On a warm Sunday 51 years ago, Australia's sitting Prime Minister disappeared without a trace.

On December 17, 1967, Australia’s sitting Prime Minister, Harold Holt, went for a swim in the ocean and never returned.

It was a warm Sunday morning and Australia’s 17th sitting Prime Minister was unwinding after wrapping up his first full year in office.

Holt had a holiday home in Portsea, Victoria, and after watching British round-the-world sailor Alec Rose arrive at Port Phillip Bay with a group of friends, Holt suggested they have a swim before lunch.

An avid swimmer, Holt waded out into the water at Cheviot Beach near Portsea, while witnesses on the shoreline watched on. The day was stormy and the water was choppy and unpredictable.

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According to the handful of witnesses, one second Holt was visible from the shoreline and the next he was gone.

Marjorie Matear Gillespie, the Toorak heiress Holt was supposedly having an affair with, reportedly said: “He’s gone, he went out to the rock and he’s gone, like a leaf on the tide, he’s gone”.

Holt was never seen again and his family and friends, and the wider Australian community still have no idea what actually happened to Holt once he entered the water that day.

It’s most likely he drowned in the choppy waters and his body was taken out to sea or eaten by marine life.

But in the 50 years since he vanished, there have been a number of conspiracy theories about what really happened to the 59-year-old. Many refuse to believe a sitting Prime Minister could disappear during the Cold War, without a sinister or calculated explanation.

Some believe Holt was actually a Chinese spy and that he swam out to a waiting submarine, which then took him back to Beijing. In 1983, a British author named Anthony Grey even published a book in support of this theory, giving it credence and earning it a lifelong place in Australian political folklore.

Others believe Holt could have been assassinated. At the time there was a general unease about the vulnerability of political leaders after John F. Kennedy was assassinated in 1963, and Australian Labor Leader Arthur Calwell was shot at and wounded during the 1966 election.

Another group of conspiracy theorists believe Holt took his own life, worried his wife would find out about his affair with Gillespie. However Holt’s wife, Zara, has stated she already knew about his extramarital affairs. The witnesses on the day also said he was in a happy, normal mood before he entered the water.

Others believe Holt simply left that day to start a new life out of the public eye.

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At the time of his disappearance Holt had been leading the country for almost two years. In that short time he had put in place key measures which helped make Australia the country it is today.

Once in office, he immediately began dismantling the White Australia Policy and ushered in a new approach to Australian migration. He also championed the 1967 referendum which saw Indigenous Australians finally counted in the census, and later established the Council for Aboriginal Affairs.

Controversially, he is also remembered for ramping up Australia’s involvement in the Vietnam War, heralding the phrase “All the way with LBJ”.