Real Sea Monsters

by Brian Dunning
June 3, 2025

I've always believed that somewhere deep down, each of us harbors some kind of fear of the water. When I was a small boy I lived about the first twelve years of my life on the beach front in southern California, and was in the ocean almost every day — but I'll admit to you right now I was never completely comfortable. And then the movie Jaws came out and made me even less so. Then several times I got tangled up in a big bunch of kelp and couldn't get out as the waves washed me back and forth. Finally, the last time I was ever in the water, I was grown with a family and went out just to duck my head underwater. The next thing I knew I was caught in a rip, couldn't touch the bottom, and was tiring quickly. If I hadn't been rescued by a lifeguard, I very likely could have died — and that's after a lifetime of knowing how to get out of rips. So, now at my advanced age and decrepitude, I am satisfied with the time I have already spent in the ocean, and do not feel compelled to spend any more. Thus, I can now, with emotional safety, talk about the real sea monsters that actually do exist — and that secretly tormented me for all the decades you have been listening to me here, and for those that came before.

What are the real sea monsters that dwell in those black depths? We know about Great White Sharks, various jellyfish, and then the unknown — what are the fish we don't personally happen to know about? What else is down there? But the real mind-warp comes when we think of what might have been down there 100, 200, 650 million years ago? What are the very worst of the real sea monsters from the entire history of Earth? Today we're going to point the skeptical eye at the fossil record, and find out.

To get started, let's draw from pop culture, from the Jurassic Park film franchise. Beginning with Jurassic World, they had a SeaWorld-type aquatic show, where the highlight was an enormous reptilian monster — the size of an airliner — which sprung up out of the water to grab its food. It was far larger than anyone had ever imagined. Was that something that really existed?

Mosasaurs

Well I hate to disappoint, but this is a case where the filmmakers might have taken some artistic license, as they did in all the Jurassic Park films. The typical size scaling of creatures in the films was about 2:1, they were twice their actual size. But they went further with Mosasaurus hoffmani. They originally had it at 70 feet long, but Spielberg liked it so much he had them stretch it to 120 feet long (37 m). So the movie version was 3x the size dimensionally; 27x the actual weight. The real Mosasaurus hoffmannii — the very largest of the clade — was 12m (39 ft) long; a far cry from the movie depiction.

Now bear with me a moment for a disclaimer if you would: All the measurements given of extinct creatures known from the fossil record have many, many size estimates given, as various paleontologists found various fossil individuals and fit them into the context of other species. Often I found the earliest estimates were the biggest, which gradually came down as cooler heads prevailed over the decades. You can quarrel with every measurement I give today because it's easy to find different ones; I had to go to a lot of trouble to find the more recent and widely accepted numbers. So don't email to tell me I got the length or weight wrong for some random prehistoric beast!

Ichthyosaurs

While Mosasaurus hoffmani looked something like a huge shark with four lobes instead of fins, Ichthyotitan severnensis was more adapted for aquatic life. Its four lobes had evolved completely into fins, and it had a bit of a dorsal fin. It was also the true king of the seas in its day, the largest ichthyosaur of all time, measuring 26 m (85 ft) long — the size of a whale.

Plesiosaurs

Plesiosaurs were the best adapted of the bunch for swimming, with much longer fins. One you may have seen from the series Walking with Dinosaurs, Liopleurodon ferox, had its size greatly exaggerated for the show. It was a short-necked plesiosaur, like most of them, and like the biggest one ever: Monquirasaurus boyacensis, which measured 11 m (36 ft) and weighed 14 mt.

Several plesiosaurs were in the same size range but probably just a bit smaller. The one you're probably most familiar with, the long-necked Elasmosaurus platyurus, was just a touch shorter at 10.3 m (34 ft), but weighed much less because it was far more slender.

That takes us to the types of real sea monsters that you might still face today, as the rest of these denizens of the deep have surviving descendants.

Snakes

The sea monsters of snakes are only semi-aquatic, meaning they spend a lot of their time in the water but not all of it. Heaven help you if you run into the biggest species today, the Green anaconda (Eunectes murinus), 6 m (20 ft) long and weighing up to 200 kg (440 lb). Terrifying as the anaconda might be, it's just a mealworm compared to the most fearsome of all time: the extinct Titanoboa cerrejonensis which was more than twice as long at 14.3 m (47 ft), nearly four times its weight at 730 kg (1,610 lb).

Turtles

Today's Leatherback sea turtle (Dermochelys coriacea) is still one of the biggest turtles of all time, 3 m (10 ft) across and weighing nearly a ton at 961 kg (2,119 lb). However the real champion is now extinct and was nearly twice the size. Archelon ischyros measured 5 m (16 ft) and weighed 2.2 mt.

And if you're thinking that turtles aren't all that scary and wouldn't really be sea monsters; well, know that Archelon ischyros was a carnivore, with a powerful jaw and a great hooked beak. Each front flipper alone was larger than a man.

Crocodiles

They're our last category of reptiles, and are probably among the most fearsome. The biggest ever extant crocodile with a size proven by a modern measurement is the Orinoco crocodile (Crocodylus intermedius) the largest individual of which measured 6.78 m (22.2 ft). Though he was nothing compared to his big brother from the Late Cretaceous period. Deinosuchus riograndensis was 10 meters (33 ft) long, half again the length of a modern crocodile, and is estimated to have weighed 5 mt. But he was also extra scary, in that fossil remains have shown that Deinosuchus often feasted on dinosaurs his own size. He'd sneak up on the shore, grab them, and death roll them into the water.

Cetaceans

Luckily the very biggest sea monster of all time is also the least likely to hurt you. Today's blue whale (Balaenoptera musculus) remains the all-time king of the seas, at 33 m (108 ft) and weighing a staggering 260 mt.

If we include extinct cetaceans looking for the biggest, the most history can do is one that weighed a sixth of that (40 mt): Perucetus colossus — and stretched only 16 m (52 ft). This is another one that you'll read much larger measurements for, but its most likely dimensions have been revised downward many times over the decades.

There was one prehistoric whale longer than Perucetus colossus: Basilosaurus cetoides which was 20 m (66 ft) long, only two-thirds the length of the blue whale. But it was also much more slender than Perucetus so it weighed only 15 mt compared to Perucetus' 40.

Fish

Let us not forget that the most notorious sea monsters in the sea — man-eating sharks — are fish. The greatest of these still living is the whale shark, which maxes out at 19 m (62 ft). That's immense, but luckily, it's friendly and only eats tiny stuff like plankton and krill. But as far as fish go, ancient history outdid today's worst sea monsters. The infamous megalodon — once classified as Carcharodon but today reclassified as Otodus megalodon, was probably just a little bit bigger — as well as having been a predator. Much uncertainty surrounds the size of megalodon, but I'd say the most accepted max size was 20.3 m (67 ft). That's still two-thirds the length of a blue whale.

Among the most interesting things about the megalodon is that in recent years, nursery sites have been discovered, where fossil remains fall mainly into two groups: full adult size, and tiny baby shark size. Several of these nursery sites have been found in Spain, the home of quince paste.

Crustaceans

While it may at first seem comforting to leave the world of giant sharks and retreat to the realm of little friendly crustaceans, they're not all so friendly. Go deep into the waters off Japan and you're likely to encounter the most frightening of all underwater sights: the Japanese spider crab (Macrocheira kaempferi), whose long, sticklike legs stretch 3.7 m (12 ft) across. It's a decapod, with eight spidery legs plus two long claws, longer than any of the legs.

But it is a fine, slender sea monster; yet the biggest can still weigh in at 19 kg (42 lb). So it's a tiny bit shy of holding the record for all-out bulk. The heaviest extant crustacean is the American lobster, the biggest on record having weighed in at 20 kg (44 lbs).

But today's biggest crustaceans do not hold a candle to the underwater horrors of our distant past. Jaekelopterus rhenaniae — not a true crustacean (but still an arthropod) — looked roughly like a lobster (that biggest American lobster having stretched to 64 cm (25 in)) — but four times longer and about 64 times the bulk. Fossils have been found at 2.5 m (8 ft) long, plus one more meter with claws extended. And while the American lobster is a bottom feeder, Jaekelopterus was the apex predator. Watch out.

Cephalopods

I'm going to give you a bit of a break from ancient ancestors of today's biggest monsters being too scary to think about. When it comes to cephalopods (squid, octopus, etc.), history cannot match today's most frightening.

The one best classed as a real sea monster is the Giant Pacific Octopus (Enteroctopus dofleini), with a maximum measured leg span of 9.8 m (32 ft). If you're in the water and that thing wants to eat you, you lose. End of story.

You're less likely to have a problem with the two largest squid that have ever existed, both of which patrol our oceans today. One is the long and slender Giant Squid (Architeuthis dux), with a mantle 2.4 m (7.9 ft) long and a total length with tentacles of 15 m (49 ft), but with a weight of merely 275 kg (606 lb). Its big brother, the Colossal Squid (Mesonychoteuthis hamiltoni) has shorter tentacles but is nearly twice the weight, with a larger and more robust mantle of 3 m (9.8 ft) and a total length with tentacles of 10 m (33 ft), but a massive weight of half a ton (495 kg (1,091 lb)).

We can all be proud — and a little bit terrified — that the fossil record shows no sea monsters worse than today's.

Invertebrates

This theme is repeated with invertebrates such as the sea jelly. The most enormous one ever known is found in our oceans today, the Lion's Mane Jellyfish, with a bell up to 2.1 m (7 ft) across, and tentacles that have been measured at 37 m (120 ft). Sea jellies don't fossilize all that well, so we have only a scant record. The biggest found so far is Burgessomedusa phasmiformis, with a modest bell of only 20 cm (8 in), and no record of tentacle length.

Colony blobs

We've saved the weirdest and freakiest real sea monsters for last — great gelatinous blobs that are colonies of tiny gelatinous blobs called zooids. They form together, generally into great big tubes. The longest of these is the Giant Siphonophore (Praya dubia) — imagine a sparkling bioluminescent rope more than 40 m (131 ft) long. Siphonophore zooids come in a variety, each performing specialized functions, and these ropes are able to move through the sea with surprising agility. See one surprise an ROV team on YouTube.

The Giant Pyrosome (Pyrostremma spinosum) is probably even weirder. All the zooids are the same, taking in water at one end and spitting it out the other. They form together into a great big tube that comes to a closed point at one end, and they're excreting water into the tube, which turns the whole thing into a big glowing blue bioluminescent jet, moving lazily around. These have been found stretching 18 m (60 ft), with the open end of the tube gaping up to 2 m (6.5 ft) if anyone wants to be brave enough to swim inside. Watch some divers have an encounter with one here.

And so with that fresh horror in mind, I am now going to recline and enjoy not being in the ocean. But for all of you who do choose to indulge, I present you the oceans on a silver platter, complete with all of their real sea monsters.

By Brian Dunning
Follow @BrianDunning