Prime Minister Harold Holt entered the surf at Cheviot Beach, Victoria on the afternoon of 17 December 1967 and was never seen again.
No body or other evidence was found, despite an exhaustive search. There were no clues to explain his vanishing. The allegations, rumours, insinuations and conspiracy theories quickly emerged and continue to this day.
Is Harold Holt still alive? Did he commit suicide? Was he drunk or drugged? Did he secretly belong to a crime syndicate? Was he a Chinese Communist spy?
The clip below combines the first news reports of his disappearance with footage of the search that quickly got underway.