Nearly every seafaring culture invented sea monsters, and a surprising number of those myths trace back to real animals. Giant squid inspired the Kraken. Oarfish almost certainly spawned tales of sea serpents. Decomposing whale carcasses washed ashore looking alien enough to launch fresh legends well into the twenty-first century. The ocean is vast, poorly explored, and home to creatures bizarre enough that they barely need embellishment to sound mythological.
The Kraken Was a Giant Squid All Along
The Kraken is probably the most famous sea monster in Western mythology, a ship-sinking, tentacled horror lurking in Scandinavian waters. For centuries, sailors’ accounts were dismissed as fantasy. Then, in 1853, a giant cephalopod washed up on a Danish beach, and the Norwegian naturalist Japetus Steenstrup recovered its beak, formally describing the species Architeuthis dux and dragging the Kraken from legend into zoology.1The Conversation. The real-life origins of the legendary Kraken The giant squid can reach lengths of up to 18 meters, yet it remains so elusive that much of its basic biology is still unknown.2História, Ciências, Saúde – Manguinhos. The Kraken: when myth encounters science
For most of modern history, researchers only ever studied giant squid from carcasses that washed ashore or floated to the surface after dying. Live encounters were almost nonexistent. That changed slowly, and it took specially designed camera platforms using low-light cameras, red illuminators, and lures that mimic bioluminescence to finally record giant squid in their habitat. Standard deep-sea tools like trawl nets and remotely operated vehicles are built to study things that sit still or move slowly, and giant squid are neither. Using these unobtrusive platforms in the Caribbean, researchers captured footage of Architeuthis dux along with other large deep-sea squid species, confirming that these animals are active, alert predators rather than the sluggish drifters some had assumed.3Deep Sea Research Part I: Oceanographic Research Papers. Studying the swift, smart, and shy: Unobtrusive camera-platforms for observing large deep-sea squid
Oarfish and the Legend of Sea Serpents
When sailors described seeing an impossibly long, ribbon-shaped creature undulating at the ocean surface, they were likely watching an oarfish. Regalecus glesne, the giant oarfish, is the longest bony fish alive. Adults can exceed 8 meters, and unverified reports put some individuals even longer. Their elongated, silvery bodies, bright red dorsal crests, and habit of swimming with their heads raised above the surface are a near-perfect match for classical sea serpent descriptions.
Despite being widespread, oarfish are deep-water animals that rarely show themselves. Between 2008 and 2011, remotely operated vehicles in the northern Gulf of Mexico recorded five encounters with apparently healthy oarfish at various depths, including the deepest verified sighting of the species, at roughly 463 to 492 meters below the surface.4PubMed Central / Journal of Fish Biology. Five in situ observations of live oarfish Regalecus glesne (Regalecidae) by remotely operated vehicles in the oceanic waters of the northern Gulf of Mexico Oarfish typically surface only when sick or dying, which means the specimens sailors encountered were likely behaving abnormally, writhing or floating in ways that would have looked all the more monstrous.
Adding another layer to the sea serpent story are giant eel larvae. Most eel larvae, called leptocephali, are tiny, transparent, and leaf-shaped. But certain species, particularly deep-sea morays, produce larvae that grow far larger than expected before metamorphosing into adults. Some of these larvae have been recorded at 30 to 40 centimeters, nearly 16 inches, dwarfing the typical 9-centimeter larva of related species.5Scientific American. The Giant Transparent Ribbons of Eel Larvae When 19th-century naturalists found these oversized larvae, some speculated they must grow into proportionally enormous adult eels, fueling stories of massive sea serpents hiding in the deep. The adults turned out to be regular-sized morays, but the speculation had already entered popular imagination.
When Whales Become Monsters
Not all sea monster sightings involved exotic deep-sea creatures. Some involved the most familiar large animals in the ocean: whales. A detailed re-evaluation of one of the most famous historical sea monster accounts, the “most dreadful monster” described off Greenland in 1734 by missionary Hans Egede, concluded that what was likely seen was an unfamiliar whale. The animal was probably a humpback, a North Atlantic right whale, or possibly one of the last Atlantic grey whales, either missing its tail flukes due to injury or, more awkwardly, a male in a state of arousal.6Archives of Natural History. Cetaceans, sex and sea serpents: an analysis of the Egede accounts of a “most dreadful monster” seen off the coast of Greenland in 1734 The account was taken seriously for centuries as evidence of genuine sea serpents. In reality, it was a case of unfamiliarity making the ordinary look extraordinary.
Whales also explain many “globsters,” the mysterious masses of decaying flesh that periodically wash ashore and prompt breathless local news coverage. A particularly striking example landed on a beach in Fortune Bay, Newfoundland, in 2001. The carcass was so decomposed and stripped of recognizable features that it was called a sea monster. Researchers extracted DNA from the remains and identified it as a sperm whale.7PubMed Central. How to tell a sea monster: molecular discrimination of large marine animals of the North Atlantic This pattern repeats with striking regularity. When large marine mammals decompose, blubber and connective tissue can create shapes that look nothing like the original animal. Exposed collagen fibers resemble hair or fur. Jaws fall away, leaving featureless mounds. The result looks alien, but DNA testing almost always points to a known species.
Mirages That Made Monsters
Some sea monsters were never animals at all. They were tricks of light. Fata Morgana mirages, caused by temperature inversions bending light over cold water, can warp the appearance of distant objects dramatically. A flat stretch of ocean surface can appear to rise into towering columns. A distant ship can be stretched vertically until it looks like a creature standing on the water. One physics analysis specifically reinterpreted the mythical hafstramb monster of the Greenland Sea as a mirage of the water surface, showing how atmospheric refraction in polar conditions could produce images that match historical descriptions of the creature.8Comptes Rendus. Physique. Novaya Zemlya effect and Fata Morgana. Raytracing in a spherically non-symmetric atmosphere
These mirages are most dramatic in high-latitude waters, exactly where many of the most enduring sea monster traditions originated. Norse and Greenlandic sailors spent long hours staring at featureless horizons in cold conditions that favor strong temperature inversions. Combine that with genuine encounters with unfamiliar animals and a cultural framework that expected monsters to exist, and it becomes easy to see how sightings multiplied.
Deep-Sea Gigantism and the Creatures It Produces
One reason the deep ocean keeps yielding creatures that look monstrous is a real biological phenomenon: deep-sea organisms in certain groups grow far larger than their shallow-water relatives. Giant isopods are the most familiar example. These crustaceans, related to the pill bugs you find under garden rocks, can grow to more than 30 centimeters in the deep sea. The giant isopod Bathynomus jamesi turns out to have expanded gene families related to thyroid and insulin hormone signaling, which may contribute to its large body size. It also shows signs of inefficient lipid breakdown, low baseline metabolic rates, and a strategy of storing food in bulk, essentially a biological approach of eating big and burning slow.9PubMed Central. Genome of a giant isopod, Bathynomus jamesi, provides insights into body size evolution and adaptation to deep-sea environment
Recent research has pushed this story even further. Deep-sea isopods appear to have acquired a gene called ND1 from microbes through horizontal gene transfer, an ancient event where the gene jumped from a bacterium into the animal’s genome. That gene, which has since been duplicated multiple times and is expressed at high levels, helps suppress normal energy production pathways. In functional experiments using zebrafish and nematodes, the gene reduced basal metabolic rate and extended survival during starvation. The interpretation is striking: these animals co-opted a microbial gene to solve the core problem of being huge in a place with almost no food.10Cell. Deep-sea megafauna co-opts microbial energy metabolism genes to withstand ultra-long starvation Paired with a distended, food-retentive stomach that allows the isopod to gorge during rare feeding opportunities, the result is an animal that can survive extreme energy scarcity while maintaining a body plan that would be unsustainable in a less-adapted creature.
Siphonophores and the Longest Animals Alive
If you want something that genuinely looks like a sea monster right now, siphonophores are hard to beat. These colonial organisms, related to jellyfish and corals, are made up of thousands of specialized individual units called zooids that function together as a single body. Some species of Apolemia can form chains exceeding 40 meters, longer than a blue whale. They drift through the deep ocean trailing fine tentacles that sting and capture prey.
Despite their size, siphonophores are fragile and difficult to study. DNA analysis of their gut contents has revealed that Apolemia species consume a wide range of prey, including crustaceans, salps, ctenophores, and larvaceans. Some appear to specialize in gelatinous prey, while others are more generalist. Their sheer number of tentacles means they inevitably entangle almost anything that drifts past, making their diet partly a function of their architecture rather than active hunting strategy.11PLOS ONE. Characterizing the secret diets of siphonophores (Cnidaria: Hydrozoa) using DNA metabarcoding A diver or submersible encountering a 40-meter siphonophore colony in the open water column would see something that looks uncannily like a living sea serpent, glowing faintly with bioluminescence and slowly pulsing through the dark.
The Monsters That Actually Existed
Some sea monsters are real not because they are alive today but because they once were. Megalodon, Otodus megalodon, was a genuine apex predator of the ancient ocean. Reassessments of its body plan suggest it could have reached about 24 meters in length and weighed around 94 tonnes. Rather than resembling a scaled-up great white shark, as Hollywood typically depicts it, megalodon likely had a slenderer body. Researchers calculated that a great white body plan at that size would have created excessive hydrodynamic drag, meaning megalodon was built differently from its most famous modern relative.12PubMed Central. Reassessment of the possible size, form, weight, cruising speed, and growth parameters of the extinct megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), and new evolutionary insights into its gigantism, life history strategies, ecology, and extinction Studies of its body-size distribution across the fossil record show that it was left-skewed, meaning larger individuals were disproportionately common, suggesting sustained evolutionary pressure favoring bigger animals.13PubMed Central. Body-size trends of the extinct giant shark Carcharocles megalodon: a deep-time perspective on marine apex predators Its extinction roughly coincided with the rise of the great white shark, and competition between the two species for resources may have played a role in megalodon’s disappearance.
Megalodon was not even the most fearsome predator of its era. Livyatan melvillei, a giant raptorial sperm whale that lived roughly 9 to 10 million years ago, had enormous teeth designed for attacking large prey, including potentially other whales. Its skull has been described from deposits in Peru, and the animal was named after both the biblical sea monster Leviathan and Herman Melville.14Zoological Journal of the Linnean Society. Macroraptorial sperm whales (Cetacea, Odontoceti, Physeteroidea) from the Miocene of Peru Modern sperm whales are deep-diving squid hunters with relatively small teeth. Livyatan was something entirely different: a surface or near-surface predator with jaws built for tearing apart animals its own size. If anything from the fossil record deserves to be called a real sea monster, this whale is a strong candidate.
Scanning the Ocean Without Seeing It
One of the reasons so many large marine animals remain poorly documented is simple: the deep ocean is enormous and nearly impossible to survey comprehensively. A promising tool for closing that gap is environmental DNA, or eDNA. Every living thing sheds DNA into the water around it through skin cells, mucus, waste, and other biological material. By collecting water samples and sequencing the DNA fragments they contain, researchers can identify species that are present without ever needing to see them.
In deep-water surveys off the coast of the North Atlantic, eDNA sampling detected 21 fish families, 23 genera, and 15 species, including several deep-water specialists and species listed as critically endangered or endangered. Among the species identified were Bigelow’s ray, Agassiz’s slickhead, the Greenland dwarf snailfish, and members of the mesopelagic community like the glacier lanternfish.15PubMed Central. Harnessing the power of eDNA metabarcoding for the detection of deep-sea fishes The technique works for animals that actively avoid cameras and nets, making it particularly useful for confirming the presence of species that are otherwise nearly impossible to observe. As eDNA methods improve and become cheaper, they will likely settle long-standing questions about the distribution and abundance of animals that have historically been known only from rare, accidental encounters.
Why the Ocean Keeps Making Us See Things
There is a psychological dimension to sea monsters that goes beyond misidentification. Researchers studying public attitudes toward the deep ocean have noted that the idea of the deep containing monsters is remarkably persistent across cultures and centuries. The concept has drawn attention from psychologists, comparative mythologists, and even neuroscientists. One interpretation, drawing on Jungian psychology, frames deep-sea monsters as projections of unconscious fears: the ocean’s darkness and inaccessibility serve as a blank canvas onto which people project anxieties that have nothing to do with marine biology.16Oxford Academic. Fear and loathing of the deep ocean: why don’t people care about the deep sea?
Whether or not you find that framework persuasive, the practical effect is real. People consistently overestimate the dangers of the deep ocean and underestimate its ecological importance. That same research highlights a troubling consequence: thalassophobia and cultural monster mythology contribute to public indifference toward deep-sea conservation. If the ocean floor is imagined as a terrifying void full of nightmares, people feel less motivated to protect it. The irony is that the real deep-sea creatures, the giant isopods with stolen bacterial genes, the 40-meter-long colonial organisms, the squid that evade every tool we send after them, are far stranger and more worth paying attention to than any sea monster a sailor ever invented.