Etched into the stone walls of Florence's historic town hall, it has puzzled passersby for centuries and inspired legends as to its origins.
Now an art historian says he has found evidence which suggests that the profile of a man's face, carved into the exterior of the Palazzo Vecchio, was the work of none other than Michelangelo.
Adriano Marinazzo, a museum curator, noticed a striking resemblance between the etched face and the portrait of a man drawn by Michelangelo that is held in the Louvre in Paris.
The two images have similar long Roman noses and weak chins and the same mops of curly hair.
Local lore has long attributed the etching on the walls of the Palazzo Vecchio to Michelangelo, but many scholars discounted the idea as romantic speculation.
The connection found by Marinazzo, from the Muscarelle Museum of Art at the College of William and Mary in Williamsburg, Virginia, now gives the theory more substance.
The piece of graffiti is believed to date to the period in which Michelangelo lived in Florence, from 1499 to 1504, when he carved his magnificent statue of David, the boy warrior who killed Goliath.
Along with the early 16th century sketch in the Louvre, Marinazzo found a cryptic clue – a piece of paper on which Michelangelo appeared to foreshadow the enigma of the carving.
"Chi dire mai chella f(osse) di mia mano," he wrote. "Who will ever say that it was by my hand?"
When Marinazzo saw the sketch in the Louvre during research for a book, he had an "epiphany", thinking to himself: "Wow, this looks like the profile."
"I showed it to my wife without saying anything," he told Artnet News. "'What do you think about this, does it look like something familiar?' And she said: 'It looks like the profile on the Palazzo Vecchio'."
The carved image is located at one corner of Florence's imposing town hall, just a few steps away from the Uffizi Galleries.
Over the centuries, there have been plenty of stories about how Michelangelo may have come to produce the graffiti.
According to one, he was watching a man led through the streets of the city to the gallows and was inspired to carve his likeness into the stone.
Another story recounts that he etched it surreptitiously, using a knife held behind his back, for a bet about whether he could get away with defacing the palazzo without being caught.
Marinazzo believes, however, that the graffiti may depict a friend of Michelangelo's – Francesco Granacci.
He was a member of a committee, which included Leonardo da Vinci and Sandro Botticelli, that gave approval to the statue of David being placed outside the entrance to Palazzo Vecchio.
"It could have been a tribute by Michelangelo to his friend, who played a fundamental role in the artist's formative years," Marinazzo wrote in a paper in the magazine Art e Dossier.
"It's interesting to note that the carving of the profile looks towards the spot where David stood."
There it remained, exposed to the elements, until it was transferred to the nearby Galleria d'Accademia in 1873. A copy stands in its place.
Other scholars said the attribution of the stone etching was credible. "I don't know how we missed it before," said Gary Radke, an art historian at Syracuse University in New York.
Michelangelo spent three years sculpting the statue of David, the biblical hero who killed Goliath with a single stone from his slingshot.
It was commissioned by the city's rulers as a symbol of the Florentine Republic's commercial and military strength and its capacity, despite its small size, to ward off bigger neighbouring states.