December 17 was a Sunday, and Harold Holt was spending the weekend at his holiday home in Portsea, on Victoria's Mornington Peninsula. Despite rough conditions - the surf was heavy and a king tide was running - Holt decided to go swimming at Cheviot Beach, a rocky cove he had visited many times before.

Holt was an experienced swimmer who enjoyed challenging himself in dangerous waters. He swam at Cheviot regularly despite its reputation for treacherous currents and undertows. Four companions watched from the beach as Holt waded into the surf. Within minutes, they lost sight of him. He never resurfaced.

The search for Harold Holt was the largest in Australian history. Navy divers, helicopters, police boats, and volunteers scoured the coastline and waters. The search area expanded to hundreds of square miles. Divers explored the underwater caves and crevices along the rocky shore.

Nothing was found. No body. No clothing. No trace of the Prime Minister. After several days, the official search was called off. Harold Holt was presumed drowned. He was the third Australian Prime Minister to die in office, after Joseph Lyons in 1939 and John Curtin in 1945.

The absence of a body fueled conspiracy theories. The most famous claimed that Holt had been picked up by a Chinese submarine - that he had been a spy for China and had arranged his own disappearance. The theory was based on nothing but imagination, yet it persisted for decades.

Other theories proposed suicide (Holt had health problems and political difficulties), murder, defection to the Soviet Union, or even abduction by aliens. All were baseless. The most likely explanation - that Holt, an aging man swimming in dangerous conditions, drowned and was swept out to sea - was also the least sensational.

A coronial inquest in 2005 found that Holt had drowned in accidental circumstances. The coroner noted that Cheviot Beach was known for dangerous conditions, that Holt had ignored his companions' warnings, and that the combination of heavy surf, rip currents, and Holt's health problems (he had shoulder pain that may have been a heart condition) created a lethal situation.

The inquest dismissed the conspiracy theories as nonsense. There was no evidence of foul play, no indication of suicide, and certainly no Chinese submarines. Harold Holt had simply been overwhelmed by the sea.

Australia responded to Holt's death with characteristic humor. The Harold Holt Memorial Swimming Centre in Melbourne was named after him - an irony not lost on anyone. The memorial pool is fully operational and regularly packed with swimmers.

Cheviot Beach remains a dangerous stretch of coastline. Warning signs note the strong currents and advise against swimming. Harold Holt's body was never recovered. He remains somewhere in the waters off the Mornington Peninsula, the only head of government in modern history to simply disappear.