Eighty per cent of the ancient city of Rome is still existing under the surface. Little by little, those who survived the fall of the Empire carried on recycling what the past had left to them.

3 hours - walking tour / by public transportation

Get ready to walk backward in time stepping down the stairs of the mediaeval Basilica of St. Clement. From 1700 to the late I century AD you'll walk around the fascinating ruins of an old church laid out in the IV century AD upon the I century AD remains of a Roman wealthy house (later transformed in a shrine) and what is supposed to be the former imperial mint. A gleaming polychrome mosaic decorates the apse of the mediaeval church, while delicate and rare IX and XI century AD fresco paintings depict some of the events of St Clement's life in the lower church as well as the shrine dedicated to Mithra, in the lowest level, shows how widely and deeply the worship of this God from Persia spread around the city.

A well preserved portion of a Roman road network and what seems to be a primitive meeting place for Christians make the undergrounds of St. John and Paul's worth visiting. Some frescoes painted inside one of the rooms of a III century AD elegant residence show symbols which look Christian: the debate is still open but the theory is very interesting.

Next to Trevi Fountain, under a block of old apartment buildings, archaeologists found the ruins of a mid I century AD Roman insula (condominium) later transformed in a wealthy house and the remains of a large water cistern to storage the water supplied by the Aqua Virgo, the aqueduct commissioned in 19 b.C. by Agrippa.