For an animal of such enormous size, the colossal squid has an extraordinary ability to keep itself hidden from human eyes. Its discovery was a gradual process, with hints of its existence stretched out over decades. Then – almost exactly 100 years ago – we got our first glimpse of these almost mythical creatures.

Because the animal lives so deep in an ocean only recently visited by modern humanity, the first clues to its existence were the occasional remains found in the bellies of whales that hunt them. Semi-digested fragments hinted at some huge, strange squid whose arms ended in clubs with sharp, gripping hooks and evoked scenes of titanic battles for survival in the ocean depths as they tussled with whales.

"It was early morning the 3rd of February, 1981, when I was working in Lazarev Sea near Dronning Maud Land, Antarctica," he wrote. "A fellow scientist rushed into my cabin and pushed me in the ribs, shouting: 'Wake up, we caught a giant squid!' With my cameras slung around my neck I ran on deck. There lay a huge reddish-brown squid. None of the crew members, several of them sea dogs who had been wandering all over the seven seas, had previously seen something like this."

"Burning with impatience to see the results of my photography, I decided to develop the films immediately on board of the vessel, rather than keeping them for developing in a professional laboratory at home," writes Remeslo, now a scientist at the Atlantic Research Institute of Marine Fisheries and Oceanography in Kaliningrad, Russia, in his account. "The quality of the photos taken that day leaves much to be desired. But the most important thing has been done anyway – to document what was most probably the world's first big specimen of the colossal squid ( Mesonychoteuthis hamiltoni ), which was raised from the depths onto the deck of a vessel and not removed from a sperm whale's stomach!"

A black-and-white image taken by Remeslo and shared alongside his story shows a pair of the Soviet ship's crew crouching next to the dead squid. The creature's two long arms can be seen in the foreground, clenched like fists. According to Remeslo, the squid measured 5.1m (16.7ft), with the mantle alone measuring more than 2m (6.6ft). The squid was described as being a juvenile female, and not yet fully grown.

"We're sitting there at Te Papa and I've got this bloody enormous thing sitting on a slab," says O'Shea, who now lives in Paris. "It's completely defrosted. I called up a couple of contacts, and I said, 'Look, I've got this colossal squid sitting on a slab here at the department. You want to come and have a look at it?’."

O'Shea was so excited that he hadn't noticed the date: 1 April 2003. Everyone mistook it for an elaborate practical joke. "Nobody took me seriously," he says. "And it wasn't until we sent them a photograph of what we were dealing with on the slab did the press converge on us… my phone didn't stop ringing for a month."

Even for someone like O'Shea, familiar with large cephalopods, the colossal squid was still a dramatic sight. "I'd never seen anything like it before," he says. "I had worked a lot with a fellow called Malcolm Clarke on a number of my documentaries in the past, and he had spent a lifetime studying the stomach contents of sperm whales – and had reported many times their beaks in the stomachs of sperm whales. I was aware of the colossal squid's existence. I couldn't have imagined it looked anything like what we had in front of us."

"The giant squid, to an extent, I was bored with, because it was just a large, very dull squid," he says. "It's got no real charismatic feature other than its size. And here I am dealing with something that's got these swivelling hooks on the arms and a beak… considerably larger and considerably more robust."

On its arms, the squid has prominent hooks. Other squid, including the giant, have teeth within the suction cups. The colossal squid's are far more prominent – curved hooks the squid uses to latch onto its prey. Incredibly, the hooks found on its tentacle suction cups can rotate 360 degrees . Scientists still don't know if the squid can swivel these hooks at will, or whether they move of their own volition when the hook latches onto prey.

San Aspiring .

The squid's decision to try and grab a quick meal proved its undoing. "It decided to scavenge a toothfish off the long line and got itself wrapped up in the backbone and trace [part of the fishing line] and was pulled to the surface," says Andrew Stewart, Te Papa's curator of fishes and one of the world's most respected fish scientists. The animal was estimated to weigh up to 450kg (990lb) and measured some 10m (30ft) in length . Some of the boat's fishing equipment had gouged deep cuts into its body, and the squid was badly injured and likely to die if returned to the ocean.

Vessels like the San Aspiring carry New Zealand fisheries scientists on their expeditions, partly in case they come across new or rare species. "They looked at this thing right at the surface, right up against the edge of the boat, and they realised that because the damage it had incurred from the backbone on the trace that, no, this wasn't going to be able to make it away under its own steam," says Stewart. "It was brought on board with great difficulty, because you're dealing with this very floppy specimen. How do you get it up out of the side of a ship and onto the deck, and then, what do you do with it then?"

"They managed to get it below deck, and they froze it in what's called a pelican bin," says Stewart, who took the initial call from the fisheries observer programme to say a colossal squid had been caught. "These are one-cubic-metre bins (35 cubic ft) that contain fuel oil and things like that. And when they get to the Southern Ocean boundary, these are brought below decks. They're emptied and cleaned, the top is cut off. They're used for putting in offal and scientific specimens. So they just put this half-tonne thing in this cubic-metre bin, and froze it as a giant, colossal squid popsicle."

Even thawing a frozen specimen this large was an issue, let along trying to preserve it. "The way these things are constructed, and the chemistry of them, it could well rot on the outside with the inside still frozen solid," Stewart explains. "So a giant wooden tank was built and lined with three layers of rubber cement, and then three layers of heavy-duty polythene plastic.

"If you freeze something, ice expands and breaks down the connective tissue and will certainly make something more gelatinous," O'Shea adds. "When we defrosted that thing, of course, the ice crystals expand, and everything blows up. Then when the ice melts, everything shrinks. As it lay on the slab, defrosting, we could see it losing bulk."

In order to preserve the body, its tissue had to be injected with a formalin solution, but getting the right mix was crucial, says O'Shea. "That was a 4% formalin solution from memory. Once I fixed it from the inside out, we then immersed it into a vat of formaldehyde/seawater solution. And then we had to monitor that thing over the course of the next 48 to 72, hours, monitoring the pH, because the minute the pH goes anything above seven, the calcareous hooks that align in the arms and the suckers start to dissolve."

The case was assembled right next to where the thawed squid was being kept in central Wellington, some 900m (984yds) from the museum itself. The museum's experts then had to work out how it could be both preserved and transported. "What do we preserve it in, display it in, and how do we get it from here down the road?" Stewart says. "We can't display it in alcohol or formalin because of the issues around health and safety and fire risk management and all that kind of stuff." A suggestion by another member of the team was to submerge the squid in polypropylene glycol. While Stewart says this is non-toxic, "they have to add a rather toxic biocide to it in order to stop any bacterial action, fungal action".

While the team were working out how to move their colossal cadaver, something elemental came to their rescue: gravity. Wellington is a hilly city, and the squid was at the top of an incline. They came up with a plan: the squid would be transported to the museum late at night on the back of a flatbed truck in the container, but with all the liquid removed in order to save weight. "It just sort of glided down late at night when there's no traffic, and the traffic lights could be set [to let it through.]" The squid was then safely unloaded and took up residence in Te Papa, an ambassador from an abyssal zone few humans will ever visit.

Specimens like the one at Te Papa offer vital clues about the biology and behaviour of this mysterious deep sea mollusc. The colossal squids that have been brought up to the surface to date have nearly always come from deep water. They have either been ensnared in cables or attacked fish caught on fishing lines, attracted by the prospect of an easy meal. Their interactions with humanity have been inadvertent, often violent and dramatically short.

Few animals are thought to prey upon colossal squid apart from sperm whales and southern sleeper sharks, slow moving but powerful deepwater sharks that can grow up to 4.2m (14ft) long. The colossal squid's size is a form of protective adaptation – the bigger you grow, the less likely you are to be eaten by something else.

Cleal said she was chosen to write the book because of her experience writing labels for exhibits aimed at children, making them as friendly and informal as possible. "They knew there's basically an insatiable desire amongst children for information about the colossal squid, whether it's in a book or an exhibition label or in a video," she says. "It constantly fascinates visitors. Everyone who comes to the museum wants to see the colossal squid.

Despite the painstaking effort that went into displaying the squid, the years have still taken their toll. "That squid is not looking at its best anymore," Cleal says. "The eyes have been removed and other body parts, there's a lot of stitches. I did want to mention that squid in the book, just to give some connection between the book and Te Papa. But that squid came to a very unhappy ending because it got itself hooked on a line and died."

With the help of squid experts like Kat Bolstad, Cleal set to work. Bringing a male squid into the story wasn't possible, because one has never been observed. "But there are things we can imagine, like what it would be like to be 2,000m (6,561ft) down, even though nobody's been down there, in the Ross Sea." She says it was all about keeping Whiti (a Māori word meaning to change or turnover) within the realms of possibility.

Cleal says the squid's enormous size and frightening appearance is part of its allure with a younger audience, but that ultimately this undersea "monster" is relatively harmless. Many descriptions of the colossal squid evoke legends of the mythical kraken terrorising sailors of old, but in reality the creatures live so deep and so far from shore that a human is unlikely to ever find themselves face-to-face with one in the water. But the fact we know so little about the colossal squid and the realm it inhabits only makes them more intriguing, says Cleal. "It's such a mysterious world, I think that captures everybody's imagination. We don't know what's going on down there."

Cleal said she was partly inspired to tell the story of the colossal squid to make children imagine what else might live in the cold, inky depths the squid calls home. "I just think that's great for kids, to be an advocate for science as a career, to think that that there are things to discover. It hasn't all been found yet? And why don't you try to become a marine biologist too?"

James Erik Hamilton was a marine biologist, a naturalist and oceanographer who spent much of his life in the Falklands and surrounding islands. He arrived in 1919 to conduct a survey of the fur seal population. He became the Falkland Islands Dependencies administrator a few years later, and spent much of the 1920s working on whaling ships or on the stations that supported them across the South Atlantic islands.

Hamilton believed they were new to science. He had them preserved and then sent them to the Zoological Department of the British Museum back in London.

While talking to O'Shea, I mention the tentacles Hamilton discovered a century ago. His response is immediate: "Have you seen them yet?" It turns out those first, species-defining tentacles still sit, suspended in a jar, on the shelves of the Molluscs Department at the Natural History Museum in London . An email to O'Shea's friend, Jon Ablett, a senior curator at the molluscs department at the museum, elicits an invitation to see them a few days later.

In these archival storage units sit dozens upon dozens of jars, each of them containing an animal – or parts of an animal – once new to science. Ablett finds the right door and opens it. There, in a jar with the words "Mesonychoteuthis Hamilton, 1925 " lie the remains of the squid that had so intrigued Hamilton a century before. The first scientific evidence of the giant lurking far below the waves.

"The way we preserve animals hasn't really changed over the last 200 years," Ablett says, noting that alcohol is still sometimes used. "With lots of invertebrate animals, especially deep-sea creatures, the preservation techniques can really distort the features and generally shrink them." The tentacles, now a century old, look lumpen and weirdly coloured, but the swivelling claws that so intrigued Hamilton are still there.

"They are, you know, half-chewed stomach remains… basically the ring of flesh around the mouth, most of bits of the arms, and that's pretty much it," Ablett says. "But he [Hamilton] was able to recognise that they were so distinct from any other known squid that they had to be a new species. And I guess sperm whales are really good at catching things in the deep ocean, much better than we were at the time, and probably even now."

The giant squid, Ablett says, throws up some intriguing questions about why some squid grow so large, when others remain relatively small. "The thing that always fascinates me is lots of the species related to the giant squid – these are the Cranchiid squids, or glass squids – are very, very small, you know, a couple of inches long. But just this one species is so large."

Ablett says there are hints in the biology as to how the squid may live their days in the deep, cold waters of the Southern Ocean. "You look at the colossal squid, it's very blobby. It doesn't look particularly sleek." This for him perhaps underlines its status as an ambush predator. "Is it hiding in these dark oceans, waiting for things to pass?" he asks.

Ablett says one thing scientists have found is that where you find colossal squid, you won’t find giant squid. The respective big beasts of the cephalopod world appear to have drawn some invisible line in the world's oceans, which neither crosses. And very cold oceans are something of a hotspot for very large organisms, he says. "It just seems to be a kind of trend with especially polar organisms, that they get very, very big. I mean, one benefit of getting massive is, of course, nothing can eat you."

The century-old tentacles preserved in the museum's jar are not the only colossal squid remains the museum has to hand. In the basement, hidden from public view, is a room filled with jars and tanks containing a bewildering array of creatures (you'll have seen it if you've watched the Tom Cruise version of The Mummy). An entire Komodo dragon, a deceased former resident of the London Zoo, floats in a large tank. The heads of deep-sea sharks leer toothily from the interior of huge jars.

In another tank – a really, really big tank – the partial remains of another colossal squid are suspended in preserving solution. Much of the rest of the tank is taken up by an entire giant squid, its long, tentacles trailing far beyond the mottled mantle. You could imagine a very long line of visitors filing past it, but this room is out of bounds to the general public. The tank had to be made by specialists usually involved in art installations. If the museum ever finds itself lucky enough to take delivery of a complete colossal squid, they might have to build another. In death, at least, the two squid might actually meet.