In recent years, archaeological excavations have been undertaken in the Oppian Hill park above the Domus Aurea pavilion with the aim of testing rehabilitation and waterproofing systems for the ancient structures, in order to intercept rainwater and thus eliminate percolations inside the underground monument.
These excavations have supplied important data on the depth and nature of the earth layer of the park that currently covers the archaeological structures and, further down, on the state of conservation of the remains of Trajan’s Baths and Nero’s Domus Aurea. As detailed an understanding as possible of what survives above the Neronian vaults and the Trajanic galleries is of fundamental importance for planning the conservation work that the monument needs.
Beneath the earth layer of the park, whose depth ranges from 3 m to 1.50 m, the first remains that come to light belong to the large terrace surrounding the central block of Trajan’s Baths, where the actual bath rooms were. Used for sports, strolling and leisure, the terrace must, at least in part, have been laid out as a garden and was bounded along its whole perimeter by a portico whose foundations have been identified in at least two places. The concrete cast to construct this vast platform covered and sealed the underlying rooms of the Domus Aurea at a constant height of about 47.30 m a.s.l., turning Nero’s residence into an underground complex.
Two different types of underfloors have been identified on the terrace of Trajan’s Baths: the first type consists of concrete screed, about 30 cm thick, in some cases with the remains of a floor in little bricks arranged in a herringbone pattern on the surface (opus spicatum) (Test Pits A – B – D).
The second type of underfloor is composed of little columns of square bricks (bessales) resting on concrete screed and supporting a floor made of a double layer of large clay bricks (bipedales) (Test Pits A-B-C-H-I). This created a cavity that functioned as a genuine ancient waterproofing system, probably employed for garden areas above usable rooms: the soil used for planting must have been laid above the floor in bipedales.
As can be seen from the plan showing the location of the test pits, this type of arrangement has been found both in the eastern and western area of the gardens, with a slight slope from north to south and from west to east to encourage water to drain away.
In some cases, archaeological studies in the Oppian Hill park have reached a greater depth, bringing to light the structures of the Domus Aurea, due to the destruction of the floors of the Trajanic terrace that had buried them from the early 1st century AD. Specifically, the major excavation in the 1970s above the pavilion’s East Wing, gravitating around the roof of the Octagonal Room (Test Pit A), uncovered a large stretch of an upper storey of the Domus Aurea, hitherto unknown.
That part of the complex extending over the summit of the hill consisted of porticoes and gardens hosting marble-clad fountains with small rooms and lightweight architecture opening onto them. To the north the area was bounded by a long canal (euripus) with a cascade at its centre that fed the fountain of the nymphaeum of the Octagonal Room on the lower floor. Perhaps partially destroyed by a fire in AD 104, this upper storey of the Neronian building was completely razed for works to construct the terrace of the baths, at a height of 30-40 cm above the floor levels.
Once razed, the structures belonging to this storey were covered by a layer of sandy earth on which the concrete for the underfloors of the terrace of the Baths was then poured. It should be noted that at the current state of research, remains of a Neronian upper storey have been identified only in this area of the park, whereas no trace has come to light in test pits further west; this suggests that the arrangement with pavilions and gardens was restricted to the area above the Octagonal Room sector. In other cases, the Trajanic underfloors are laid directly onto the upper surface of the vaults of the Domus Aurea.
In Test Pit D, the underfloor of the Trajanic terrace rests directly on the upper surface of the vault of the Neronian room beneath. At a period after Trajan, both the vault and the Trajanic concrete were cut to create a channel serving as a drain for a monumental fountain whose remains were found in Test Pit B, built on the terrace of the baths during the Severan period.
A unique instance is the roof of the Neronian corridor (Cryptoporticus 19) which forms the northern edge of the pavilion on the Oppian Hill. The outer surface of the vault, brought to light in the two soundings at the western edge of the area investigated (F and G), is arranged as a terrace with two parapets at the sides and at a significantly lower height than the vaults of the other Neronian rooms. Originally, the terrace was equipped with skylights that supplied air and light to the underlying cryptoporticus, subsequently closed by a waterproof sheath.
These features of the vault were noted already in 1882 by De Romanis, a scholar who in the 19th century excavated some of the rooms of the Domus Aurea and published a plan with some sections: he suggested the construction in the Trajanic period of a counter-vault on this terrace to raise the room to the level of the terrace of the Baths. However, at the current state of research no remains of this counter-vault have been found in Test Pits F and G.
The excavations were carried out with archaeological assistance from Società GEA S.C.ar (Test Pit B) and Cooperativa Archeologia, for all the other soundings.