On Sunday, December 17, 1967, Australian Prime Minister Harold Holt donned his snorkel gear and walked into the water at Portsea's Cheviot Beach. He was never seen again.

The official explanation - that 59-year-old Holt drowned accidentally in a wild sea - did not satisfy British author Anthony Grey, who in 1983 published a book The Prime Minister Was a Spy .

In it, he made the bizarre claim that Holt had been an undercover agent for Communist China. According to Grey, Chinese frogmen had placed the co-operative Prime Minister in a plastic air bubble and propelled him from the waters off Cheviot Beach to a submarine waiting a kilometre offshore. Holt, said the writer, was alive and enjoying asylum in the People's Republic.

Another theory - which gained considerably more credence - was that Holt had suicided. According to The Harold Holt Mystery , a television documentary aired in 1985, the Prime Minister had been in a depressed state, but Holt's widow and his press secretary scorned talk of suicide. He had been cheerful, with absolutely no sign of depression.

The drama may not yet be over. A retired Labor politician, who in his time was a very senior minister, has told more than one journalist that he has, in a hidden file, the stunning truth about Harold Holt's disappearance. So disturbing is the revelation that file contains, says the ex-minister, that he has specified in his will that it must not be published until after his death.