Early on the 25th of April, 1977, a Japanese fishing vessel named the Zuiyo-maru was trawling for mackerel, around 30 miles off the New Zealand coast. Little did they know that entangled in their nets, over 300m below the surface, was something far larger and more mysterious than its normal catch—something that would set the scientific world alight with speculation.

When the nets finally broke the surface, the crew were met by an enormous rotting carcass unlike anything they had ever seen before. As it was hoisted clear of the water, two tons of pallid grey flesh hung before them, with a vast gaping jaw and deep red muscles running down the spine. Even in its decomposing state, whatever the creature was, it measured 10m from nose to tail. Some claimed it was a decaying whale, others that it might be a giant sea turtle with its shell peeled off; some even whispered that it might be an as yet unidentified sea monster.

The captain and crew agreed that the stinking carcass should be thrown overboard, otherwise they risked spoiling their valuable fish catch. Fortunately, a quick thinking shipmate managed to take a handful of photographs and measurements, as well as collecting a few pieces of ‘horny fibre’ from the fin. Over the coming years, these tiny clues would eventually help explain the origins this mysterious creature from the deep.

Back in Japan, an eminent professor declared it was the remains of a supposedly extinct plesiosaur. The plesiosaurs were a group of marine reptiles from the Triassic and Jurassic periods that lived throughout the world’s oceans, but they were thought to have become extinct 66 million years ago. The unusual photographs, and accounts from the crew that the creature had a neck, seemed to support this claim. Such was the excitement among the public that officials from the fishing company sent the fleet back out to try and find the dumped carcass.

Fortunately, analysis of the tissue samples finally revealed the truth. Biochemical tests confirmed that the horny fibres were made from the same material as shark fins. The scientific community eventually agreed that the carcass was almost certainly a badly decayed—and rather large—basking shark (Cetorhinus maximus).

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Basking sharks are the second largest fish in the world’s oceans, though off the coast of New Zealand they have now become extremely rare. Curiously, they also decay in a very unusual fashion, with the jaws and gill arches falling away, making the remains look like they have a long neck and small head. Once flesh on the body and internal organs have fallen away too, it leaves just the pectoral and pelvic fins which in turn look larger and more distinct. The end result is a carcass that looks remarkably like a plesiosaur—and with a bit of imagination this helps to keep the idea of long lost sea monsters alive.

Indeed whilst the Zuiyo-maru carcass is among the best known, there have been numerous other supposed ‘sea serpents’, such as the ‘Stronsa Beast’ of the Orkney Islands, the Scituate ‘monster’ of Massachusetts and of course the Loch Ness monster. Many of which have turned out to be basking sharks, too, or in the case of Loch Ness, probably just a log!

So whilst we might have uncovered the truth of the Zuiyo-maru carcass, out there in the unexplored depths of the oceans there is much more still to discover.