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It was December 17, 1967, when Prime Minister Harold Holt mysteriously vanished while swimming in the sea in Victoria.
Mr Holt, a keen outdoorsman and spear fisherman, went missing from Cheviot Beach at Port Nepean, becoming the third Australian prime minister to die while in office.
His body was never recovered and he was presumed to have drowned, however the intrigue into the location of his body sparked wild speculation about what really happened to him.
Some conspiracy theories suggest he died in a botched kidnapping attempt while another popular rumour claims he was a Chinese spy and was picked up by a submarine and lived the rest of his life in Beijing.
Was Harold Holt a spy?
Futurist Dr Richard Hames however has rubbished claims he was a spy and said there were two obvious reasons as to why this theory would not be plausible.
"Submarines – that's very unlikely indeed," he told Yahoo News Australia as part of its Conspiracies Unpacked series.
"They couldn't operate in those waters at the time, I don't think they could operate in those waters today actually, it's pretty choppy."
Dr Hames said the real killer of that conspiracy theory for him however was the fact Holt and his wife intensely disliked Chinese food.
"You wouldn't choose to go to a country voluntarily if you hated the food," he said.
Dr Hames said he did not believe there was any evidence the prime minister was a Chinese spy and conspiracy theories coincided with the height of the Cold War.
What happened to Harold Holt's body?
With Mr Holt's body never found, there were theories some knew where it was.
"There was an Indian mystic Dadi Balsara who wrote a letter to the government soon after Mr Holt went missing suggesting he knew precisely where to find the body buried in the sand," Dr Hames said.
"The government did take notice although I don't think it was followed through and the body was never found."
Dr Hames said a number of conspiracies surrounded the whereabouts of the prime minister's body, with one man at the time claiming he had been asked to take Mr Holt's dead body the day before he officially disappeared and bury it.
The futurist, however, believes Mr Holt did just tragically drown in the beach.
"No submarine, no spy, not plausible," he said.
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