ROME (AP) — Work to upgrade Rome's public transport has again run up against an ancient problem: archaeological ruins.

Culture ministry officials on Monday showed reporters where work on the city's third subway line unearthed barracks for Roman Praetorian guards dating from the second century.

While construction workers poured concrete at the planned Amba Aradam metro stop, an archaeologist just a few meters away brushed dirt from a small bronze bracelet.

The barracks, discovered nine meters (about 30 feet) below street level, cover 900 square meters (9600 square feet) and include a long hallway and 39 rooms decorated with black-and-white mosaics on the floors and frescoed walls.

"It's exceptional, not only for its good state of conservation but because it is part of a neighborhood which already included four barracks," said Rossella Rea of the Culture Ministry. "And therefore, we can characterize this area as a military neighborhood."

Archaeologists have also found a collective grave at the barracks, where they have so far discovered 13 adult skeletons along with a bronze coin and a bronze bracelet.

Officials hope to incorporate the discovery into the new metro station, which is scheduled to open in 2020. Work on Rome's Metro Line C has been beset by delays due to corruption probes and funding shortages since launching in 2007.