These are the big ones. These are the sea monsters whose size not only matches ships but dwarfs them. When these are introduced, all you'll see is an overhead shot of a small boat on top of a much larger shadow in the water and "small boat" ranges from fishing ship to aircraft carrier. These beasts aren't just big, they're leviathan.
They'll live in the deepest depths and their appearances are saved for the end of days when the world is to be shattered by their movements. If they're not Eldritch Abominations, they're the next best thing. There will be little shyness about their inspirations, they'll be unabashedly named after the Norse legend of Kraken or the biblical Leviathan. The Kraken tends to take the form of a tentacle lacedgiant squid or octopus, while the Leviathan is more variable, ranging from a colossal whale to a massivesea serpent. However, since Our Monsters Are Different, the title of kraken or leviathan may be given to all kinds of terrors of the deep.
The leviathan in the bible is often taken to be a view of a whale and is sometimes translated directly as whale (intriguingly the "big fish" that swallowed Jonah was often interpreted as a whale or leviathan but nowadays people are having a lot of fun with it as a Megalodon). Indeed, the modern Hebrew word for whale is לויתן, or leviathan. Strangely, the actual biblical description of a leviathan suggests that it has legs and is able to move on land, and seems to refer to armour plates or scales (in fact, if one ignores the Word of Dante and the fire-breathing, it sounds quite like a crocodile).
Kraken on the other hand are believed to be inspired by giant squidnote although they are usually depicted as octopuses and were originally described as more crab-like than cephalopodic, and when most people think of a kraken, they tend to imagine a a squid-like creature that is simply many times bigger than a real one. However, since actual Nautical Folklore says that the Kraken could be mistaken for an entire chain of islands, one can reasonably infer that it was imagined to be much bigger than most film depictions.
Interestingly a common theme in sea monster stories is that of two contrasting sea beasts that are each other's mortal enemy and in real life sperm whale, the largest predatory whale and deepest diver, are believed to hunt giant squid.
In video games, Kraken often have Cognizant Limbs, with their tentacles being considered separate from the body as enemies.
If you were looking for the book by China Miéville, see Kraken.
Namor can also summon a giant whale called Giganto, using a special horn. In Fantastick Four1602, Benjamin calls the Giganto counterpart "Leviathan".
In The Cadanceverse, the ponies searching for the Elements must deal with one of these as they journey through the Everfree.
In Star Wars Episode One, the Naboo ocean seems to be filled with monsters each getting bigger and bigger as you go deeper and considering the fact that the planet is meant to be hollow and full of ocean that gets you some pretty big fish. Also the oceans of Kamino. Ah hell, any planet with an ocean in the Galaxy Far Far Away is bound to have sea monsters in it.
Word of God states that the creature from Cloverfield is a sea monster and not an alien like some believe (the 'falling object' was a satellite that awoke the creature, not the creature itself). Though said creator seems to have taken on the anger of the co-Gods by saying as such, since the others went with the alien explanation, or at the very least wanted viewers to make up their own truths.
The Kraken from the Pirates of the Caribbean series gets bonus obfuscation points for being called "Davy Jones' Leviathan" once. Maybe they just meant leviathan as in "really big thing". The Pirates visual dictionary actually contains a picture of the entire Kraken rather than just its tentacles, and it seems to combine both monsters. Imagine either a whale with tentacles where its head should be, or a squid with a head long enough to act as a tail.
According to Atlantis: The Lost Empire, the Biblical Leviathan is actually a giant mechanical lobster that guards the entrance to Atlantis and looks nothing like the Biblical Leviathan. The Kraken was actually featured in the first act of the film's sequel.
The Liopleurodon ("Swimming Sharptooth") from the 9th The Land Before Time movie is a clear leviathan.
Pacific Rim: While called "kaiju" (the term for Godzilla and other strange, gigantic monsters in Japanese film), most of the monsters in the film are closer to this tropic thematically.
The "Leviathan" mentioned above is described in the Book of Job (Ch. 41, all of it), in the Old Testament of The Bible, and referenced elsewhere. Unlike most sea creatures, the Leviathan has an affinity for fire or something reasonably similar. Job 41:8 ("Lay thine hand upon him, remember the battle, do no more.") seems to indicate a reasonably high encounter survival rate, considering that this creature is essentially a seagoing dragon.
Leviathan represents the sin of Envy. "Envy lurks in the hearts of men, as Leviathan lurks beneath calm waters."
Job also makes reference to something called a Behemoth, which has similarly entered the lexicon as a catch-all term for "crazy-great-big-thing". It's thought to be a hippo. Or a titanosaur of some sort, what with the tail like a cedar and all.
Also, the great fish that ate Jonah.
In the Midrash there's a legend or myth that states that the giant fish which swallowed Jonah narrowly avoids being eaten by Leviathan.
The sixth Artemis Fowl book features a kraken. It's actually a peaceful bivalve creature that lives off of microorganisms and krill filtered out of seawater (just like a blue whale in fact). You still don't want to be on top of it when it sheds its skin, though...
Codex Alera has its own leviathans, something like enormous turtles with no shells. Word of God is they're descended from plesiosaurs, and the smallest ones are forty feet long. The larger ones... well, they're probably the single greatest threat to oceangoing ships. Alerans have to use watercrafting to get around them, and the Canim maintain elaborate maps of their territories to avoid making them angry by sailing in too close.
In the Scott WesterfeldLeviathan trilogy, the eponymous beast is actually a airship, that looks sort like the biggest whale you'll ever see. In the next book the Behemoth (who is supposed to be a land creature) actually live in the sea, the picture of it puts it up with Lovecraft's beasties. Also their are the normal British navies' Kraken, which are massive squid/octopus monsters that can pull a weakened ironclad under given time.
Taylor Anderson's Destroyermen series has World War II warships fall into an Alternate Universe where the Cretaceous extinction event didn't happen. Among the sea creatures in this world is the "mountain fish" (apparently actually a whale), so big it can wreck a steamship — by biting it. At one point, it's stated that the locals' massive city-ships known as "Homes" are almost as big as mountain fish — and a Home is the size of an Essex-class aircraft carrier.
For bonus points, the sailors of the Empire of New Britain actually call the beast the Leviathan, likely referring to the Biblical one.
The final book of the Illuminatus! Trilogy, Leviathan, has the main characters coming face-to-um...something with the eponymous sea-monster, a titanic single-celled organism that's survived and grown since the Paleozoic Era.
The Enterprise fish from The War Against the Chtorr, part of the ecological Alien Invasion. These massive fish roam the oceans eating everything natural or man-made to fuel their constant hunger and enormous growth. Immune to Bullets thanks to their massive layers of blubber, the only way to destroy them is with a low-yield atomic torpedo. A short story about the Enterprise Fish reveals that their huge size is explained by them being a colony of creatures. When they get too large, the host 'fish' dies and breaks apart into smaller creatures.
Steve Alten writes a lot of books with these; his novel Meg was about a Megalodon (sixty foot prehistoric shark) that makes it to the surface after surviving down in the Marinara Trench. Hilarity Ensues. It suffered from some sequelitis though. His more recent novel The Loch deals with, of course, the Loch Ness Monster. In a twist however, Nessie isn't a peaceful plesiosaur but a giant eel with a taste for tourists.
The Leviathan makes an appearance during The End of the World as We Know It in Good Omens. Since this particular apocalypse has all the Biblical imagery being filtered through a boy who has recently overdosed on Green Aesops, its immediate action is to attack a Japanese whaling ship.
The Kraken stirred. And ten million sushi dinners cried out for vengeance.
Invoked with the title character of Moby-Dick. Although Starbuck insists that he's actually just a particularly large and aggressive whale that should probably be left alone, Ahab - and, increasingly, Ishmael - see the whale as some kind of ineffable Eldritch Abomination - the personification of fate itself, God, Satan, or a combination of all three. Also, among the many, many epigraphs of the book, are quotations from the Bible in reference to Leviathan and the thing that swallowed Jonah, because Leviathan, Moby-Dick the whale, and Moby-Dickthe book are all utterly impossible to fully comprehend.
Drowned Wednesday from the Keys to the Kingdom series was cursed to turn into a massive, omniphagic whale.
In Sennar's Mission a gargantuan sea monster similar to a kraken is met on the way to the Submerged World. Its hunting method consist in wait for ships and then make them slither on his body to the gaping mouth in the middle, using tentacles if necessary. However, the beast is never named.
One of Larry Niven's Svetz the time traveler stories is Leviathan!, in which he is sent back in time to catch a whale, but the first "whale" he latches onto with his tractor beam is just too big to bring back — as it's the Biblical Leviathan, not a whale at all. After realizing that actual whales would have to exist to feed a predator that massive — so much of the knowledge of the past is lost in his world that they only have vague descriptions of animals that may once have existed — he brings one back: Moby Dick, just after he sank the Pequod.
In Wild Mage, during the attack on Pirate's Swoop, Daine tries to convince a pod of whales to attack the Carthaki backed pirate fleet. When they refuse and leave, Daine sends her awareness far out after them, and wakes the Kraken. He's more than happy to destroy any fleet she wants. She knows full well it's a deal with a demon, but she's desperate and agrees to his offer. He's described as an octopus with too many arms that are a mile long each, with a body that's a mile and a half. He's also fast: Daine found him out past the Copper Isles, a four day sail. He made it to Pirate's Swoop in the space of a morning.
Prince Roger: Large aggressive fish make ocean travel a near-impossibility on the planet Marduk.
In The Book of the New Sun, Erebus and Abaia are unseen aquatic antagonists who are said to be the size of mountains. Mankind's continued existence is apparently due to them counting only enslavement, rather than destruction, as a victory.
Further, Severian reads a Future Imperfect story in which the legend of the Minotaur is conflated with the USS Monitor, the ironclad battleship supposedly being the armored head and shoulders of an aquatic monster.
In A Song of Ice and Fire the Greyjoy family uses a Kraken as their house symbol. As yet no actual Krakrens have been seen in the series, but rumours are beginning to surface that they are appearing.
One episode of Jurassic Fight Club featured a battle between a Megalodon shark (see above) and its more recently-discovered contemporary, the "biting sperm whale".
Beast Legends did an episode featuring a digitally animated kraken incorporating the nastiest features of the Giant Octopus, Giant Squid and Colossal Squid, scaled up to 200ft long from the tip of its head to outstretch tentacles, which take up half the body length them selves.
If we go with the view that Space Is an Ocean, Moya's species of giant living ships called "Leviathans" count as examples of this trope.
The Leviathans on Supernatural are not giant sea creatures, but rather are perfectly capable of living on land and while they are seldom seen in their true form, there's no sign that this true form is any bigger than the human disguises they use. But there are good mythological reasons for calling them by the name "Leviathan": Their backstory is related to biblical mythology; they are Eldritch Abominations that are all about having a predatory appetite; and when their true form is shown, the view isn't very clear, but it definitely has More Teeth than the Osmond Family.
Jormungandr the World Serpent of Norse myth is also under the sea, long enough to wrap around the world, some legends say twice.
Greek mythology has several large marine creatures. They were typically sent by the sea-nymphs to ravage land-dwelling kings or queens who offended them, and the royals tried to appease the monsters by sacrificing their beautiful daughters, which the heroes then had to rescue. One particularly worthy of mention being Ketos, the monster who was slain by Perseus. It is very obvious that Ketos was what the Kraken was meant to be in Clash of the Titans, but they opted for the creature of the Norse myths instead.
There's also Scylla and Charybdis from The Odyssey. Scylla was a six headed, snake necked creature that sat atop a cliff and snatched sailors from passing ships. The reason they didn't just sail around was because of the creature opposite the cliff, her cousin/aunt, Charbydis. Charybdis was a giant mouth under water. Every time she opened her mouth, a whirlpool formed to suck everything down. The original rock and hard place, or alternatively, "devil and the deep blue sea."
Another example was the sea serpent Heracles fought when he promised to rescue King Laomedon's daughter Hesione in exchange for a pair of magic horses. Heracles eventually killed the monster by letting it swallow him and then cutting his way out from the inside with his sword, but Laomedon tried to cheat him by refusing to give him Hesione and the promised treasure. Pissing off the Hot-Blooded Heracles was never a bright idea, and in this case Heracles exacted his vengeance by invading and looting Troy, killing the king and all his sons.
Another example would be the unspecified sea monster sent by Posiedon to help the Greeks in the Trojan War; too bad for Odysseus he forgot to give Posiedon props for that when it was over.
Of course the best example to be found in this mythology would probably be the sea-monster gods Phorcys and Keto, children of Gaea and Pontus, the Ocean. The spawn of Phorcys and Keto included Echidna, the alleged "Mother of Monsters," the Gorgons, Scylla, the Graea, the sea nymphs, and everything else scary about the sea that wasn't one of Poseidon's offspring.
And then, of course, there's the Kraken and the Leviathan. The former emerged in medieval Scandinavian, especially Norwegian Nautical Folklorenote It is not yet found in Norse Mythology., while the latter is the Christian edition of the Canaanite Lotan, an aspect/friend/ally/whatever of the sea god Yam.
Apep might also count. He was a colossal serpent that stalked the ancient Egyptian underworld, hoping to devour the sun god. According to The Other Wiki one of his nicknames was "World Encircler", making him similar to Jormungandr above.
"Belly of the Whale" by Burning Sensations. The video shows the band and others partying in a set that's loosely modeled on a giant whale's mouth, complete with a water-slide entrance.
Mastodon's second album, Leviathan, particularly the song "Megalodon".
Alestorm, a Scottish Pirate Metal band, has a song called (and glorifying) "The Leviathan" on their album Black Sails At Midnight.
From the same band, Death Throes of the Terrorsquid is a song about a group of pirates setting out to slay the Kraken/Leviathan.
In Rifts, the biggest Sea Monster is the Lord of the Deep, also referred to as both Kraken and Leviathan, a massive abomination resting at the bottom of the Marianas Trench with tentacles that can reach for thousands of miles.
The splatbook Elder Evils has the Leviathan, an immense world-spanning sea monster made out of pure chaos which was a side-effect of the creation of the world. It sleeps at the bottom of the ocean and will eventually fade away into nothingness, provided nobody wakes it up. Unfortunately, since this is Elder Evils we're talking about...
Kraken and another Leviathan also appear in the various Monster Manuals, the former being a particularly Giant Squid with magical powers, the latter being a really, really big whale. Supposedly.
Forgotten Realms got Leviathan — the superpowered whale working as Earthmother's divine minion for sea missions. She did never spawn avatars (until merging with Chauntea), such improved natural beasts were enough to deal with her problems.
While it never appeared in the original novels, the Dragonlance modules based on the Chronicles trilogy give us the King of the Deep. The King of the Deep is a nightmarish sea monster with the body of a huge fish covered in silver hairs, the head of a giant squid, and a pair of long, deadly lobster claws, created when ten (or twelve in the updated 3rd Edition version) of the corrupted priests of Istar offered themselves to the Queen of Darkness, who turned them into this monster.
In Chaosium's Stormbringer supplement Demon Magic, the adventure "Sorcerer's Isle" had a Megalodon that could sink ships by biting through their hulls and a giant whale-like demon named Lvthn.
Warhammer 40,000 has a few, though many are rather obscure to the majority of the fandom:
The Space Wolf homeworld has a massive kraken (said to be a Tyranid offshoot) and sea serpents straight out of Norse myth, appropriate given the Space Wolves' Viking theme, and a sea monster is said to live on the planet Armageddon, where it attacked Ork ships. Given the nature of the setting, it's a safe bet that most world with any oceans have at least one.
The chronologically second and third of the three major Tyranid Hive Fleets so far are named Kraken and Leviathan, respectively (the first being named Behemoth, incidentally). Hive Fleet Kraken lived up to its namesake, as it made it debut by extending itself into many "tendrils" across a vast swath of galactic space, allowing it to consume a great many more worlds than Behemoth did with its blunt "single monolithic mass" approach; it even allowed it to evade Behemoth's fate of total destruction by surviving through numerous splinter fleets. As for Leviathan, look closely at the image◊ with the Hive Fleet's tendrils approaching the galaxy from below the galactic plane; if Leviathan is the sea monster, then the Galaxy is the ship that it dwarfs with its size. And of course, these three fleets are just the vanguard of a larger Swarm that's predicted to arrive within a century of Leviathan's own arrival, i.e. within the first tenth of the 42nd millennium... assuming that the Time of Ending hasn't overtaken the Galaxy by then, that is.
The New World of Darkness has the Leviathan as the Kerberos of the Ocean of Fragments. It dwarfs nearly everything else on this list, and pretty much qualifies for Eldritch Abomination status on size alone, even disregarding that it's an unstoppable force of nature that literally the entire human race has nightmares about. To give some idea of the scale, no-one has any idea what it actually looks like or what it is — it's so massive that the most anyone's ever seen of it is a vast, seemingly-infinite wall of flesh that gives no hints to its form. The best guess anyone has is that it's some sort of impossibly gigantic cephalopod, but that's just because it has tentacles.
There's also a fan-brew game on RPG.net, Leviathan The Tempest, which is all about playing the Fish People in human form descended from primordial gods of the ocean. They have the ability to assume ungodly large forms, but don't do it except for dire circumstances as it tends to rend the Masquerade in two and drives humans into a state of holy terror.
And humans would utterly crush any Leviathan who tried it.
Exalted name drops the latter with Leviathan, a Lunar from the First Age who's spent the past few millennia stuck in his spirit shape as an orca, dwelling on his failure to protect his Solar mate, and serving as a god to/tormenting the inhabitants of a sunken city. Appropriately enough, he has a custom shapeshifting Knack that not only makes him gigantic, it allows him to serve as his own military force in mass combat.
It also has a kraken, although said kraken is made of magma.
The Warhammer world has buckets of these, as detailed in the Seas of Blood supplement to the early 90s naval warfare spin-off Man'O'War and the 2011 Pirate-themed spin-off Dreadfleet. Warhammer's giant sea monsters include the classic giant squid Kraken and the horrific Black Leviathan (a humongous deep-sea angler fish that could swallow small ships whole), as well as the Narwhal-like Behemoth, the giant crab Promethean the Sea Dragon, the giant merman Triton, the Sea Elemental and the giant shark Megalodon. Dreadfleet has its own zombie sea monsters - the Sea Giant, Bone Hydra and Leechwyrm - and even a ship made from the rotting undead carcass of an Orb Leviathan (possibly the same species as Man'O'War's Black Leviathan, maybe not). Finally smaller, but still huge, sea monsters are available from Forge World to use in land-based Warhammer armies in the shape of the Merwyrm and its variants.
The thing called Kraken is a giant alien from another dimension. During a mission in Cimerora you must fight off tentacles of the actual Kraken.
Cap'n Krak'n Jumbo Seafood sign in Independence Port. Complete with octopus holding a spoon and fork and wearing a bib.
EarthBound has the Kraken, who in this game is less squid and more sea serpent.
Resistance 2 features the Kraken. It's actually kind of puny compared to some of these, but it's enormous and tentacley. It also features a creature called a Leviathan, but that one's more Kaiju than Giant Swimmer, despite living in the mostly-flooded Chicago.
Final Fantasy III has the Nepto sea dragon, who roams the oceans of the Floating Continent. It functions as a Beef Gate preventing you from sailing out of the Bay of Nepto, and its rage can only be appeased by placing the gemstone Eye of Nepto in the Nepto Statue in the Nepto Temple. If the party tries to sail away regardless, the ensuing battle is unwinnable, and the party must run (if they can.)
In Final Fantasy IV, Leviathan is a feared monster that swallows ships whole. When Cecil and his companions set sail from Fabul, Leviathan does just that, scattering the party and taking Rydia to his kingdom, the Land of Summoned Monsters, where the party can later fight him and his Queen, Asura, to earn the right to summon them.
Final Fantasy V has an additional sea monster, the plesiosaur-like Syldra. She is Faris' pet sea dragon, who can carry the pirate ship on her back even when the destruction of the Wind Crystal keeps all other ships from sailing. Unfortunately, the boss monster Karlabos/Karl Boss drags her under, and Syldra is further weakened by saving the party from the sinking Walse Tower. In a Tear Jerking scene, Faris experiences a Heroic BSOD upon Syldra's death, but the latter's spirit would reappear in the third world and offer her help as a Summoned Beast.
The Updated Re-release of Final Fantasy VI added Leviathan to the Esper roster, by having the sea serpent preying upon ships sailing between South Figaro and Nikeah. Defeating the creature will reduce it to Magicite, which the player can equip.
The third chapter in Tales of Monkey Island is called "Lair of the Leviathan". The chapter starts with Guybrush and his ship being eaten by a giant Manatee, but the title Leviathan turns out to be an even bigger female Manatee, which the first one has to conquer without being eaten.
In World of Warcraft, The Lurker Below, a boss in Serpentshrine Cavern raid whose ingame model and is actually called "kraken". Its brethren are called kraken as well.
In Cataclysm, however, there is the more traditional (i.e. giant squid) kraken Ozumat. And that's not even mentioning Nespirah... who is a giant... underwater... something.
Also from the same expansion, the Whale Shark. Nearly ten million hit points, quarter million damage bites and an achievement for killing this beast just because you CAN, in other words, you have to kill him especifically for the Bragging Rights Reward since he doesn't gives loot of any kind.
Mists of Pandaria have a few of their own as well. Three squid with the same model as Ozumat - one involved in a quest chain, another swimming in the waters off the coast of Townlong Steppes, and a third seen beached on the coast of the Thunder King's Island. As well as a giant boss eel (and its children) that drops a Bragging Rights Reward.
Um, Kyogre anyone? It's an obvious Expy of the Biblical Leviathan.
The entire Weather Trio was meant to be one big Expy. Groudon is meant to represent the Behemoth, which according to some sources is mortal enemies with the Leviathan. Rayquaza is meant to represent the Ziz, whose job is apparently to calm Behemoth and Leviathan down when they get going.
Dragon Quest IX: at Port Llaffan, you meet Jona, a girl with the ability to call Leviathan... a gift the other townsfolk abuse by making the great whale provide the whole town with fish, completely giving up their own fishing efforts.
Age of Mythology has creatures called Leviathan and Jormund, but they're no bigger than most of the other Myth Units. The Kraken is also a present myth unit.
The Jormund is supposed to be the offspring of the real World Serpent though. The snakey fellow himself couldn't show, what with blocking out the entire screen...
A humongous Zerg monster from StarCraft II is called a Leviathan. It can fly, shoot a wide variety of things (even other zerg air units), and has tentacles. It is also shown to be twenty or thirty times the size of a Terran Battlecruiser in some images. The Zerg appear to use them as interplanetary transports, and Kerrigan uses one in Heart of the Swarm as her mobile base. Concept art indicates that it was originally going to be a whale-like creature larger than a protoss Mothership that could swarm planets with clouds of some kind of creature. There is also a zerg creature from the novels called a Behemoth, which is a docile manta ray-like creature of similar size (it is hard to tell how big the Leviathan is supposed to be) used for transport.
You can create both of these in Scribblenauts. The Kraken is a giant squid-like creature, the Leviathan manifests as a long sea serpent.
Wing Commander: Prophecy: The Terrans assign Reporting Names to the ships used by the new alien race attacking through the new wormhole in the Kilrah System. These names are pretty much all names for aquatic creatures from Earth, given that the alien ships all look like they would be at home under the ocean. The capital ships are named for mythological sea monsters, including the Kraken and Leviathan.
Endless Ocean : Blue World has both of these. There, the Leviathan is a large albino sperm whale with a back story lifted from Moby-Dick. The Kraken appears in the form of Kraken Jr., a young giant squid.
In the Boreas seabed mission of Guild Wars, players have to fight a giant kraken like creature as the final boss. In addition, "Leviathans" are large sea serpent like enemies that the players fight at points in the mission "the deep", as well as being frozen in jade at oher locations.
The aptly-named LeviathanDownloadable Content for Mass Effect 3 introduces a race of enormous, tentacled aliens that live in the galaxy's deepest oceans. Their species is billions of years old, and was formerly the apex race in the galaxy. Their true name is never given, but they are called "Leviathans" by other races. They were the race that created the Catalyst, and their faulty parameters were the reason the Catalyst started the Vicious Cycle. The first Reaper Harbinger was created from the Leviathans. The few remaining survivors blame themselves for this programming oversight.
Wapsi Square features a creature bearing some resemblance to the kraken. It is ancient and large enough to drop a U-boat on a beach. At one point in the past, Bud made a pet out of it and named it Stinky.
The SCP Foundation has plans of varying effectiveness to contain and/or prevent everything from multiple zombie apocalypses to holes in reality that periodically spew monsters. Their plan for this? When you cut through the formal scientific sounding terminology, it amounts to "Hope to hell it doesn't wake up, and if it does, hope it's friendly. The thing is just too damn big to do anything about."
Magical Trevor 4 featured a Kraken with the face of a haddock.
This Cracked.com article discusses various unsolved mysterious and possible explanations for them. One of the mysterious is the Bloop (an extremely loud sound from underwater), and the possible explanation is that it's Cthulhu.
In an episode of Martin Mystery, a Leviathan was guarding a treasure and attacked anyone who tried to steal it.
On Catscratch, Gordon goes hunting for the Kraken, who will supposedly grant a wish to whomever would defeat it in battle. In a later episode, the Kraken is revealed to be from another dimension.
Sinbad: Legend of the Seven Seas features a leviathan which the crew mistake for an island, complete with vegetation. It isn't actually aggressive, and is actually quite useful, as they can use it to tow the ship where they need to go. This leviathan gets points for being one of the comparatively few depictions that actually is big enough to be reasonably mistaken for an island.
There's a fossil whale species named ''Livyatan melvillei (from the Hebrew spelling of Leviathan and the author of Moby Dick). And it is pretty scary-looking◊.
For a while researchers theorized that the stories of the Kraken were exaggerations of Giant Squid sightings, but more recent discoveries might prove that the tales were not so exaggerated; specimens as large as eight meters (26 feet) have been caught, and it's theorized they can grow to 10 to 13 meters in total. They've also been shown to display aggressive hunting behaviors, in contrast to the hypothesis that they are slothful drifters.
Then there's the Colossal Squid. A live specimen was caught in 2007◊, and initial estimates put its total length at 10 meters (33 feet) long before the tentacles shrank post-mortem. It's theorized that there may be even larger examples of the species in the wild, as the beak of the live specimen was smaller than what they'd found in the stomachs of sperm whales.
There have always been unpleasantly large cephalopods throughout the history of the world:
One was Cameroceras, an Ordovician (about 440 million years ago) genus of primitive shelled cephalopods which had long, conical shells ranging from 3 to 11 feet in length.
An ammonite from the Cretaceous of Germany was Parapuzosia, it had a coiled shell with a 6 foot diameter. It had numerous cousins in the US that were slightly smaller, about 3 to 5 feet in diameter.
From the Niobraran Sea, of Cretaceous Kansas, we have Tusoteuthis, a 20 to 35 foot long (with tentacles outstretched) relative of the modern-day vampire squid. Of course, it was not even close to being the worst of the worst of the denizens of the Niobraran: a fossil of a predatory salmon, Cimolichthys, showed that the beast choked to death while trying to swallow a Tusoteuthis, tailfirst.
Much is made about how the bus-sized, 20-foot placoderm fish Dunkleosteus was the world's first vertebrate superpredator. It's often mentioned that Dunkleosteus ' bite was among the strongest of any vertebrate living or extinct, and that it was a voracious cannibal. What isn't mentioned is how Big D had several, equally voracious relatives, including the evocatively named Dinichthys and Gorgonichthys, who were only slightly smaller (by about a few feet, or so). And then there was the 30 foot long Titanichthys, the largest placoderm ever, though, it was a basking shark-like plankton eater with "ineffectual mouthplates."
Then there's the Liopleurodon. It stalked the seas of the mid to late Jurassic. Its maximum size is controversial, but modern estimates are in the 20-30 foot range, about the size of a killer whale. Some other estimates, though, put it at over 50 feet long, which would make it the largest known predator in Earth's history.
Also, the Mosasaurs of the Cretaceous, which were about as close to the mythical sea-serpents as you can get.
The bizarre and spectacular Deep-Sea Siphonophore◊s definately deserve a mention - while the Siphonophores you're familiar with are probably the ever-dreaded Portuguese Man o' War, and the By-the-wind-sailor, deep in the ocean lurk some SCARY BIG guys. Being cnidarians, all of them have poisonous stings, some even being dangerous to humans. Fortunately the chance of coming across one is ludicrous, as they are both rare and strictly deep-sea dwellers.
The deep-sea siphonophore Praya dubia is one of the better known. Praya colonies form long strings of individuals, some colonies have been recorded at 50 meters long - and that's only the ones they've found. The colonies average out at around a length of 40 meters, though there have been unconfirmed sightings of far longer colonies.
This one Appears to be related to the Praya dubia and features a beautiful red glow. The size on this one appears unclear - some shrimplike creatures dart past the camera but they seem to be much closer to the camera.