draping fins that resemble a voluminous robe, allowing it to swim through the fresh water bubbles it hangs out near without being harmed, since its fins hold big blobs of salt water.

Coral studded Ax-fish - So called because it sweats calcium out of its body to coat itself in a salty, protective shell, after which it waits at the bottom of fresh water bubbles and simply eats anything that dies from the water, it's head shaped like a battle-axe. It is 2 feet long.

Coin Flounder - So called because it has wide flat scales that resemble golden coins. It is about 3 feet wide and a little over 4 feet long, extremely flat wide and narrow, the creature's entire body is only slightly thicker than a slice of bread, which is why it has such large powerful scales.

Barrel Eyed Slog-fish - So called because it's telescopic eyes point upwards and resemble clear barrels. The slog fish rustles around in the mire of the seabed searching for food and has a sudden pop-out double mouth that can bite something up to half a foot away.

Lantern-bearing Goblin-fish - So called because of its green skin tone, pointed piggish facial structure, and bright yellow eyes, the lantern-bearing goblin fish flexes and twists its light lure in a way that resembles the lantern waving code for "come here". Lantern-bearing goblin fish weighing over 900 pounds have been caught before.

Something unthinkably terrible, something so bad the only real moral choice is sink the ship in a self-destruct before the whole world suffers the curse.

We're talking junji itoh 'you are definitely going to die, its unavoidable, but you might fix the problem even though you died' levels of BAD.

Building on :

>A famous, deep-diving oceanic explorer disappears in the hole, and the PCs are sent down to find out what happened to him.

>He and his crew went further down into the hole than anyone else before him, and in the ruins at the bottom they found an artifact.

>As they brought it aboard, it started to sprout glassy strands and tendrils, infecting the crew with a beautiful, horrible eldritch infection.

>It, the artifact or the infection locked within it, was kept in check by the weight and the chill of all the water it was buried under in the hole.

>Now, it seeks to rise to the surface, where it may spread freely across the surface of the earth, infecting all with it's glassy tentacles.

>Knowing he could never allow his ship to return to the surface, the captain and crew sabotaged their sub before succumbing.

...

Hell of a way to go. Incidentally have you read the hp lovecraft story "dagon"? It's about something similar, though I don't think it features an artifact, I do know it features a submarine discovering an ancient undersea inhabited city at one point.

It would leave a huge sub full of zombie-like sailors, crudely puppeted by the prehensile, glassy growths that have infected them, drifting in limbo near the bottom of the hole.

I haven't ready that particular story yet. I will now, though.

Another thing this reminds me of is a B-movie called Deep Rising. They weren't puppets, but the dead bodies were revoltingly well designed.

If you're in a fantasy game:
> Mermaids. Sexy, but in a horrifying way. They're out to feed on your flesh.
> Abandoned ship. Not so abandoned. Beware the curse.
> Kraken. Wants your meat but won't get you. Will break your ship and leave you on the uninhabited island.
> Hostile aborigens. The island is not so uninhabited. They hate aliens.
> Weresharks. Want your flesh and might get you. Or might be on your crew.

If you're in a modern/recent past game:
> Abandoned cruise ship. Fresh from Bermuda Triangle. Lost years ago, but dinners in the restaurant are still fresh.
> fpbp - Cthulhu stuff could be added
> Undersea research facility. You're isolated from the world. But you are not alone here.
> Another undersea research facility. Built by someone else. Research subjects are you.

Oh my God, fuck that anomalocaris thing. I want nothing to do with a human-sized sea bug.

Here's the mind blower, user; Anomalocaris had some of the most advanced eyes to have ever been present on a life form on earth.

There is absolutely no hiding from it, ever, short of total denial of sight options like hiding completely behind something or releasing a cloud of ink.

Fucking end me, wrong post.

I'll admit, I love the idea of a rusted art deco hulk.

Hallucegenia was another crazy as fuck sea thing, and one of the most "outsider type" looking life forms I've ever seen.

Sounds like Sphere mixed with The Thing. Neat.

Oh boy, that thing looks all kinds of unsettling.

Do you think it would be more or less unsettling if it was on an utterly deserted island's coast, rather than floating around?

I'm seeing Made in Abyss, but in a literal abyss (makes sense since the curse if ascension in the manga is conceptually based on diver's sickness and barotrauma). The big hole could contain artefacts, or lifeforms with unique and valuable properties, so people dive there to explore it, but also deadly monsters and environmental hazards.

And now, 7 islands you really don't want to land on.

1. Fog Island - Located over 1700 miles from the nearest land, Fog Island is a barren, rocky island with no land plants and no permanent inhabitants. It gets its name because it is a semi-volcanic island and water flowing through large cracks in the ground layer of the island causes fogbanks nearly 330 days a year.

2. Resignation Island - Enclosed by ice sheets for more than half the year, Resignation island is a nearly barren island that produces enough food for about 10 people per year, its highest point is Mount Folly, a mountain with large amounts of visible pyrite, giving it a false gold gleam in light.

3. Exile's Rock - A mostly barren island that produces enough food for around 5 people a year, it gets its name because it has a rock formation in the water near it that resembles a flailing drowning person, and it has a history of insanity, suicide, and sorrow. It is 1300 miles from the nearest land.

4. Thorpe's Island - Despite its scenic beauty, white sands beaches, and tropical climate, Thorpe's Island is inhabited by 4 very hostile aboriginal tribes, all 4 of which practice both headhunting and cannibalism.

5. Leadwood Island - So called because of its temperate woody climate, it is uninhabitable because all fresh-water sources on the island pass through a massive reservoir of Lead, meaning all freshwater sources are severely lead poisoned.

6. Burial Island - So called because it was a popular marooning site and had very numerous graves on it, though it has no inhabitants. It is warm and temperate in spring and summer, but bitterly cold in fall and winter.

7. Scuttle Island - One of the creepiest islands in the world, Scuttle Island was used for several hundred years as a graveyard of badly damaged or simply discarded-as-obsolete whaling ships.

Running a campaign with undead being raised by a lich for shits, giggles, and conquest. A session where they're teleported to some lake and stalked by an undead whale would rock. Maybe start them in an air pocket, thousands of feet below surface, and those with darkvision can just BARELY make out a massive shape swimming around them...

Damn good stuff user.

Crew is sent to map routes under the ice on the pole, no surfacing possible. For whatever strange reason they have to dive deeper and deeper to continue - the ice seemingly stretching down to impossible depths. When they finally traveled straight down for a day or two the pressure on the hull becomes nearly unbearable, when they try to turn around they notice that the icecover is seamlessly trapping them - the only way possible further down.

Book Related, it's great, It's about a floating pirate city made of lashed-together ships, and all the conspiracies and plots going on in it. One of the biggest conspiracies in the book is
A plan to summon a leviathan from another plane of existence through a blue hole, and then use it to propel the city

Forgot the picture.

>anomalocaris
Fuck you for reminding me of this:
exhentai.org/g/775255/97810c12ff/

Now i'll have to go and bang my head against a wall untill i forget it again.
I'll send my medical bills to you.

You think that's bad, check out eurypterids, or sea scorpions (which technically are closer to horseshoe crabs than scorpions, but still distantly related to actual scorpions). The largest of them were longer than a man, and had crushing pincers and bladed tailspikes. They were the dominant predators before the evolution of large armoured fish, and survived all the way to the end of the Permian, when they died along with 90 % of all life on Earth.

My own personal preference would be for it to still be afloat somehow after all these decades. A wrecked ship on an island feels a little more plausible, and therefore a little less unsettling than a ghostship adrift on the cold seas.

I've also thought that having a whole floating graveyard of haunted ships ranging from galleons to oceanliners would be a cool setting for a horror game.

> Monument Isles
>A cluster of tall volcanic spires, each one having been laboriously carved over the course of centuries into a monumental figure gazing out mysteriously over the turbulent seas. The scaffolding used to create these figures still encase several unfinished spires and lays in wreckage around the bases of the others, making passage between them very treacherous.

...

scuttlers and clickers

Risk of drowning and accurately portraying the effects of being underwater on your eyes.

If you're looking for some great encounters, I'd recommend looking at the Aquatic tag on SCP foundation. Plenty of nasty beasties and disturbing wrecks. I personally have always found the corpses encrusted in/mutated into bottom dwelling animal life to be on the disturbing side.

Also, another thing to keep in mind: INSANITY. People dont tend to do well mentally when locked inside a metal tube with the same people for long periods of time, especially when there is nothing but black outside the potholes. Make the players question what they are seeing. Did they really just see something the size of an airliner slide past or was it just their imagination? Did the sonar just pick up dozens of contacts for a few seconds or was it just a paranoia induced hallucination?

How many people are ON the submarine, anyway? It might be a good touch to have one or more of the PCs just start catching glimpses of someone they don't recognize passing in the corridor or standing at muster. There shouldn't BE anyone they don't recognize on the submarine.

>That puts me in mind of some location, a stretch of the abyss where a sub crew must maintain silence lest they attract the attention of something dark and terrible. No sonar. No radio. No talking or making any loud noises inside the sub at all
Oh, sure, like Hunt For Red October. And every other submarine movie ever made. nothing wrong with that... some classic tropes are classics for a very good reason

>Zombie Whales
Remember when Uncharted Seas used zombie whales as the submersible option for their !Tomb King faction? I remember that.

Oh yeah, agreed, that was a great little piece of ocean fantasy adventure / horror. Although fair warning, ol China is a bit polarizing here on /tg. I personally love his stuff (that setting in particular) but you'll likely get a few people popping up saying he's a no talent hack before close of thread.

Here's an idea: keep track of your player's sanity scores and compare their Perception/Spot/whatever checks to it. The lower the sanity, the better the chance their minds will conjure up something to see even if there's nothing there. Nothing major, maybe just the hint of a toothy maw or glassy eye. Something to make the players aware that they cannot always trust their senses.

These are neat, although I stand by the notion that the things actually living in the ocean tend to be weirder and more horrible than what our imagination can conjure, even if they really scary-looking stuff is usually too small to be a dangerous (although in a fantasy or alien setting that could be very different). A few examples of freaky animals that would fit right in the pages of a monster manual:

Goblin Shark - Named for its long pointy nose and sharp hooklike teeth, the it can grow up to 6 m long. The nose contains large amounts of electricity receptors that lets it locate prey even in complete darkness by sensing its hearbeats, and the jaws can spring forward to capture prey.

Stoplight Loosejaw - Aside from having an exceptionally large mouth relative to its body, this fish has two exremely unusual traits. It has large red light organs behind its eyes. Most deep sea fish can't see red light, as it is the first wavelenght to be absorbed by water, giving the fish "searchlights" that lets it illuminate its prey while remaining unseen. The second unusual trait is that its lower jaw is on a hinge and can spring forward to impale prey.

Black Dragonfish - A black serpentine fish with a mouth full of long sharp teeth which can open to an angle of almost 180 degrees. The fish waits with its mouth open for prey to come close enough, then strikes while snapping its jaws shut like a beartrap. Unlike many deep sea fish, it migrates closer to surface to hunt, but only during the night.

>Gulper Eel - A relatively large deep sea fish, which is little more than a huge gaping mouth and a sacklike stomach attached to a long tail. The inside of the mouth is completely black, making it nearly invisible in the darkness. It hunts by slowly swimming around with its mouth open, or by floating in place with a glowing lure at the end of its tail suspended in front of the mouth. Anything that swims into the open mouth gets swallowed whole.

...So, time for deep sea horror thread?

What PF book is this critter from?

If you take a more supernatural angle things can get fun.

>Voices over the radio, loved ones long gone trying to provoke the crew into speaking or crying out.

>Electrical fluctuations that threaten to sound alarm bells and that plunge the sun into darkness.

>Shadows and specters slithering, through the blackness, waiting for the silence to be broken.

Yeah, that's definitely something that happens in submarines pretty regularly. Being stuck in a metal tube simultaneously cut off from most of humanity and unable to get any private time isn't ideal for ones mental wellbeing, and can lead to nervous breakdown. Especially if you're also in a dangerous situation.
Also even when entirely sane, its easy for your senses to play trick on you. I've read William Beebe's description of his deep sea exploration (Beebe was the first person to dive into the abyss in a bathysphere), and he recounts how both he and the other person in the bathysphere felt the surroundings outside were a brillian blue, so bright they would've sworn you could read in the light, only to realise when they looked away from the window that it was too dark to see anything. He also reported on almost all of his dives seeing something large moving right at the edge of his field of vision, beyond the range of their searchlight. Initially he assumed it was just his imagination, but it kept happening, and he thought it might be some animals. However, in reality it was probably an optical illusion,

The question in my mind is how would the PCs effectively fight a zombie whale? Whales are huge creatures, even with modern tools it still might take 10-20 harpoons going deep to really kill the beast dead.

Thank you, I'm putting more together soon, and will post it in thread.

What if it was on Resignation Island, Fog Island, or Burial island? These are some of the most remote places in the world, and far, far away from any shipping lane. The mystery of how it even got there in the first place would need to be explained, and with a ship on fog or burial island, there'd be enough shelter for more than 1 or 2 people to live there. See for the full details.

Is leviathan a divine beast meant only to be released at a god's command?

Plot twist: the sub travelled through time at some point during the sub-polar mapping mission, it is now the year 51,000 BC.

Some of the times your senses play tricks on you it shouldn't be a hallucination but should be actually real instead, mixing real encounters with hallucinated encounters, which brings up the question of; do you get hallucinated experience points if you resolve a hallucinated encounter, or not?

I always wanted to run Dark Heresy in an underwater Hive spire with attached cave system/mines.
As the cultists/demons take more areas get flooded and electricity gives out adding to the claustrophobia. It's a matter of time before there is a mass panic.
Cave diving with demons is easily in the top 10 of most horrifying things ever.

And now 3 islands you really (really really) don't want to land on.

1. The Dissipation isles - previously one large island, an volcanic eruption less than 2 hours after it was discovered changed it into 3 large islands, North Island (rocky, barren), West Island (trees and small plants, some animals), and South Ridge (mountains that go straight down to water). The highest point on the dissipation isles is Suicide Cliff (South Ridge).

2. Crag Island - So called because from a distance it resembles 3 large islands, but it is actually 1 big mountain that collapsed inward to make the island. A treasure ship was sailed into lagoon cave, and how to get it out is a big time mystery, the lagoon cave is said to have a complicated series of traps, including one that is believed to tug a support beam and collapse the cave with no warning if it's not properly solved.

3. Hauberk Island - So called because a mixture of gravel and solidified magma covers nearly its entire surface. Utterly barren and uninhabited, though many strange carvings have been found on stones there, including carvings of large trees, people throwing spears, a carving depicting a simple stone-age saw, and depictions of some sort of monkey, the lowest point on Hauberk Island is the Magma Lake, an extinct volcanic hole that is a fresh-water source.

>The question in my mind is how would the PCs effectively fight a zombie whale?
Get into its belleh, and then destroy its ambergris-phylactery while fending off necromantic carrion-feeders and parasites.

Not a bad solution, thanks user.

A few fun and fabulous nasties of the waters for you.

Snakehead Trapjaw - So called because it's head resembles a pit-viper's head, the Snakehead Trapjaw's jaw can expand to be seven times the size of the creature's mouth when closed, and has a complicated system of 4 joints on both left and right sides of it's mouth, allowing it's mouth to open in a strange 'wings extended sideways' sort of look.

Grunting Burchard - A bottom feeder that displays aggression by belching suddenly, the heaviest Grunting Burchard ever found was 500 pounds, and the fish is generally 3-5 feet long.

Sentinel Crab - So called because it appears to have eyes on it's backside and it's backmost legs have a 'claw that cannot flex' portion of the limb that makes it difficult to tell which way the crab, which only grows to a 4 pound maximum weight, is facing.

Eichman's Deep Sea eel - Eichman's deep sea eel, so called because it lives so deep that only 21 specimens have ever been caught in history, is an absolute-bottom dweller with a large mouth like that of a gulper eel, the lazy, flabby creature grows to be about 6 to 8 feet long, and mostly swims casually about with its very large baleen encrusted mouth open.

Shredtooth Hackfish - So called because it's teeth, which grow to be 9 inches long, are both sawtooth and razorblade, the Shredtooth grows to be 3 feet long and weighs around 10 pounds, but travels in swarms similar to a piranha.

Conquering Grumpfish - so called because it's body shape and colors resemble that of a set of platemail, and because it has a 'grumpy/serious' expression due to the shape of it's face. It grows to be around 700 pounds, though this takes a long time.

Eventually they discover a passage through the ice to what they assume is the suface that only leads them to a vast ice cave filled with the broken wreckage of other lost vessels.

Agreed for sure, but it is fun to invent new things.

No, I never played that, but it sounds cool.

What if the passage lead to a Vrill / Hollow Earth region that has air and animals/plants/people?

Man, those things in your picture remind me of that one jonny quest episode.

I've beed dredging some more images of deep sea fish, including species I haven't had much material before.
This here is the lancetfish, a large (over 2 m long) midwater fish, which is often found in the "twilight zone" (area where most light has been absorbed by the water). They're notable for the huge dorsal fin, and also from being cannibalistic, frequently eating smaller members of their own species.

Here's another twilight zone inhabitant, the telescopefish, so named for its bulging eyes that look like the sights of a pair of binoculars (I guess binocular fish would be a better name. The again, binoculars are just two telescopes stuck together). Its cientific name (Gigantura), comes from its other notable trait, a tailfin (lower lobe of the tailfin, specifically) that is notably longer than the rest of the body.
Not the highly distended stomach: like many deepsea fish the telescope fish can swallow prey its own size or larger.

And here's a snipe eel, a fish whose body becomes so thin its gut actually has to do an u-turn and head back towards its head since the rear of the body becomes too narrow to fit an intestine.

Closeup on the head of a dragonfish or closely related genus, showing the light organs under the body.