Wouldst thou … learn the secret of the sea?
Only those who brave its dangers
Comprehend its mystery!

Poet Henry Wadsworth Longfellow was talking about sailors in that 1850 verse, and it was indeed the mariners of ancient times who braved the uncharted dangers of the ocean, encountering mysteries aplenty.

But they didn’t always fully comprehend what they saw. In a time when uncharted areas on maps were decorated with dragons and other mythical creatures, sailors of old likewise embellished their stories with accounts of monsters of unfathomable size dwelling in the depths of the oceans.

Back on shore, storytellers and even early scientists were more than happy to spread the tales, sometimes pointing to massive fossils or outsized remains washing ashore as proof of the existence of sea monsters. Over the centuries, countless legends of sea creatures grew across virtually every culture in the world.

5 Famous Sea Monsters

Today, science tells us that sea monsters aren’t real, but their legends could well have grown from eyewitness accounts of some of the largest, most majestic — and sometimes most terrifying — creatures in the underwater world. Here are a few of our favorite sea monsters of literature, lore and legend, as well as some thoughts about the real-life giants of the deep who may have inspired the ancient tales.

1. The Leviathan

Although the word has evolved to be a generic descriptor for something particularly large, the stories of the Leviathan refer to a specific sea monster, who is one of the oldest such myths in human records. The Leviathan originated from ancient Jewish legend and is mentioned several times in the Old Testament.

Depending on the source, the Leviathan has the appearance of a large sea serpent, often a multi-headed one, like the famous Hydra of Greek legend. Sometimes it’s described as being crocodile-like. It also has the attributes of a dragon, including the ability to breathe fire.

Read More: The Mysterious Origin of Dragons

Many scholars view the Leviathan in strictly metaphorical terms, with the monster symbolizing chaos or evil, but over the centuries “leviathan” is a name that’s been given to real-life giant sea creatures, especially whales.

In fact, the original Hebrew spelling of the name, Livyatan, figures into the official name of Livyatan melvillei, a prehistoric whale discovered in 2008, which lived around 12 million years ago. The creature was a type of sperm whale with 14-inch-long sharp teeth. Paleontologists determined it grew to nearly 60 feet in length — big enough to hold its own against the massive megalodon shark, which would have existed around the same time, and competed with this real-life Leviathan whale for resources.   

2. The Kraken

As sea monsters go, names — and bodies — don’t get much bigger than the mighty kraken. This sea monster first appears in the historical record around the 12th century A.D., and was described then as the biggest monster ever to swim the world’s oceans. Early documents claimed that the kraken was huge — as large as 1.5 miles in circumference — and known to haunt the waters that figure prominently in Nordic culture (mainly around Norway, Iceland and Greenland).

Interestingly, some legends suggest that the kraken didn’t really prey on mariners and their ships so much. Instead, they were said to focus on eating huge amounts of fish, but were so massive than when they submerged, they could create whirlpools that sucked down human vessels in a dramatic act of collateral damage.

Read More: Startling New Sea Creatures

Like the Leviathan, at times the kraken was described as having many heads and claws. Other descriptions gave the kraken decidedly cephalopodic features, including an enormous head and multiple, massive tentacles.

As it turns out, krakens with those kinds of features do actually exist. Since the 1850s, when their giant beaks were first discovered, scientists and sailors alike have determined that the legendary kraken was most likely based upon an actual living sea monster, namely the giant squid, whose existence has been captured on film only in the last decade or so.

3. Colossal Squid

The giant squid may be, well, giant. But it is still smaller than the colossal squid, another enormous form of sea life which may have been mistaken for krakens and other sea monsters in history, but which is also all too real.

Living mainly in or near Antarctica, the colossal squid is just about the largest deep-sea predator that we know of. It can and has taken down whales of humongous size.

Read More: Why Is the Elusive Colossal Squid So Hard to Come Across?

Indeed, for many years, the only evidence we had of its possible existence were the outsized tentacle scars it left behind on surviving whales.

It’s only within the past 20 years that researchers have more conclusively confirmed the existence of these giant cephalopods, which possess up to ten legs or tentacles, can weigh up to 1,000+ pounds, and boast lengths of up to 50 feet.

4. Giant Whales

In Norse lore, the hafgufa has a special place in legend as a canny and rapacious fisherman. According to stories apparently based on eyewitness accounts, the massive creature was known to belch up portions of its previous meal in order to provide attractive bait for fresh fish.

Evidently possessed of enormous funds of patience, the hafgufa would float and wait, its giant maw left wide open until a sufficient number of fish came to feed, then it would snap its trap of a mouth shut, capturing a fresh meal.

Current research has determined that this kind of “trap feeding” is consistent with the hunting habits of some modern whales, which currently hold the title as the largest living creatures known to inhabit our planet’s oceans. If you want to see an actual “monster” of the deep alive today, whales will certainly fit the bill, although the huge mammals are hardly monstrous.

Read More: A New Whale Species Is Fighting for Survival

Where once ancient man might have categorized them as evil creatures from the deep, we now know that whales, like giant and colossal squids, are no malevolent beasts of fiction and legend, but instead are majestic animals that need to survive, as we all do.

That doesn’t mean that some whales didn’t have an attitude, especially when mariners insisted on hurling harpoons at them. In 1820, the whaling ship Essex found this out in dramatic fashion, when an 85-foot-long sperm whale — a good 20 feet longer than most of its kind — decided to attack the ship. The assault capsized the Essex, forcing the 20-man crew to abandon ship.

After months at sea — and indulging in a fair bit of cannibalism — eight members of the crew survived. Their story was the sensation of the era, and eventually inspired an obscure writer named Herman Melville to compose a novel based on the event. You might know it as 1851’s Moby-Dick.

5. Honorable Mention: The Loch Ness Monster

When it comes to aquatic cryptids, it’s practically an international law that you must include a mention of the infamous Loch Ness Monster, or “Nessie,” as she is known to her friends. Nothing compares in fame and intrigue to this mythical beast, even though the creature is associated with a cold and deep but nevertheless land-locked freshwater lake.

Entering the historical record in the 6th century A.D., the Loch Ness Monster has many theories attached to it. One of the most enduring is that the massive monster is one (or perhaps several) prehistoric creatures — the plesiosaur is often suggested as the actual species that somehow still survives in the loch in modern times.

Other theories assert that the creature first entered the famously deep loch during an unspecified era in ancient history, when the loch was actually connected to the North Sea (and still is, via some tributaries). Still other theories purport that there is somehow still a deep-water channel open from the loch to the ocean, and that occasional sightings of the monster over the centuries are actually multiple sightings of several different creatures from the same species.

Read More: Palm-Sized Sea Creature Named the World’s Oldest Animal

In fact, there is no conclusive proof that Nessie is real, and indeed there is a large body of evidence debunking many of the historical sightings over the years.

And yet, given that it took many centuries for giant and colossal squids to be confirmed as real, it might be premature to suggest that creatures like Nessie do not — or never have — existed. After all, more than 70 percent of the planet’s surface is covered by water, and barely 5 percent of that area has been explored by humans.

So, who’s to say there aren’t an abundance of undiscovered monster-sized sea animals still dwelling in the depths? Such creatures might yet be living in obscurity until modern mariners discover them. Then, writers and poets, like Longfellow more than a century ago, can share the secrets of the sea with the rest of the world.

Read More: Are Mermaids Real or Did Sea Creatures Inspire Mermaid Folklore?