There are fears a mammoth 18-metre shark is still lurking within the deep.
With the Megalodon existing approximately 3.6 million years ago, there has been a long-believed conspiracy that in fact the monster never became extinct at all.
Murky footage has emerged online of hulking predators underneath the water while some have reported eyewitness accounts off the coasts of Indonesia where fisherman have long spoken about mega sharks.
An old wives' tale in Mexico is that "the black demon" attacks boats.
However futurist Dr Richard Hames has debunked the mystery, saying there's a very good reason as to why it could no longer live in waters today.
Despite the coelacanth fish being discovered again after it was considered extinct, Dr Hames doubted the same could be true for the Megalodon shark.
"The Megalodon lived in warm waters and in fact it was the cooling of the Earth at that time that caused a lot of sea mammals to go out of existence and that was food for the Megalodon," Dr Hames told Yahoo News Australia as part of its new Conspiracies Unpacked series.
"It probably couldn't survive in today's waters anyway."
Explaining recent Megalodon sightings
With murky footage emerging online of what some claim is the shark, said to weigh 16 tonnes, Dr Hames suspects it is fake.
"It's so easily manipulated," he said.
"Visual data now you can fake anything."
While sharks are cartilaginous, meaning they don't have skeletons and their bodies instead rot away leaving only their teeth and jaws as evidence of existence, there are suggestions there is limited proof the Megalodon became extinct.
However Dr Hames debunked the claim.
"The teeth are huge, the Megalodon had 18-centimetre serrated teeth in two rows," he said.
"Sharks, depending on what they eat, they get a new set of teeth roughly every two to three weeks so the Megalodon would have got through a whole load of teeth. They shed about 40,000 during a lifetime.
"The marks that would have been left on whales, for example, would have been clear, and also at the bottom of the ocean we'd be seeing a lot of Megalodon teeth and we're not.
"Some of the evidence simply is not there."
Questions over unexplored ocean
With 1.35 billion trillion litres of water on Earth, conspiracy theories argue there is still room for mystery with much of that still unexplored.
Dr Hames admitted there was a "little, tiny room for mystery" but did not buy into suggestions the Megalodon could be lurking in unexplored waters.
With the Mariana Trench in the western Pacific Ocean about 66,000 feet deep, slightly more than Everest is tall, there are other claims there is enough room for mysterious creatures to exist.
"There's enough room, but again, it's very cold down there and the warm waters the Megalodon needed don't exist in the trench," Dr Hames said.
"It was an amazing predator and it was very successful, but unfortunately, when the conditions around you change or that life becomes impossible, then you go extinct."
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