Following the work undertaken between 1998-99, and the discoveries made in those years, funding was obtained under the L. 396/90 for Roma Capitale, which financed campaigns in 2003-2006 and 2007-2008.

A museum is to be created in the gallery below the south-western exedra of Trajan’s baths (the so-called “cryptoporticus”), where in 1998 both the building with the large fresco of the “Painted City” and that with the wall mosaic of the “Muse and Philosopher” were discovered. With this in mind it was necessary to continue with the main objective, that is the guaranteeing of the best conditions for the conservation of the archaeological materials housed in this area. Prior to the excavation inside the gallery below the south-western exedra, the programme of works foresaw the waterproofing and consolidation of the vault prior to the removal of the late 18th century levels and the excavation of the stratigraphy above the vault itself. These interventions were to terminate with the re-laying of the preceding floor level.

However, the situation uncovered by the excavations conditioned future excavation strategy. It was thus decided to proceed with a general renovation of the Trajanic level, waterproofing the vault’s extrados with a thick layer of opus signinum and reconstituting the line of the perimeter wall of the portico above the gallery.

A trench dug inside the exedra revealed the presence of numerous remains both of the architectural structure and the facings, and it was decided to complete its excavation down to the original level. Excavation of the late 18th century dumps covering this entire area, relating to the “Fabbrica del Salnitro” (known as the Powder magazine) installed in this corner, revealed the presence of late antique dumps below, often cut by pits and cuniculi made during the Renaissance period in the search for treasure and building materials. The dumps relate to successive phases of the bath’s abandonment, datable to between the 6th-7th century A.D. Two isolated burials and scarce traces of occupation dated to the late antique period.

The excavation of these dumps brought to light the original floor of the exedra in slabs of giallo antico and pavonazzetto marble (some of which still preserved); low steps ran along the edges of the walls, originally faced with white marble. The semicircular exedra (diameter circa 30 m), with two orders of rectangular niches, is traditionally identified with the library; the façade was articulated by four columns (of which the robber trenches were found) and by two semi-pilasters or semi-columns which stood against the two brick-faced ends of the exedra, delimiting the façade. The exedra faced onto a portico circa 10 metres wide which ran around the perimeter of the baths’ enclosure. The discovery of at least three travertine bases, which had supported the vertical elements of the portico (pillars or more probably columns), made it possible to reconstruct a colonnade with an intercolumniation of circa 3 metres looking out onto the green area surrounding the rooms dedicated to bathing. The portico stood exactly above the gallery (so-called cryptoporticus) which had been built to function as a foundation in the points where there were no pre-existing structures to use as a base for the terracing.

Above the gallery, in correspondence with the building with the fresco, a passage which had opened in the vaulted roof led into the room below, part of the building’s original entrance. Its exploration revealed a large mosaic with a grape harvesting scene. The mosaic, previously only seen by video camera, was preserved on a side wall and must have been part of a larger surface decorated with mosaic, which with various pictures and registers decorated the entire vault. Thus the mosaic was part of the decorative scheme of the same large building, probably constructed in the Flavian period, in the last quarter of the 1st century A.D. Its façade was decorated by the fresco of the “Painted City”, at a height of almost fifteen metres from the ground, at the side of the grandiose archway, over ten metres wide, which constituted the main entrance. During 2009 both the vault and the mosaic itself were restored and consolidated.