On December 17, 1967, Australia’s Prime Minister Harold Holt went swimming at Cheviot Beach near Melbourne and was never seen again.

The news of his disappearance was announced on television and radio across Australia.

The Prime Minister was presumed drowned and a memorial service was held in his honor, attended by world leaders including UK Prime Minister Harold Wilson and US President Lyndon Johnson.

Conspiracy theories abound as his body was never found, including one book which claimed he had been taken by a Chinese submarine.

Fifty years after his disappearance, Holt’s lengthy career as a progressive and ground-breaking Australian politician has been completely overshadowed by the mysterious nature of his death.

Holt became Australia’s Prime Minister in January 1966, only two years before he vanished.

He had been a loyal minister under the country’s longest-serving leader, Sir Robert Menzies.

Holt was a younger, 59-year-old man who was a breath of fresh air compared to Menzies, who was a stuffy Anglophile.

Despite his relatively short time in office, Holt secured a series of significant achievements, including switching Australia’s currency to dollars and cents from the complicated pound and pence system.

Another landmark of his time as a leader was the historic 1967 national referendum, which paved the way for indigenous Australians to finally be counted among the country’s population.

Frame said Holt was also one of the first politicians to work to end the infamous White Australia policy, which severely restricted immigration from non-European countries, and to reach out diplomatically to his nation’s Asian neighbors.

As Prime Minister, he built a close personal friendship with then-US President Johnson, famously declaring at the White House during a visit he was “all the way with LBJ” in the early stages of the Vietnam War.

The day Holt vanished was hot and humid Sunday, perfect for a swim. Witnesses on the beach told police the tide was unusually high with a strong undercurrent that day.

Despite that, Holt began to swim farther from the beach, into deeper water, a 1968 police report said.

One of the last things Holt had said to the group before he went into the water was, “I know this beach like the back of my hand.”

“He was not seen again.”

Amateur sleuths, police, and even the Australian army descended on the beach in an attempt to find the missing leader, or at the very least his body.

The search involved up to 300 people at times but was finally called off on January 5. No trace of Holt was ever found – all that was left was his pile of clothes sitting on the sand at Cheviot Beach.

His disappearance made Holt’s death part of Australian folklore and provoked furious debate.

There was wild speculation in the media that Holt had committed suicide, distraught over an allegedly fractious marriage or worn out from his job.

Police in their 1968 report found this was highly unlikely. Other theories had a more international flavor.

“(Some claimed) there was some sort of foreign power involved, that he’d been picked up by a Russian or Chinese mission and been whisked away against his will,” he said.

There was even a widely scorned book, titled “The Prime Minister Was A Spy” by British journalist Anthony Grey, which alleged Holt had been a Chinese spy, and had evacuated to a Chinese submarine from Cheviot Beach at the end of his mission.

Holt’s disappearance sent the governing Liberal Party into turmoil, resulting in a series of short-term, ineffective leaders, and eventually, the return of the Labor Party to government in 1972, after 23 years out of government.

The Liberal Party itself did not fully recover from Holt’s death, according to Warhurst, until 1975 when future Prime Minister Malcolm Fraser took control.