This is a herm...
and this is another herm...
A herm was a bust of Hermes, who as you surely know was the Messenger of the Gods.
Athens was riddled with hermae. There was a herm at every cross-street in the city. Many houses installed a herm outside their front door. In the Agora was a platform with a hundred or more of them.
Hermes, as Messenger of the Gods, was protector of travellers. By placing his bust anywhere a traveller might pass, the superstitious Greeks were doing their best to protect anyone out on the streets.
Interestingly, Hermes was also protector of thieves, presumably because as an occupational hazard thieves often need to travel quickly at short notice.
In Athens these busts would have been set atop a short pillar, head height at most, and if you looked to the base of the pillar you would probably have seen the carving of an erect phallus pointing up at you, another symbol of good fortune.
These pictures are from the Met., Roman copies of Greek originals, and they are very good quality indeed compared to most hermae. Think of all the cross-streets and houses in Athens: there were thousands of these hermae. Top sculptors would have reserved their valuable time for more profitable work. Probably most hermae were churned out by low-end sculptors and journeymen learning their trade.
One morning in 415BC, Athens awoke to discover every herm in the city had been damaged. Someone had obviously gone about the city overnight destroying all the divine good luck symbols, and considering the many hundreds of hermae involved it could only have been a calculated act of sabotage. This incident has gone down in history as The Mutilation of the Hermae.
Athens was paralyzed with fear. This wasn't mere sacrilege; to most people the Gods were as real as a smack in the face, and a God's cult statue was a place the God could inhabit. The mutilation of the hermae was like kicking the God Hermes in the balls a hundred times over.
The Athenians expected direct and dire divine retribution at any moment. A frenzied search for the culprit began at once. In the panic it only became necessary for someone to suggest a culprit for the accused to be arrested, and more than one of these unlucky men were executed, but the panic went on. Fairly soon debtors were accusing their creditors as a novel means of debt cancellation.
Then suspicion fell upon a fascinating scapegoat: Alcibiades, the first cousin once removed of Pericles. Indeed Pericles, though dead by this date, had been legal guardian of Alcibiades as a child. Alcibiades was brilliant, daring, wealthy, handsome, clever, opportunistic, egotistical, dissolute and utterly self-serving. This was the sort of insane thing Alcibiades might do for a joke.
Now Alcibiades was guilty of any number of crimes in his life, but this probably wasn't one of them. Nevertheless the mud stuck, and even though by then he'd departed to lead an invasion of Sicily, he was recalled to stand trial.
Alcibiades wasn't a complete moron; he turned tail and ran, straight to the Spartans with whom Athens was at war. In revenge, Alcibiades advised the Spartans how best to defeat Athens, and his advice was good.
And so the bad luck of the mutilation came to pass, because the man charged with the crime contributed to the downfall of Athens.
It must be added Alcibiades changed sides again later, and Athens took him back before expelling him once more. If he were alive today, Alcibiades would be a junk bond trader, or a used car salesman, or a world leader, or possibly all three at once.
The real culprit of the Mutilation of the Hermae and the motive for it remains ones of the ancient world's greatest unsolved mysteries.