Its head resembled a dog’s, its downturned nose a camel’s, and at the end of its reptilian body was the tail of horse. Witnesses say it was covered in a thin white film. When the remains of a strange creature were pulled from the stomach of a sperm whale, most of those present agreed: it was a sea monster – or at least something unknown living in the depths off Canada’s west coast.

Crews at the whaling station in the archipelago of Haida Gwaii assembled a platform of wooden boxes and laid out the 3-metre (10ft) carcass, using a white sheet to display the curiosity that had baffled veteran whalers.

A photo of the creature, called the “Cadborosaurus”, by locals, appeared on the front page of a local newspaper on 31 October 1937, adding to the growing lore that a marine cryptid – a creature unknown to science – and at times supposedly measuring three times as long, stalked the waters.

Samples of the mysterious discovery have long disappeared and all that remains are a handful of black and white images.

John Kirk, president of the British Columbia Scientific Cryptozoology Club, is adamant the carcass was of an unknown species hiding at the edge of human understanding in the emerald depths of the Salish Sea. He cites first-hand accounts of the discovery, including an interview with a flenser who helped remove the carcass.

“The scientific world, of which we are a part, is always looking for excuses not to allow new animals into the catalogue. And quite frankly, I find that notion absolutely cockamamie,” he says.

One of the few pieces of the carcass sent for identification was shipped to a museum in Victoria, 400 nautical miles (740km) south-east of Haida Gwaii. It was disposed of after the museum’s director – not a trained zoologist – suggested it was from a foetal baleen whale.

“We lost a massive discovery here because of misidentification,” Kirk says. “And I think it’s a horror story of how flippant scientists can sometimes be in regard to these kinds of things.”

But almost 90 years later, many scientists say the images simply show a decomposing basking shark – a giant and ancient fish that once thrived off Vancouver Island before being slaughtered to virtual local extinction. Today, the basking shark shares the cryptid’s fate: often misidentified and rarely, if ever, seen by veterans of the ocean.

Because sharks have no bones, the change from living creature to carcass is a profound transformation. When basking sharks decay, their massive gill-basket – the structure that most defines the lumbering fish – collapses. What remains is a long neck-like structure and a small head. The breaking down of muscle fibres and cartilage can give fins a furry, feather-like appearance.

“With a long spinal cord and a small head at the end, it looks like a mythological sea serpent,” says Ben Speers-Roesch, a professor of marine biology at the University of New Brunswick. “Unless you know what you are looking at or have familiarity with it, it’s not intuitive what this creature might have been.”

In 1977, the Japanese fishing trawler Zuiyō Maru pulled a creature from the depths off the coast of New Zealand that resembled a long-lost dinosaur. The discovery prompted a flurry of excitement from scientists who claimed it was an unknown species, until an analysis of amino acids in the muscle tissue suggested it was a basking shark.

Speers-Roesch says this is called the “pseudo-plesiosaur carcass” phenomenon, when decomposed basking sharks appear to have a long neck, small head, and large paddles – all hallmarks of a prehistoric plesiosaur.

He concedes that the 1937 photo from Canada diverges from typical carcasses because of how it was displayed.

“The mystery has persisted because it has elements that are not as easily identifiable as a basking shark. It does look a little bit different in a couple of ways,” he says, adding that on rare occasions, young basking sharks have been found in the stomach of sperm whales. “But so much of the carcass captures what we know about basking sharks and how they decompose. Even if you’re well educated, you can really make poor interpretations of what you are observing.”

With no surviving samples and only a handful of photographs, the question of what the mysterious creature was can never be resolved definitively. But the real story, say scientists, is not so much the mystery of a species that might exist, but the tragic end of one that did. Now, amid a push to protect the waters of the Pacific and the vulnerable species that remain, a rare sighting of a basking shark in 2024 has renewed interest in the species and in the long-forgotten campaign that wiped them from the coastal waters.

In 1955, the federal government devised a plan to kill off the sharks. Its strategy relied on a large blade fixed to the bow of a patrol ship, called a “razor-billed shark slasher” by local media.

“Because basking sharks fed at the surface, crews could lower the blade and run them over,” says Scott Wallace, a former fisheries scientist who wrote the 2007 federal government report that determined the basking shark was endangered and largely extinct in British Columbia waters. “They simply cut them in half.”

The sharks’ only crime was to have inadvertently wandered into nets laid by anglers to catch salmon.

“Anything that interacted with salmon or the salmon fishery was killed off as a way of thinking that they could again manage the ocean,” says Wallace, who co-authored the book Basking Sharks: The Slaughter of BC’s Gentle Giants. “And it wasn’t long before basking sharks were added to the official ‘nuisance list’ and they became a target.”

At the same time, officials were shooting seals and sea lions around fishing grounds and river mouths. In the early 1960s, Canada’s fisheries ministry installed a .50-calibre machine gun on an island, a weapon typically used against armoured vehicles and low-flying aircraft. Its sole purpose was to kill orcas. The high-powered gun was never used, but the message was clear: the ocean’s giants were something to be destroyed, not respected.

The government estimates it killed at least 413 basking sharks with the boats over the next 14 years. At the same time, as many as 1,500 sharks may have been killed by entanglement. There was also a brief attempt to create a commercial fishery for their liver oil, but experts say it is unclear how many sharks were slaughtered from that effort. All told, as many as 2,600 – more than 90% of the population – was eradicated.

Now, federal law makes it illegal to kill, harm or capture a basking shark if it is found off the coast of British Columbia. The government has an official recovery strategy and action plan, and the sharks are protected by the strongest wildlife protections in Canadian law. But Canada’s fisheries department also concedes it could take 200 years for the sharks to recover to a healthy population.

“There are periods of time – often decades – that they [basking sharks] just disappear,” says Wallace. “And then all of a sudden, they return. Maybe it’s the ocean conditions – we really don’t know. But there is a chance, and it’s slim, that they’re still in the area, out of human view.”

While the tragic story of the demise of the basking shark is indisputable, the “pseudo-plesiosaur carcass” phenomenon is not enough to sway some that the mysterious creature in the 1930s photograph is something that is identifiable.

“The carcass is most certainly not a basking shark,” says Kirk. “And it’s not a reptile. Whatever it is, it must be a mammal because it possesses hair and it doesn’t resemble any of the orders of marine mammals today in those waters.”

For Kirk and cryptozoologists, the prospect of something eluding scientific acceptance is a powerful draw and underscores their relentless hunt.

In 2010, while walking his dog, Kirk says he encountered an unknown creature in the waters where British Columbia’s Fraser River empties into the Salish Sea. Its head was “pointed” and the neck extended from the water “like a stovepipe” with a hump curled behind. The encounter was a “breathtaking experience” and left him stunned.

“But I will tell you: in my nearly four decades living in the region, one thing I’ve never seen is a basking shark,” he says. “Not sure if I ever will.”

Speers-Roesch understands the urge to look at a century-old photograph and believe something unexplored still lurks in the ocean. But the nexus of human senses and the natural world is a space rife with error. Under the right conditions, a carcass can look nothing like the living animal. When a dead racoon washed up on a shore on Long Island, in New York state, word spread quickly that the bloated, hairless carcass was an unknown creature, called the “Montauk monster”.

“Humans will always be curious about the unknown,” says Speers-Roesch. “There are still profound mysteries and species in the ocean that we know very, very little about, and probably even species of relatively large animals that are yet to be discovered or described. But when you actually look at the creatures that are known to exist in the ocean, it’s even more spectacular and powerful.”