Restoration works at Colle Oppio bring to light what is believed to be the largest wall mosaic ever found in the Roman world, alongside a rare fresco of an unidentified walled city.Restoration and enhancement work on the great exedra and underground gallery of the Baths of Trajan at Colle Oppio has been completed, Rome city council announced.  The project focused on the upper spaces of the south-western exedra, roughly 30 metres in diameter, and the underground gallery beneath it, which houses two exceptional ancient artworks.   The works were carried out under the PNRR-funded Caput Mundi programme. Painted City fresco and the Great Mosaic The Painted City is a fresco of around 10 square metres offering a bird's-eye view of a walled city with towers and buildings shown in perspective. Dating to the first century AD, it belonged to a building that predates the Baths of Trajan. Rome's cultural heritage superintendent, Claudio Parisi Presicce, said the identity of the city depicted remains unknown, though it must have been a well-known one.   The Great Mosaic is an imposing wall decoration, uncovered so far to a height of around five metres, depicting an elaborate architectural scaenae frons, or theatrical backdrop, with statues and figures across two registers, including a bearded philosopher, a centaur and a poet being crowned with laurel.   According to archaeologist Francesco Pacetti, who worked on the excavation, it is believed to be the largest wall mosaic ever found in the Roman world, and excavations are continuing to find its full extent; a newly revealed section depicts grape harvesting and winemaking.   Further excavation also uncovered an additional mosaic band showing a port city, likely depicting the so-called "maritime villas" motif, along with traces of a marble-clad wall and a decorated marble niche standing at 2.2 metres high, suggesting the original room reached some 12 metres in height. Both discoveries are dated to the second half of the first century AD.The exedra's two rows of niches are thought to have held books and documents, with the surrounding steps used by those attending public readings and gatherings, while a portico once ran along the underground gallery below.Alongside the restoration, works have included a new reception area and bookshop, a walkway linking the two sections of the gallery above the Great Mosaic room, new lighting, paths and viewing areas, a tiered seating area for events, and a mechanised link between the upper and lower levels. Structural and seismic surveys were also carried out on the exedra.The Baths of Trajan were commissioned by the emperor Trajan, designed by Apollodorus of Damascus, and inaugurated on 22 June 109 AD. Covering around 60,000 square metres on Colle Oppio, the complex incorporated earlier structures, including parts of Nero's Domus Aurea, and was surrounded by a large portico used for sport, leisure and cultural activities as well as bathing.   Public access will begin on a trial basis, with small guided groups, to monitor the impact of visitors on the delicate artworks. Free guided tours in Italian, by reservation, will be available from 2 October, with six daily slots of up to 15 people at 10.00, 11.00, midday, 13.00, 14.00 and 15.00. Bookings can be made by calling tel. 060608.Rome mayor Roberto Gualtieri said the intervention returned a remarkable place at the heart of Colle Oppio to the city, describing the restorations and new archaeological discoveries as further evidence of how work on Rome's heritage continues to bring fresh fragments of history to light.