Mayor Roberto Gualtieri presented the recently restored Painted City fresco and Great Mosaic that were discovered in an underground gallery during excavations in the 1990s of the Baths of Trajan on the Oppio hill near the Colosseum.
The works are opening to the public for the first time, initially in small groups of 15 and during a trial period of weekend access starting this month. The aim is to determine how the delicate works respond to increased humidity and climactic impacts before allowing more general access.
Gualtieri said the fresco, which features images of a walled port city from a birds-eye view perspective, was the largest wall fresco of its kind from ancient Rome and as such needs to be open to the public.
After being hidden underground for centuries, its blues and terra cotta colours are still clear but faded.
"They are truly unique in the world, both from the point of view of artistic beauty and the historic and documental importance," he said at the unveiling on Tuesday.
Rome's cultural superintendent, Claudio Parisi Presicce, said the grandeur of the port city as depicted in the fresco, with buildings, a theatre and temple, public piazzas and towers, was particularly well conserved.
The restoration work included techniques to preserve and improve visibility, remove surface deposits and consolidate the grouting to keep the decorated surfaces intact.
"We don't know what city it refers to, but it must have been a particularly well-known one," he said.
The mosaic is 15 metres high and features intricately decorated columns and architectural elements and figures over four different levels. The top levels are up-close images with people that are almost like theatrical scenes, while at the bottom are views of the port city's layout from above.
The Baths of Trajan, which were dedicated by Emperor Trajan in 109 AD, extended over six hectares and were at the time the largest bath complex in the world.
They incorporated an earlier baths complex, the Domus Aurea, and were designed on a different trajectory to take better advantage of the water temperatures to optimise the thermal experience for users.
The complex now open to the public includes a grand semicircular upper-level "library" which archaeologists believe was part of the baths complex and allowed users not only to practice sport and bathe but also to participate in cultural activities.
