A planned hi-tech driverless underground railway line set to bring desperately needed transport links to the historic heart of Rome has run into a minefield of Roman remains.

Planners aim to send the new C line under the city centre at a depth of 30 metres, well beneath the archaeological treasures that litter Rome. Stations will also be built deep underground, but even the simple task of digging entrances and exits is proving a headache and could mean the scrapping of the Largo Torre Argentina stop, which serves crowded tourist sights such as the Pantheon.

"This is unfortunate but acceptable," builder Federico Bortoli told Corriere della Sera, after workers ran into the corner of an imperial Roman public building.

The C line's builders have offered archaeologists a rare glimpse at Rome's imperial past and are obliged by law to slalom around valuable finds.

Rome's two existing tube lines neatly skirt the heart of Rome which sits in a loop in the Tiber between the Capitoline Hill and the Vatican. The gap should be filled by the 15-mile C line, which will link the Colosseum to St Peter's before reaching into the suburbs. But the second stop at risk on the crucial, one-mile city centre stretch of the line could be Chiesa Nuova, further down Corso Vittorio Emanuele and handy for Piazza Navona, the popular gathering spot built on the site of a former Roman circus.

Digging proceeds, but the mood on site has changed since archaeologists found a line of amphorae which may have been used to hold plants in the garden of a villa of a well-to-do Roman. Further excavations will determine the final decision on whether the stops can be built.