There is one extraordinary exception. The Church of Santa Maria Antiqua in the forum in Rome was buried by a landslide after an earthquake in 847. Some of the precious relics it contained were retrieved, and moved to a new church, Santa Maria Nuova, near the Arch of Titus. Santa Maria Antiqua was then completely forgotten. Almost 800 years later, in 1617, a church in the baroque style, with a convent attached, was built where Santa Maria Antiqua had once stood. In 1702, a group of builders started digging in the garden of the convent in an attempt to salvage ancient blocks of marble. They came across what turned out to be the ruins of the apse wall of Santa Maria Antiqua-but although the discovery generated some enthusiasm from locals, Pope Clement XI, who had authority over the land, wasn't in the least interested. He ordered the site to be filled in. That put an end to any attempt to excavate the medieval building. Two hundred years later, archaeologists excavating under the forum decided it was time to see if they could identify Santa Maria Antiqua: its existence was known not just from the episode in the 18th century, but from a guidebook to Rome for pilgrims written in the eighth century. The archaeologists started digging underneath the Baroque church, and were astonished by what they found: there were classical pillars and arches, but most amazing of all, there were walls covered with more than 200 square metres of early Byzantine frescos. They appreciated the extreme rarity of those pictures, and in a reversal of the practice of previous centuries, the Baroque church was demolished so that the medieval one could be brought to light.    

And what a church it is. It was constructed using what had probably originally been a very grand vestibule that the emperor Domitian built between 81 and 96 AD for the entrance to his palace on the Palatine hill. Its conversion to a church took place during the sixth century. Rome's population was by then a small fraction of what it had been at the height of the pagan empire — it had been sacked several times by various different armies, including Ostrogoths and Vandals — and the forum was turning from the vibrant centre of the world's greatest city into a malarial marsh. Over the course of the next 300 years, the walls of the church would be frescoed several times: one part of the apse wall has seven different layers of fresco, each one corresponding to a different decorative scheme.

The first two layers belong to the pre-Christian period. There is then a layer of fresco depicting the Virgin in the guise of a Byzantine empress, with a crown and jewels of a kind similar to those found in the portrait of the empress Theodora in the early sixth-century mosaics in the church of San   Vitale in Ravenna. Perhaps the most remarkable frescos, here and elsewhere in Santa Maria Antiqua, date from the first half of the seventh century. The remnants of images datable to this period — they include a figure identified with the Virgin, and "Solomone", the mother of Maccabees — do not have the characteristics thought to be typical of Byzantine art. Far from being stiff and two-dimensional, they are vivacious and naturalistic, and seem to have been painted rapidly, with prominent brushstrokes and subtle gradations of shading that create the impression of movement. They were commissioned by Pope Martin I, whose pontificate lasted from 649 to 653.

Martin I succeeded in angering the Byzantine emperor Constans by condemning as heretical doctrines to which Constans was committed. Constans had Martin deposed, arrested, imprisoned and then exiled to the Crimea. The figures from this period are a beautiful reminder that the traditions of painting that came down from the ancient world, and whose distant origins can be seen in the wall paintings of villas in Pompeii, did not disappear quickly, but lasted for centuries.