When archaeologists started digging under St Peter's basilica in 1939, exploring the ancient necropolis there, they found numerous pieces of red plaster marked with graffiti in Latin and Greek.
One piece, only a few centimetres long, bore the Greek inscription Petros Eni (Peter is inside) and confirmed that the basilica did indeed mark the ancient tomb of St Peter, a site visited by pilgrims for almost two millennia.
That piece of plaster takes a privileged place in the exhibition Petros Eni running at the Vatican which looks at the building of the largest basilica in Christendom and shows off some of the art associated with it.
Among the artists whose work is on show are Rembrandt, El Greco and Caravaggio, who all produced stunning portrayals of St Peter, and Raffaello and Titian, who painted the popes involved in building the basilica.
Renaissance giant Michelangelo is also a key figure in the exhibition.
He was appointed chief architect of the basilica project in 1546, when he was 70. He spent the last 19 years of his life on it and designed the dome which is one of its defining characteristics.
The original wooden scale model he built of the dome is one of the exhibits, along with a wooden crucifix and a number of letters and sketches which provide a view of the minutiae of his work.
One of the letters is a stiff complaint to the panel in charge of construction, demanding that they pay his representative and work supervisor immediately. Otherwise, Michelangelo threatens to take the matter up with the pope himself.
According to Cardinal Francesco Marchisano, arch priest of St Peter's Basilica, the goal of the exhibition is "to help visitors see for themselves the labour that the construction of the new basilica cost and to appreciate its artistic, historical and religious value".
The first stone of the basilica was laid by Pope Julius II on 18 April 1506. From start to finish the complete construction took 120 years and about 10 architects.
Parts of this tortuous story are told through the plans and diagrams that a succession of architects produced for popes who were determined to build something magnificent to mark the centre of the Christian faith.
But more interesting for the non-specialist are documents such as a shopping list made out in 1549 for a feast celebrating completion of the main vault in the basilica.
Written by the panel of elders in charge of administering building work, it calls for 150 kg of veal, 30kg of 'fine sausage', 30 Pecorino cheeses, bread "from the German baker" and 12 barrels of Corsican wine.
Apparently this was for a gathering of about 90 people.
As well as exploring the building of the basilica and the art it inspired, the exhibition has an archeological slant which focuses on the site where St Peter's was built.
There are artefacts, such as the Petros Eni fragment from the ancient burial ground, along with other pieces of tombs and graves from Roman families in the first century.
There are also drawings and artefacts of the first St Peter's basilica, which was built by Roman Emperor Constantine in the third century AD.
The final section of the show aims to inspire visitors with the examples of saints who have made pilgrimages to St Peter's basilica (old or new) central events of their lives.
On display are Mother Teresa's sandals and a tunic allegedly worn by Saint Francis of Assisi when he was blessed with the stigmata. There is also a diary of St Thére'se of Lisieux in which she describes being forcibly removed from the presence of the pope by guards after he failed to grant her request to enter a convent at the age of 15.
Petros Eni is showing in the Braccio di Carlo Magno, left colonnade of St Peter's Square, until March 8.
