Do sea monsters exist? Depends what you consider a monster. Show a sperm whale, a Komodo dragon or a deep sea anglerfish to someone for the first time, and they might find them just as sensational as any creature from fiction or myth. And even scientists believe that when it comes to demystifying our planet’s most hostile reaches, we’ve probably plenty more bizarre life to find. (Read: World’s largest cave fish discovered in India.)

One thing seems inevitable, though: when it comes to large and unexpected creatures announcing themselves to science, the chances are they will come from the depths of the ocean. Much of the nooks and crannies of our oceans remain unexplored, and with new species discovered with every major expedition into the deep – a place challenging simply to get to for interested humans – our seas are fertile territory for new discoveries.  

The Carta marina et descriptio septentrionalium terrarum – named in Latin as per cartographic conventions of the time – was the first detailed map of Scandinavia, printed in 1539 and created by Swedish cartographer Olaus Magnus. On it reveals the convention may believed that every animal on land had a marine equivalent – and that in the waters, monsters lurk.  

As for sea monsters in the popular imagination, countless renderings in literature and on the big screen has proven – with the exception of perhaps Splash! and the watery characters in Disney’s Luca – that they’re rarely friendly. And the mystery around the deeps, combined with the imaginations and fears of those who sail them, have built up an impressive cast of deep-sea creatures that have terrorised the waters of history for centuries. (The Walt Disney Company is majority owner of National Geographic Partners).

Some of these legends have been so intriguing and historically tenacious, science has taken a long, hard look. Some may have been exaggerated from animals we now know are real. And just occasionally, nature has produced a real-life surprise of its own. Here are some of the more famous sea monsters from history – and the case for their real equivalents.   

Colossal cephalopods

To sailors of the icy northern seas in the Middle Ages, the kraken was no joke. Born from the shapeshifting world of Scandinavian mythology, this creature – from the Old Norse term for octopus – is a tentacled creature that lurked in the ocean between Norway and Greenland, occasionally rising to make a meal of ay ship foolish enough to get in its way.

The fable of the kraken was richly developed: by fishing over where one lay on the seafloor, the catch would be good, as fish were attracted to its regurgitations. The animal itself (there were thought to be at least two) was immense: the size of an island, confusing sailors by appearing and disappearing in the mist. Descriptions of the kraken suggest a compound of creatures and conditions at fanciful sizes, seeming to bear hallmarks of the giant squid, basking shark, sperm whale and crab.  

An 1801 drawing by Pierre Denys de Montfort envisions a colossal octopus that reportedly attacked a group of sailors off Angola. Even the biggest cephalopods are some way off the scale depicted: the giant Pacific octopus can have a leg span of around 4 metres, whereas the largest giant squid ever examined was just over 13 metres from mantle to tentacle-tip. The latter could conceivably reach much greater lengths. 

Photograph by Pierre Denys de Montfort / Wikimedia Commons

While belief amongst skittish sailors was apparently universal, naturalists took it seriously too: the kraken made it into Carl Linneaus’s first version of the Systema Naturae – categorised Microcosmus marinus – as well as a Norwegian natural history of 1752, where author Erik Pontoppidan described the creature as ‘round, flat, and full of arms, or branches’; in the same volume he also found space for mermaids and sea serpents, if anything confirming that knowledge of the seas at the time was sketchy at best.

It’s forgivable: in the times before submersibles and diving gear, most ideas concerning large sea animals were based on partial glimpses at sea or hugely bloated carcasses washed ashore – so it’s understandable a certain conjecture was at play for much of the early days of seafaring. Maps of the age depict waters teeming with all manner of ship-menacing monsters, and the idea would take many years to stubbornly sink.

At the bottom of this image, a shark is attacked broadside by a deep sea squid in a remote camera image taken at 2,400ft in the Kaikoura Canyon, a mile-deep trench off South Island, New Zealand. Squid – such as the Humboldt squid – are formidable predators, but their deep sea incarnations are little-observed. This never-seen-before behaviour suggests the squid are masters of their environment, and some theories suggest gigantism is entirely possible amongst animals which rarely leave the depths. 

Photograph by Emory Kristof / National Geographic Image Collection

As late as 1809, botanist George Shaw spoke soberly of the kraken in his zoological lectures to the Royal Institution, citing European relatives of the ‘enormously large’ species of cuttlefish (he was probably confusing them with squid) in the Indian Ocean as being possible culprits for the legend: ‘A modern Naturalist chooses to distinguish this tremendous species by the title of Colossal cuttle-fish, and seems amply disposed to believe all that has been related of its ravages.’ He goes on to describe a then-recent attack on a boat in ‘African seas’ where three sailors were seized and killed by such a ‘monster.’ A tentacle cut-off during the struggle was the thickness of a ship’s ‘mizzen-mast, and the suckers the size of pot lids.’

Down-scaling sailor-seizing, ship-crushing proportions, later descriptions steadily de-sensationalised the animal, to the point where it might be recognisable as creatures we now know exist – but remain shadowy.

Rare Video of a Live Giant Squid

The giant squid, for instance – like its bulkier but shorter southern counterpart the colossal squid – retains a heady mystique, with only a handful of sightings of the living creature on record. What we do know is extrapolated from analysis of fearsome, predatory relatives such as the Humboldt squid, the occasional carcass, and the scars observed on creatures such as sharks and sperm whales that do battle with it in the deep.

With toothed suckers, a fearsome beak and eyes the size of dinner plates these invertebrates resemble many of the accounts of the kraken. Proportions still fall somewhat short, however: the largest giant squid specimen so far recorded measured 13 metres in length, with some speculating they could reach 27 metres, and others suggesting bigger still. All are conjectures, though – which means, in the style of any true sea monster, we really don’t know what might be down there.

This eerie image by Edo-period Japanese artist Utagawa Kuniyoshi depicts the Umibozu rising from the sea to confront ‘the sailor Tokuso.’ Depictions of the Umibozu vary but the rounded, dark head, prominent eyes are recurrent. Appearing when the water is calm and causing a storm, it has been attributed to brooding stormclouds or other atmospheric phenomena.  

Sinister manifestations

Amongst the more ominous of mythical sea phenomena is Japan’s umibozu – who appears out of the night-time seas as a black apparition, often as the waters are becoming rough. With a rounded head resembling a Buddhist monk’s shaved scalp – hence the name, meaning ‘sea priest’ – the umibozu is variously referenced in Japanese folklore from as early as the 17th century, though its origins are ambiguous.

In folklore, the umibozu is said to fortell an oncoming storm, and its legend often mingles with that of the funa yueri – the souls of drowned sailors – in that it asks for a ladle with which to fill the boat with water to sink it. Explanations for the phenomena include storm waves, ominous mammatus or thunderhead clouds, and even mirages.

Giant sharks

Monstrous sharks were once very real, which is perhaps enough for some to believe they still might be. There is a fairly good case for that too, but probably not for the creatures of your expectations. We would know, for instance, if Megalodon – an 18-metre prehistoric shark with teeth the size of salad plates – was still on the rampage nearly 4 million years after its last appearance in the fossil record. (Read: Megalodon is definitely extinct, and great whites may be to blame.)

Bite marks in drifting carcasses, and a seafloor breadcrumb trail of those tell-tale teeth grown and shed on endless rotation, would all give us plenty of evidence that a temperate-water predator with such a demanding diet was still loose in the seas. But colder, deeper waters home to more adaptable creatures could hold plenty of surprises.

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