Immediately outside the Porta Maggiore, one of the city-gates of Imperial Rome as well as a point of convergence of the more important group of aqueducts of the Roman Empire, lies, hidden below seven metres from the level of the Via Prenestina, this intriguing monumental complex, accidentally brought to light in April 1917, after a subsidence of the ground along the railway line Rome-Cassino.
The complex consists of an entrance corridor leading into a vestibule, which gives access to the main hall of a basilican-type building. The architecture follows an east-west orientation and was originally provided with an external entrance: a long barrel-vaulted gallery which, from the east, aboveground, descended with a considerable slope along the northern side of the Basilica and then bent at a right angle, thus reconnecting to the vestibule.
Only a stretch of the corridor, which was presumably decorated as well as the rest of complex, remains: the one that leads into the vestibule, which is characterized by a quadrangular plan covered by a domical vault with a skylight.
The vestibule gives access to the the main room: a rectangular hall subdivided by six pillars into three barrel-vaulted aisles. The nave, wider than the aisles, has an apse at the bottom.
In the three-aisled hall, the white colour of the stucco decoration predominates: the apsidal conch is decorated with a depiction of Sappho in the act of plunging from the Leucadian cliff; in the panel at the centre of the vault of the nave stands the figure of Ganymede, who was kidnapped by a winged Genius. On all of the three barrel vaults, moulded cornices frame geometric panes decorated with scenes from mythology, mystic rituals or daily life.
On the walls, above an ample red-painted faux-wainscoting, are large panels with stylized landscapes.
The elegance and the organic nature of the decorative scheme make the Basilica a unitary work of art ascribable to the first decades of the 1st century CE, both for the choice of the subjects and for the style of the execution.