Roman nobles flocked to the center, built by the emperor Hadrian in 123 A.D., to hear poetry and speeches while seated on terraced marble seating in three massive halls, they said.
"Hadrian's auditorium is the biggest find in Rome since the Forum was uncovered in the 1920s," Rossella Rea, the archaeologist supervising the dig, told Britain's The Guardian newspaper.
Brick walls and areas of gray and yellow marble flooring have been exposed at the bottom of an 18-foot hole in Piazza Venezia, one of Rome's busiest thoroughfares, during excavations to build a new subway line crossing the heart of the city.
"We don't have funds for these kind of digs so this has come to light thanks to the new line," Rea said.
The discovery of the ruins has complicated plans for a subway station at the location, but Rea said the 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.