After passing through the portal, a few steps lead down into an irregular space covered by a groin vault. On the vault it is still possible to see the traces left by the reed mats on which the mortar was poured to create the vaulting. Built in the 12th century, this first forebuilding originally formed a true vestibule opening onto a portico.
On the left, one can admire a splendid monumental trough-shaped sarcophagus, strigilated—that is, decorated with undulating fluted grooves—made of white marble and dating to the late 3rd century. Near the curved ends are scenes of two lions attacking two wild boars. The rear face is only roughly worked. This is a pagan sarcophagus, whose original placement is unknown; it may have been transferred to the abbey to house the burial of a distinguished abbot or another important figure.
