Almost 50 years ago, archaeologists searching for the ruined house of Augustus found a tiny clue buried deep in 2,000 years' worth of rubble overlooking the Forum in Rome.
By Christian Fraser
BBC News, Rome
The single fragment of painted plaster, discovered in masonry-filled rooms, led the experts to unearth a series of exquisite frescoes commissioned by the man who would later become Rome's first emperor.
On Sunday following decades of painstaking restoration, the frescoes in vivid shades of blue, red and ochre went on public show for the first time since they were painted in about 30BC.
One large room boasts a theatrical theme, its walls painted to resemble a stage with narrow side-doors.
High on the wall a comic mask peers through a small window.
Other trompe l'oeil designs include an elegant garden vista, yellow columns and even a meticulously sketched blackbird.
Builders' names preserved
The Rome authorities have spent nearly 2m euros preserving the four Augustus rooms - thought to comprise a dining-room, bedroom, an expansive reception hall at ground-level and a small study on the first floor.
The quality of the work has been compared with that in Pompeii
Experts say the frescoes are among the most splendid surviving examples of Roman wall paintings, on a par with those found in the towns of Pompeii and Herculaneum.
Archaeologists believe they may have been painted by an Egyptian.
In the large entrance hall, graffiti on one wall is believed to have been left by the builders, who seem to have sketched out geometric designs, possibly for mosaic floors, and left their names.
In 31BC Augustus - or Octavian, as he was then known - had triumphed over the combined forces of Mark Antony and the Egyptian queen Cleopatra at the Battle of Actium.
The victory brought Egypt, and with it immense wealth, into the empire.
But if the frescoes on the walls are exquisite, their surroundings, though impressive, with vaulted ceilings, are less than palatial.
The Roman historian Suetonius described how Augustus lived in a modest house on the Palatine before he assumed supreme power and built a sprawling imperial complex higher up the hill.
The great-nephew and adopted son of Julius Caesar, he took the name Augustus on becoming sole ruler in 27BC after the civil wars that followed Julius Caesar's assassination.
His rise ended the Roman Republic and marked the beginning of the Roman Empire. He died in AD14.
Some of his interior decoration was found intact when the Italian archaeologist Professor Gianfilippo Carettoni finally broke through to the rooms in the early 1970s.
Other frescoes had to be pieced together from fragments found by a team led by Irene Jacopi, the archaeologist in charge of the Palatine Hill.
The art is so delicate that no more than five visitors at a time will be able to enter the rooms. Nevertheless, they are expected to attract large crowds.
Recent archaeological work in Rome has boosted tourism by as much as 40%, according to the city authorities.
The Palatine Hill has been giving up new finds for years, although much of the site is off limits to visitors and under threat of subsidence.
The minister of culture, Francesco Rutelli, said that 12m euros had been set aside to help protect the extensive ruins.
Mr Rutelli, who is standing for election as mayor of Rome next month, described the opening of the Augustus rooms as an "extraordinary event, the fruit of decades of work".
"Ancient history is here," he said. "Every day you have a new discovery. It's incredible - in the very heart of the city, in the middle of the traffic and ordinary life."
In November last year archaeologists located a grotto deep beneath Augustus's imperial palace that may have been the shrine where ancient Romans worshipped Romulus, the founder of the city according to legend.
Next year archaeologists hope to open to the public Augustus's mausoleum - once a monument in white travertine marble that is now an overgrown ruin.
From Monday entry to the Roman Forum will no longer be free. Instead, visitors must pay 11 euros ($16; £8) for a combined ticket that will give entry to the Forum, the Palatine Hill and the nearby Colosseum.
Officials say the proceeds will fund increased security and restoration work around Rome. "There are exciting new finds every month," said Mr Rutelli, "and we need this money to preserve these treasures for future generations".