Archaeologists have presented the completed excavation of a 900-seat auditorium under Rome's Piazza Venezia, which they are hailing as the city's most significant discovery since the Roman Forum was unearthed 80 years ago.

The ancient arts complex or "Athenaeum", which lies 5.5m underground, dates to 123 AD. It comprises three halls whose 13m-high arched ceilings and terraced marble seating once provided space for Rome's noblemen to listen to poetry and philosophy. Its construction is believed to have been funded by Emperor Hadrian, who was a keen patron of the arts.

After the fall of the Roman empire, archaeologists believe that the complex was used to smelt ingots and mint coins during the Byzantine era, while from the 16th- to the 19th centuries one of the halls served as a hospital cellar. An earthquake in 848 AD led to a large part of the structure’s roof collapsing onto the floor of one of the halls, where it still remains.

The archaeologists' discovery follows five years of excavations and came about as a result of digging for the capital’s third underground line, the troubled Metro C, part of whose route was designed to run from the Colosseum to St Peter’s.

Work began on the project in 1990 but its construction has been hampered consistently by the extensive archaeological heritage buried beneath the city, leading to the scrapping of plans for two stations so far. In February 2012 the line's spiralling costs prompted the Audit Court to describe it as "the most expensive and slowest public works project in Europe and the world."

The new discovery will force engineers to rework their plans yet again and may even result in the abandoning of the line’s last proposed station in the centre. However Rossella Rea, the archaeologist in charge of the excavation, told The Guardian newspaper that the Piazza Venezia station and the ruins could coexist. "I believe we can run one of the exits from the station along the original corridor of the complex where Romans entered the halls," she said.

The archaeological site, which is currently surrounded by a taxi rank, is expected to be opened to the public in three years.