Rome’s largest ancient wall mosaic was hiding under the Baths of Trajan

A vast mosaic that decorated a grand hall centuries before the Baths of Trajan rose above it has come into public view for the first time, after conservators finished reopening the underground gallery beneath Rome’s Colle Oppio. Known as the “Muse and the Philosopher,” the composition covers roughly 160 square metres and is regarded as the largest ancient wall mosaic yet found in the city, and among the largest surviving from the Roman world.

The find sits inside a roughly 70-metre gallery in the south-western corner of the baths complex, a structure originally built to support the portico above and the exedra known as the “Great Library.” Visitors now reach it by a raised walkway installed specifically to protect the fragile surfaces below.

A bath complex built over an earlier hall

The Baths of Trajan were inaugurated in AD 109 on a roughly six-hectare artificial terrace designed by Apollodorus of Damascus, the emperor’s court architect. That terrace buried earlier construction on the site, including portions of the Domus Aurea, the palace Nero built after the great fire of AD 64. According to the Sovrintendenza Capitolina, the mosaic hall predates the baths and was not part of Nero’s palace; many of its tesserae are missing, likely stripped and reused when the Trajanic complex went up around it.

A theatrical backdrop rendered in stone and glass

Across two registers, the mosaic renders an architectural scene modelled on the painted backdrop of a Roman stage, a form known as a scaenae frons. Columns, capitals and architraves are picked out with garlands, vegetal scrollwork and Dionysian motifs, framing statues that include a centaur alongside figures identified as Muses, a bearded philosopher and Apollo. One seated male figure at the centre of the composition has a face specific enough that researchers suspect it depicts a real individual rather than an idealised type. Work carried out in 2010–2011 and 2013–2014 showed the surviving panels belong to a single hall oriented north to south, with an east wall around 15 metres long and a south wall of about 10 metres facing a large nymphaeum.

PNRR-funded digging reaches a maritime scene

Renewed excavation under the national PNRR recovery programme, run under the banner Caput Mundi, pushed the dig lower and exposed a further decorated band beneath the theatrical scene. That register shows a row of buildings that appear to front the sea, a motif the Sovrintendenza Capitolina describes as belonging to the “maritime villas” decorative tradition. Below that band, excavators found traces of a wall once faced with large marble slabs and a niche about 2.2 metres tall that was probably built to hold a statue. The floor of the hall has not been reached, but researchers estimate its walls originally stood close to 12 metres, with the whole space dated to the second half of the first century AD.

A painted port city on the opposite wall

The same gallery holds a second major find: a large fresco known as the “Painted City,” which offers a bird’s-eye view of a fortified port. Turreted walls, a main gate, a harbour, street grids, a theatre, a temple, a quadriporticus and dense residential blocks are all rendered with enough detail to show individual windows on the housing. First uncovered in 1997 and recently restored, it is considered the largest surviving Roman wall fresco. Archaeologists have not identified the city it depicts and remain uncertain whether it shows a real settlement or an imagined one. “If it did exist, it was extremely important,” Claudio Parisi Presicce, Rome’s heritage chief, said of the mystery city.

Excavations older than they look

Much of the mosaic first came to light during digging in 2011, and earlier work inside the gallery, carried out between 1990 and 1992, had already uncovered remains of the Polveriera Camerale, a saltpetre works that operated on the site at the end of the eighteenth century. Excavation of the mosaic is continuing, and researchers have not yet determined whether an adjacent scene of grape-treading, visible on a separate mosaic panel accessible only through a hatch in the portico above, belongs to the same building phase.

A multimillion-euro restoration opens the gallery in stages

The work that made the gallery safe to walk through was carried out under the PNRR-funded Caput Mundi programme, which municipal heritage officials have costed at roughly €2.7 million for the restoration and public fit-out of the exedra and underground gallery at Colle Oppio. That funding covered structural consolidation of the ancient masonry, conservation of the mosaic and fresco surfaces, new lighting, and the raised walkway that now lets visitors move through the space without touching the original floors. The gallery, part of a newly created Museum of the Baths of Trajan, is being opened gradually, with small guided groups admitted on selected days and advance booking required; the first free guided visits were scheduled for the weekend of 12 and 13 September. Alongside the mosaic and the fresco, the museum displays large lead pipes, known as fistulae, that once carried water through the baths’ hydraulic system. The reopening, reported this month, has been framed as one step in a longer campaign to open more of the baths’ substructures, many of which have been documented by archaeologists for decades but never made accessible to the public.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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