The giant squid has three hearts, blue blood and a donut for a brain. It has eyes the size of Frisbees and a tongue made of knives. It tangos with sperm whales and has been the inspiration for countless stories about sea monsters.

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A frozen giant squid on display at Australia's Melbourne Museum. Image: Fir0002/Flagstaffotos

But for all the fascinating facts there are to learn about this animal, one question tops all others when you type "giant squid" into the Google machine: Just how giant can the giants get?

The answer is surprisingly tricky. For starters, it's not like we run into giant squids all that often. The boneless behemoths, technically of the genus Architeuthis, inhabit the twilight waters of the ocean's middle depths and don’t usually come up to the surface, despite all the tales of krakens snatching sailors from the decks of their ships.

Monster stories dating back to the Bible or Homer's "The Odyssey" may in fact be references to the giant squid – and although we’ve probably been catching glimpses of these creatures for hundreds of years, it wasn't until 2006 that we caught one of the buggers alive and on film. It took several more years for cameras to capture the squid in its natural, deep-sea habitat.

Without live specimens to observe, scientists have had to make do with the occasional rotting carcass. Sometimes these are found washed up on beaches or dredged up by fishing nets. Other specimens have been retrieved from the bellies of sperm whales that have previously preyed upon the giants. Unfortunately, the whales are able to digest their squid sashimi rather quickly, and the only thing usually left behind is a belly full of disarticulated beaks. (This whale apparently stopped at the giant squid buffet before getting hauled in.)

But even these meagre scraps can teach us quite a bit about the monsters.

Charles Paxton, a statistical ecologist at the University of St Andrews in Scotland, recently reviewed all the giant squid length measurements he could find in an attempt to estimate the creature’s maximum length. (The Journal of Zoology published his findings last month.)

So, how big does Paxton think giant squid can grow? From the top of its mantle to the bottom of its tentacles, his estimates suggest we could be looking at about 20 solid metres (~65 feet) of tentacles, suckers, and eyeballs the size of human skulls.

This would be much longer than a school bus – the universal comparison measurement for long things. But remember, says Paxton, when envisioning super-sized cephalopods, we’re only talking about length. Even the beefier colossal squid wouldn’t tip the scales against a full-size people mover.

Of course, no one has ever actually seen a squid 20 metres long. And looking across the internet, you'll find that records for the longest measured giant squid are all over the place. The Smithsonian Institute says just 13 metres (~43 feet). National Geographic goes up to 18 metres (59 feet).

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Size squabbles: Charles Paxton believes a 20-metre giant could be lurking in the depths, but squid expert Steve O'Shea pegs his top estimate at a more conservative 15 metres.

Some records frequently bandied about online date back to the 1800s and are considered by some scientists to be, well, less than trustworthy. After all, a giant squid corpse has a consistency somewhere between al dente spaghetti and snot. And no one has studied how far those tissues will stretch if you’re really motivated to give them a tug.

Steve O'Shea, a world-renowned expert on giant squid, remains sceptical of Paxton's estimates and maintains that 15 metres (~49 feet) is the absolute, most generous, pie-in-the-sky upper end of their length. "Anything is possible," says O’Shea. But the likelihood of there being a 20-metre giant squid is "so exceedingly remote that you couldn't justify the effort in writing about it."

O'Shea's biggest problem with Paxton's estimates is that he includes data from some of those more sketchy nineteenth-century carcasses. When I relayed this qualm to Paxton, he re-ran the numbers without any of those outlier data points and still came up with a maximum length of just over 20 metres with a 99.9% prediction interval. (Then he bet O’Shea a few bottles of Kraken Rum that he'd be proven right within the next ten years.)

Basically, Paxton is saying that while most giant squid are nowhere near 20 metres long, statistics predict that a super-massive cephalopod could be out there in the same way that most adult male humans on earth are more or less me-sized (1.75 metres or 5 feet, 9 inches), but occasionally specimens on the scale of Hafþór Júlíus Björnsson and Manute Bol do emerge. Of course, we should probably reverse that analogy – because in the world of giant squid, it's the ladies who get super large.

However massive these giants get, they must do so quickly. Scientists estimate lifespan by counting the rings within giant squid statoliths, the tiny, mineralised structures found in invertebrates that provide the creatures with a sense of orientation (like our own inner ear). And by those accounts, even the oldest geezers of the deep may top out at just five years of age. Other experts say 12 years.

Are you sensing a pattern here? When it comes to many aspects of the giant squid's life, speculation is the name of the game. So let's talk about some things we know.

Even the Brienne of Tarth of giant squids will have begun life as a teeny, tiny paralarva, probably no larger than a gummy bear. And even as mini-monsters, Architeuthis are formidable predators. They use a pair of super-long tentacles equipped with hundreds of teeth-ringed suckers to lash out at prey. Those suckers are also good for defence, as evidenced by the telltale scars seen on sperm whales, one of the only creatures capable of preying upon full-grown giant squid.

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Prey is gripped by the feeding tentacles, before being passed to the parrot-like beak and the terrifying, tongue-like radula within.

Once hooked by the tentacles, prey is reeled in and subdued by the other eight arms and then gnashed to pieces by the parrot-like beak. But the horror does not end there. Behind the beak is the giant squid's radula, a tongue-like muscle built out of thousands of microscopic cheese graters.

Put it all together and death by giant squid is like something doodled in a Bond villain’s dream journal. In a matter of milliseconds, a prey fish is slammed, sucker-punched, dragged, squeezed, sheared and then ground into a fine pulp. But unlike 007, the victims have no hope for escape. In fact, according to the analysis of stomach contents found in some carcasses, even giant squid are not safe from giant squid.

Within Architeuthis, things get even weirder. Whereas we have four chambers of heart muscle all cobbled together, the giant squid has three separated chambers: one pumps deoxygenated blood into each of two gills, while a third, more central heart routes the newly oxygenated blood to the rest of the body. When picturing it, remember that squid blood is blue. This is due to the presence of a copper-based compound called haemocyanin, as opposed to our own iron-based haemoglobin.

But perhaps the most alien thing about Architeuthis is its brain, which is shaped like an oversized Honey Nut Cheerio on a string. That string, by the way, is the oesophagus, because the giant squid's ancestors were little more than glorified feeding tubes with shells. Here's a clip from "Inside Nature's Giants" that explains this in more detail. (Bonus: The guy with his fingers in the squid brain is none other than Steve O’Shea.)

You see, for all we still don’t know about the giant squid, there’s quite a bit that we do. We even know what giant squid taste like. And according to Dr Kat Bolstad of the Auckland University of Technology, it ain’t good.

“Their tissues are full of ammonium, which saves them energy by aiding in buoyancy,” says Bolstad. So while the texture may be similar to the fried calamari you’re used to, the flavour would be akin to that of toilet cleaner.

Happily for the giant squid, being unappealing to our palate means they likely won’t go the way of tuna, sturgeon, swordfish and any number of other species we’ve fished to the brink. If world’s fishing fleets turned their attention and technology toward the species, it’d be about a week before we had an answer to the question of giant squid maximum length.

Perhaps some mysteries are best left in the deep.