Under the basilica of the Flavian Palace itself is one of the most interesting rooms, a hall probably dating from Caligula's time, and called, from motifs used in its decoration, the 'Hall of Isis'. On its walls remnants of scenes from mythology and Roman legend may still be traced in the thick darkness by the fitful light of incandescent lanterns. Below its vaulted roof, painted with an almost baroque design of wavy bands of blue and rose with spirals of golden ribbons, is the frieze of sacrificial vessels, lotus, and sacred asps of Egypt which suggested the name of Isis, whose mystery cult Caligula revived in Rome. A curving brick wall, which cuts ruthlessly through the hall, belonged to Nero Domus Transitoria.
The paintings of this room, discovered in 1724, were drawn by several artists, and were engraved and printed by George Turnbull, together with copies of other ancient frescoes, in two books. But despite the interest the pictures had created at the time, the room was forgotten by all except a few devotees of Roman antiquities until it was re-excavated in 1912.
South of the basilica are two other underground rooms, belonging to the Domus Transitoria, which were also discovered in 1724 but remained visible throughout the nineteenth century under the name of 'Baths of Livia'. These are not the same as the half-buried chambers once called the 'House of Livia' but now generally known as the House of Augustus. Charlotte Eaton spoke of seeing here by 'the glimmering of some wax tapers . . . the gilded ceiling of these splendid dungeons still shining in the passing ray, and painted with figures designed with exquisite taste and correctness'. Today these rooms, like the 'Hall of Isis' and the still earlier Republican chamber, the 'Hall of the Griffins', discovered in the 1930's, are kept covered for protection and shown only by special arrangement.
Considerably farther to the south and west, and closer to the Palace of Tiberius than to that of Domitian, lies a little half-buried house discovered in 1869. It has borne the names of Livia, widow of Augustus and mother of Tiberius, and of Germanicus, father of Caligula, but is now believed to have been the modest dwelling of Augustus himself, preserved unharmed as a State monument through the building orgies of later emperors. The frescoes of mythological scenes upon its walls are among the best-known ancient paintings in Rome, though exposure to the light has dimmed their colours sadly.
The dividing line between the Palace of Tiberius and the other palaces on the hill runs roughly parallel with the modern road leading up from the Forum. No remnants of the upper stories now stand on the site of the Palace of Tiberius, but a few feet above the level of the present Farnese Gardens a low ridge marks the line of the half-subterranean Cryptoporticus built by Nero as a link between the Palatine and his Golden House and enlarged by the addition of cross branches. Openings high up in the massive vaults let in a cool, dim light as grateful now on a summer day as when the imperial household used it as a passageway between the buildings on the hill.
Who else may have used it since, or for how long, remains a field for fascinating conjecture. Something at least has long been known of some buried chambers beneath these gardens, for Bernard de Montfauçon wrote of them late in the seventeenth century: 'All the hill is full of subterraneous passages, the Entrance into which is purposely stopt up.' Whether he meant the Cryptoporticus or some of the corridors in the Palace of Tiberius, or simply spoke in general of all the substructures beneath the Farnese Gardens, is a matter for speculation.
Most of the rooms in this palace, begun by Tiberius and extended by his successors, especially Domitian, still lie buried beneath the gardens. Those which can be seen today are shut off from the light not only by the earth above their vaults but also by the massive substructures built in front of them toward the Forum. Caligula, successor of Tiberius, began these substructures, and later emperors extended them so far that they must have almost completely darkened the rooms behind, unless they were lit by interior courts. Long ago, at least as early as the mid-sixteenth century when the Farnese family levelled the ground above them, the great brick arches were filled with earth to more than half their height, while above them spread, in all their glory, the gardens which were fit successors to the palaces on the imperial hill.
The Farnese began these gardens about the middle of the sixteenth century, when Cardinal Alessandro II, kinsman of Pope Paul III, bought the site of the Palace of Tiberius and much of that covered by the Flavian Palace. Having dug for works of art and demolished considerable remnants of ancient masonry, they employed the best architects and gardeners of the day to lay out the gardens which were among the sights of renaissance and baroque Rome. Extending from the top of the hill well into the Forum, they contained no palace but were dotted with pavilions and towers. The retaining wall at their foot ran through the middle of the ancient House of the Vestals, and its high terraces covered these ruins and those of many buildings on the hillside. The casino opposite the great entrance gate which Vignola built was squarely in front of the northern entrance to Nero's Cryptoporticus. All that remained visible of the Palace of Tiberius were the immense arches of the substructures which supported the northwest corner of the gardens and perhaps a few fragments of crumbling wall.
These gardens passed presently to Farnese kin, the dukes of Parma, in whose hands they remained until they were inherited by the Bourbons of Naples in 1731. It was during the last years of their ownership, between 1720 and 1730, that the dukes of Parma excavated the centre of the hill in search of buried works of art and discovered the painted rooms beneath the Flavian Palace.