Here's something about it from Zenit.org today.The Basilica of Many Levels
Not only was it a week to remember forgotten martyrs but it was also a week to commemorate the rediscovery of forgotten origins. On June 3, San Clemente, one of most interesting churches in the city, organized a concert in its 12th-century atrium to celebrate the 150th anniversary of its excavations.
The Basilica of San Clemente was dedicated to St. Clement, the third Pope after Peter, and author of the Epistle of St. Clement to the Corinthians, one of the oldest pieces of Christian writing after the New Testament (roughly circa A.D. 90).
From the Renaissance era to the 19th century, however, everyone believed that the elegant basilica with the 13th-century mosaic decoration was the same place mentioned by St. Jerome in 392 as preserving "the memory of St. Clement to this day," although the relics were not brought until 500 years later by St. Cyril, who died in Rome in 869 and was also buried in the church.
In the 17th century, when the Dominicans were expelled from Ireland, they sought refuge in Rome. They were given the Basilica of San Clemente, and they have been in that church ever since.
In June 1857, Dominican Father Joseph Mullooly, prior of San Clemente, began an excavation under the present day church. In 10 years, he discovered a fourth-century basilica directly under the floor level and, pressing further, he found two buildings dating from the first century.
Modern explorations have uncovered some charred remains of structures under that level, identified as buildings destroyed during the great fire of Rome in A.D. 64, the same fire that the early Christian community was blamed for, and the cause of St. Peter's arrest and martyrdom. With three layers of Christian history in a single structure, San Clemente literally became a time elevator for the vicissitudes of Christianity in Rome.
From the cold embers that brought about the death of St. Peter, the next level brings us to polytheistic world of imperial Rome. One structure was a "domus," a single family dwelling with faint traces of once-lavish decoration in the moldy plaster carving on the vault. This home was donated to the cult of Mithras, a Persian god popular among the Roman military. In the ancient dining room, visitors can see the couches arranged around an altar for the ritual sacrifice to the god.
Next door, another religious group was meeting discreetly. Although many scholars claim the other first-century edifice under San Clemente was some kind of public structure, other archaeologists have put forward another intriguing theory.
It has been suggested that the large and austere building was owned by T. Flavius Clemens, a wealthy senator who was killed together with his wife Domitilla by his cousin the Emperor Domitian for being a Christian. In A.D. 96, Christians could not own land, so wealthy Roman converts bought land, keeping the titles in their names, but allowing the Christians to use the property. This is the origin of the titular Churches.
Since the first Popes were all Jewish converts, and Jews in Rome were generally freedman who took the names of their former masters, Clement may have been a former slave of T. Flavius Clemens. And the modest building 75 feet below ground level may have been Clement's home as Pope.
With the legalization of Christianity, the old makeshift Christian buildings were transformed into capacious churches ready to house the large congregations of Christians -- some estimates suggest that there were as many as 250,000 in Rome by 313.
Climbing up the narrow mildewed stair from the lowest level of San Clemente, one is forced to recall the first moment the Christians came out from underground and were able to step into the light.
The first basilica was built in the fourth century, hard on the heels of St. John Lateran, the first Christian church built in Rome. It was a big, almost ungainly building, covering the entire space of the older building below. The walls of the ancient titulus still support the church today.
The Basilica of San Clemente lasted for almost 800 years. It was given beautiful liturgical structures as well as artwork. The sixth-century marble choir can still be seen in the topmost level, and beautiful frescos, narrating the lives of St. Clement and other saints, brightened the walls. But this church was sacked by the Normans in 1084 at the height of the Investiture Controversy.
The ancient church had been destroyed, but undaunted, the Romans filled in the debris and built a new church on top of it. The elegant Basilica of San Clemente still stands today, and the splendid décor was enhanced by the magnificent concert held to celebrate the Church's constant commitment to stand by its flock through persecutions, disasters and destruction.