Hundreds of years ago, sailing the vast ocean must have been a terrifying feat. We still have much left to learn about our seas today, so it makes sense that ancient mariners let their imaginations go wild when thinking about the creatures that might be lurking in the waters below.
Many mythical sea monsters may have been based on real marine life sightings – here are a few of the animals behind the lore.
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10 terrifying sea monsters
The Kraken
Scandinavian folktales tell of a terrifying beast called the Kraken which emerged from the depths, wrapped its arms around ships in a crushing embrace and dragged the vessel and its hapless crew down to Davy Jones's locker.
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Many believe this legend was inspired by a real-life counterpart: the giant squid (Architeuthis dux). This enormous cephalopod lives in the deep ocean and can grow to around 13 metres long. Despite being legendary for many years, these ocean giants are rarely seen. The first video of a live giant squid wasn’t captured until 2006.
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Moby Dick
The tall tale of a sperm whale charging headlong into a boat and sinking it might sound fantastical, but the plot of Moby Dick was inspired by real life events. Author Herman Melville heard of a whaling ship called the Essex being rammed by a sperm whale and the story ended up in his novel.
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Scientists recently captured this headbutting behaviour on camera for the first time when they filmed sperm whales charging into one another. Exactly why they have these face-first fights is not yet clear.
Porphyrios
Porphyrios was a huge whale that is said to have spent 50 years terrorising boats off the coast of Constantinople. There are some clues hinting at what species it might have been but this mystery has never been fully solved.
Some say that this terrible sea monster must have been a sperm whale because of its impressive size (around 13 metres long) while others suggest that its hunting behaviour (it eventually stranded itself trying to catch dolphins) means it must have been an orca, which can beach themselves on purpose while hunting.
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This is all confused by the fact that Procopius, who told the story, was writing about historical events. He may have misinterpreted things or exaggerated elements of the tale for the sake of a good yarn. Was Porphyrios an orca, a sperm whale, or something else altogether? Maybe we’ll never really know.
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Doomsday fish
It is said that when oarfish washes up an earthquake, tsunami or some other kind of natural disaster is on the way. That’s why these serpent-like fish are known by some as ‘harbingers of doom’ or ‘Doomsday fish’, even though there’s no scientific proof of this.
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These strange animals, also known as ribbonfish, are very much real but you can see at a glance why they might inspire mythical stories. They have long, shimmering silver bodies that can stretch to around seven metres and flamboyant red crests on their heads like a jaunty feather sticking from a cap.
Jaws
Duh, duh. Duh, duh… even the theme tune of the Jaws movie strikes fear into many people. The movie plays a large part in why people see great white sharks as mindless man-eaters.
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The reality is that – while it’s sensible to be cautious in waters that might have predatory sharks – the chances of being bitten by a white shark (or any species!) are incredibly low. According to the International Shark Attack File, there were just 65 unprovoked shark bites and 29 provoked bites recorded in 2025 – that’s all around the world and by all species of shark.
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Hafgufa
In 2011, marine scientists documented whales doing something called ‘trap feeding’. They saw baleen whales remaining still at the surface of the water with their mouths wide open – and the fish swam in, soon to be gobbled up.
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The researchers thought this was a new behaviour but there was something familiar about it. When Old Norse stories from the 13th century described a monster called hafgufa, it sounded eerily similar to these trap feeding whales: the fish scurry into the hafgufa’s gaping mouth until it snaps shut its jaws and swallows the lot of them.
Lyngbakr
Another 13th century Norse fable tells of the lyngbakr (or heather back). Its deadly strategy? It mimics an island.
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Imagine how relieved an exhausted sailor would be to step foot on solid ground again after months at sea. So, when they saw a huge mound rising from the ocean, covered in heather, it’s natural that they thought they’d found an island.
But as soon as they set foot on its back, the lyngbakr would plunge below the waves, bringing the down mariners with it and drowning them all. The story isn’t real but you could imagine how a large whale could have inspired it.
Hydra
Greek mythology is renowned for its magnificent monsters and the Hydra is one of the most frightening. This sea snake had far too many heads to start with (some say three, others say nine) and if you lopped one off two grew back in its place.
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Clearly, this isn’t a real animal but there are real freshwater organisms called hydras that are related to jellyfish and corals. Unlike the mythical hydra, they are tiny. But they do have the ability to regenerate their whole body from small parts.
Mermaids
One of the most famous maritime legends is that of the mermaid: magical beings with the tail of a fish and the body of a stunning woman.
Given their tendency to lure sailors into treacherous situations with their beautiful looks, it seems odd that the legend of the mermaid may have been inspired by early sightings of dugongs. These gentle sea cows are gorgeous in their own way but look more like a floaty potato than a pin-up girl.
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But take one look at their enchanting tails and you’ll see how the myth emerged. When I saw a dugong’s tail fluke disappear slowly below the waves in Bazaruto, Mozambique, I was immediately convinced I’d spotted a mermaid.
Unicorns
While unicorns are more likely to be seen prancing around a verdant meadow than splashing in the sea, the origins of this mythical beast are surprisingly nautical.
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Narwhals are known as ‘unicorns of the sea’ because of their long horn-like tusks but these real Arctic whales weren’t just named after the legendary creatures; they helped to inspire them. In Medieval times, narwhals were hunted and their tusks passed off as ‘real’ unicorn horns. This might explain why old depictions of unicorns show such a long, spiral horn.
