For centuries, fishermen from Norway and Greenland have told tales of a terrifying sea monster: the kraken. Supposedly, this vast creature has giant tentacles that can pluck you from your boat and drag you to the depths of the ocean. You can't see it coming, because it lurks deep beneath you in the dark water. But if you suddenly find yourself catching a great many fish, you should flee: the kraken might be beneath you, scaring the fish towards the surface.

In 1857, the kraken began to move from myth to reality, thanks to the Danish naturalist Japetus Steenstrup. He examined a large squid beak, about 8 cm (3 in) across, that had washed up on Denmark's shores several years earlier. Originally he could only guess at the overall size of the animal, but soon he was sent parts of another specimen from the Bahamas. When Steenstrup finally published his findings, he concluded that the kraken was real, and it was a species of giant squid. He named it Architeuthis dux, meaning "ruling squid" in Latin.

Only after Steenstrup had described the creature could scientists begin to unravel whether there was any truth to the old myths. Was this huge squid really as dangerous as the legends had led people to believe? Where did it come from, and what was it up to in the dark depths of the sea?

The kraken has held a grip on people's imaginations for hundreds of years. The Norwegian writer Erik Pontoppidan described one in detail in his 1755 book The Natural History of Norway. According to fishermen, Pontoppidan wrote, it was the size of a "number of small islands", and its back appeared to be "about an English mile and a half".

Sail too close and the Scylla would try to eat you

Its grasping tentacles were only part of the problem. "After this monster has been on the surface of the water a short time, it begins slowly to sink again, and then the danger is as great as before; because the motion of his sinking causes such a swell in the sea, and such an eddy or whirlpool, that it draws everything down with it."

Different cultures had different names for similar-sounding monsters. Greek mythology describes the Scylla, a six-headed sea goddess who ruled the rocks on one side of a narrow strait. Sail too close and she would try to eat you. In Homer's The Odyssey, Odysseus was forced to sail close to Scylla to avoid an even worse monster. As a result, six of his men were lost to Scylla, who swung them up onto her cliff and "bolted them down raw".

Even science fiction writers have got in on the act. In Twenty Thousand Leagues Under the Sea, Jules Verne describes a giant squid that is distinctly kraken-like. It "could entangle a ship of five thousand tons and bury it into the abyss of the ocean." So does the real giant squid live up to its legendary counterparts?

Since Steenstrup's initial discovery, about 21 more species of giant squid have been described. None were from live animals, but rather from parts, or occasionally whole specimens, washed up on shore.

Even now, nobody is even sure how big giant squid can grow

For example, in 1933 a new species called A. clarkei was described by Guy Colborn Robson, from a near-intact specimen found on a beach in Yorkshire, England. It was "referable to none of the species hitherto described", but was so badly disintegrated that Robson could not even determine its sex. Others have been described after they were found inside the bellies of sperm whales, which evidently ate them.

Giant squid are thought to grow to a length of 13 or even 15 m, including their tentacles. One estimate suggested they could reach 18m, but that could be a serious overestimate, says Jon Ablett of the Natural History Museum in London, UK. That's because squid tissue can act like rubber in the midday sun, so when a squid washes up and dries out it can stretch out.

It's telling that, even now, nobody is even sure how big giant squid can grow. Whole specimens are hardly ever found, due to the squid's elusive nature. They spend much of their time at depths of 400 to 1000m. That may be partly an attempt to stay beyond the reach of hungry sperm whales, but it's a partial success at best. The whales are perfectly capable of diving to such depths, and a giant squid is almost defenceless against them.

The squid does have one advantage. Its eyes are the largest of any animal: they are as big as dinner plates, up to 27cm (11in) across. These giant peepers are thought to have evolved especially to spot the whales at great distances, giving the squid time to take evasive action.

In turn, giant squid prey upon fish, crustaceans and smaller squid, all of which have been discovered in the stomachs of those analysed. One giant squid even turned out to have the remains of other giant squid in its stomach, suggesting they sometimes resort to cannibalism – although it's not clear how often.

The squid look like they should have no trouble catching prey. They have two long tentacles that could grab their victims. They also have eight arms, covered with dozens of suction cups lined with horny rings with sharp teeth. If an animal gets snared by enough of these suction cups, it could never escape, says Clyde Roper, a retired giant squid hunter at the Smithsonian Institution in Washington.

This sounds like a terrifying ordeal. But none of it is proof that giant squid are active predators. Some big killers, such as sleeper sharks, move slowly to conserve their energy. They only scavenge for food when it presents itself. In theory, giant squid might do the same.

This idea was blown out of the water in 2004. Determined to observe a live giant squid in the wild, Tsunemi Kubodera of the National Science Museum in Tokyo, Japan teamed up with whale expert Kyoichi Mori. Together they used the known locations of sperm whales as guides, and snapped a photo of a live giant squid near the Ogasawara Islands in the north Pacific.

He captured the first video footage of giant squid

Kubodera and Mori lured the giant squid with bait, and discovered that it attacked horizontally by stretching out its tentacles in front of it. Once the squid had caught something, its tentacles coiled "into an irregular ball in much the same way that pythons rapidly envelop their prey within coils of their body immediately after striking," according to their report.

Eight years later, Kubodera went one better. Working with a film crew, he captured the first video footage of giant squid.

The key to this, according to team member Edith Widder of the Ocean Research & Conservation Association in Fort Pierce, Florida, was stealth. She suspected that the electric thrusters that power most submersible cameras were scaring away the squid. Instead, she came up with a contraption called Medusa, a battery-operated camera attached to a lure. Medusa emitted a blue light, designed to mimic the light produced by a crown jellyfish called Atolla. When this jellyfish comes under attack, it uses its light to lure any bigger creatures lurking nearby to swoop in and attack the attacker.

Dainty feeding

The footage from the first eight-hour dive was largely blank, but during the second attempt, the enormous arms of a giant squid suddenly flashed up on the screen. "We were screaming our heads off," says Widder.

The squid only took very small, delicate bites

After a few more tries they saw the squid in full and watched it wrap its arms around the whole camera platform. It precisely aimed for the place where a predator would have been, confirming that it was indeed an active predator.

To further entice the squid, Kubodera dropped down a smaller squid as bait. He and two others then spent 400 hours in a cramped submarine, to get more footage and see the creature with their own eyes.

A giant squid did attack, but the bait "wasn't shredded the way you would have thought," says Widder. The squid fed for 23 minutes, but it only took very small, delicate bites with its parrot-like beak, gradually munching away. Widder thinks that giant squid cannot gobble their prey down quickly, because they might choke.

Giant squid are clearly not quite the scary monsters they have been painted as. They only attack their direct prey, and Roper believes they are not naturally aggressive to human beings. As far as we can tell they are more gentle giants, says Roper, who calls them "magnificent creatures".

Male giant squid have an external penis up to 1m long

Though they have been known for over 150 years, we still know almost nothing about their daily behavioural or social patterns, eating habits or where they travel on a typical day or year. As far we know they are solitary animals, says Roper, but their social lives remain mysterious.

We don't even know where or how often they mate. We probably can't extrapolate from smaller squid. Whereas in most cephalopods, the males have a modified arm for storing sperm, male giant squid have an external penis up to 1m long.

In a bid to uncover their mysterious mating habits, two Australian researchers analysed several female giant squid in 1997. Their findings suggest that giant squid mating is a violent, scattergun business. They concluded that the male uses his muscular and elongated penis to "inject" packets of sperm called spermatophores directly into the arms of females, leaving shallow wounds. Later studies suggest that the spermatophores do this partly of their own accord, using enzymes to break through the female's skin.

It's unknown how females access this sperm to fertilise their eggs. She might rip her skin open with her beak, or the skin covering it may break down and release the sperm.

What is clear is that giant squid are very successful at producing offspring. They seem to live in every ocean, apart from the polar regions, and their population must surely be large if they can satisfy the cravings of so many sperm whales. There are probably millions of them out there, says Widder. She says humans have clearly been exploring the deep ocean in a way that scares them off, or we would have seen more of them.

Giant squid living on opposite sides of the planet can be almost genetically identical

What's more, it emerged last year that all 21 species described since 1857 actually belong to the same species. A study of the DNA sequences from 43 tissue samples taken from around the world showed that what seemed to be separate species were all freely interbreeding.

This may be because young squid larvae are transported all around the oceans in powerful currents. That would explain why giant squid living on opposite sides of the planet can be almost genetically identical. Ablett says the mistake is understandable, as so many of the supposed species were originally described from incomplete parts.

"The entire world's population of giant squid may have evolved from a relatively recent population expansion, after a preceding population crash," says Ablett. Nobody knows what caused their population to shrink. The genetics only tells us that the population began growing some time between 110,000 and 730,000 years ago.

So if the giant squid is not a true monster of the deep, are there any other contenders?

It has swivelling hooks to help it catch fish

The colossal squid, first described in 1925, looks like a promising candidate for a gigantic sea monster. It might grow even larger than giant squid. The largest specimen ever captured was only 8m long, but it seems to have been young so it may not have reached its full length.

Instead of teeth on its suckers, it has swivelling hooks to help it catch fish. But unlike the giant squid it seems not to be an active predator. Instead, the colossal squid floats around and uses its hooks to ensnare prey that stray too close.

What's more, the colossal squid only lives in the Antarctic seas, so they can't have been the inspiration for the Scandinavian kraken legends.

Much more violent are the smaller Humboldt squid, which are known as "red devils" because of the colour they flash when in attack mode. They are more aggressive than giant squid and have been known to attack humans.

They certainly couldn't drag fishermen from boats

Roper once had a lucky escape, when a Humboldt squid came "gouging with her sharp beak through my wetsuit". Some years before, he had been told the story of a Mexican fisherman who fell overboard into a school of actively feeding Humboldt squid. "As he reached upwards for his mate to pull him aboard, he was attacked and pulled beneath the seas and was never seen again, having become a meal for the hungry school of squid," says Roper. "I considered myself quite fortunate to have come out of the water in more or less one piece."

However, while the Humboldt squid is clearly dangerous, even at maximum length they are hardly bigger than a human. So they don't pose a serious threat unless you happen to be in the water with them. They certainly couldn't drag fishermen from boats, as the kraken legend claims.

All in all, there is little evidence of a truly monstrous squid living in the ocean today. But there is reason to suspect that squid reached stupendous sizes in the distant past.

According to Mark McMenamin of Mount Holyoke College in South Hadley, Massachusetts, during the early dinosaur era there may have been whopping squids up to 30m long. These prehistoric krakens may have preyed on ichthyosaurs, giant marine reptiles that looked a bit like modern dolphins.

McMenamin first presented this idea in 2011. He found nine fossilised vertebrae from ichthyosaurs, arranged in linear patterns that, he argues, resemble the pattern of the "sucker discs on cephalic tentacles". He suggests that a kraken "killed the marine reptiles and then dragged their carcasses back to its lair" for a feast, arranging the bones in near-geometric patterns.

There were periods in the past when squids became very large

It's a speculative idea. In its defence, McMenamin points out that modern cephalopods are some of the most intelligent creatures at sea, and that octopuses have been known to collect rocks in their dens. However, his critics point out that there is no evidence that modern cephalopods stockpile their prey.

McMenamin has now found a fossil that he believes to be part of the tip of an ancient squid-beak. He presented his findings at the Geological Society of America. "We think we're seeing a very close connection between the deep structure of a particular group of modern squid and this Triassic giant," says McMenamin. "What it tells us is there were periods in the past when squids became very large."

However, other palaeontologists remain to be convinced. So far, it is not clear if there truly were gigantic squid in the seas of the past.

Today's giant squid, however, seemingly has all the ingredients needed to make a monster. But rather than the reality of the animal, it's our perception, muddied by stories, that keep the kraken alive.

We may never quite know what's down there

Perhaps the squid remain so mysterious, almost mythical, because they are so elusive and lurk so deep. "Humans need their monsters," says Roper. Giant squid are so big, and are such "creepy-looking animals", that it is easy to turn them into the violent beasts of our imagination.

But even if giant squid are gentle giants, the ocean itself remains deeply mysterious. Only 5% of it has been explored, and new discoveries are still being made. The megamouth shark is over 5m long, and has a face no one would ever forget, but it was only discovered in 1976.

We may never quite know what's down there, says Widder. It's perfectly possible that there is something much bigger and scarier than giant squid lurking in depths far beyond human reach.