Part of the ceiling of Roman Emperor Nero's Domus Aurea collapsed on Tuesday.
Some 60 square meters of the baths built on top of the Golden House by the emperor who succeeded Nero, Trajan, came down because of seepage from recent heavy rains, civil protection experts said.
The area where the collapse occurred, a tunnel that was once part of the baths, has been cordoned off because it is close to the entrance to public gardens above it, they said.
The Domus Aurea, built by Nero soon after the great fire in Rome in 64 AD, has been shut since 2005 for work to make it more stable.
It was closed after masonry fell from flaking walls and a high level of dangerous seepage was detected.
The current project aims to open up 2,600 square metres of the site.
When the Domus was completed, it actually stretched for 50 hectares and covered most of the neighbouring Palatine and Celian hills as well.
Nero was reputed to have remarked that finally he was beginning to be "housed like a human being".
After Nero's suicide in 68 AD the Flavian emperors who succeeded him proceeded to bury all trace of his legacy.
For more on the Domus Aurea, see here, here, here and here.