Rome's other great underground adventure lies out near the Appia Antica, to the southeast of town.

BBC: Going underground at the Colosseum

Christians were buried by the Via Appia too, in the catacombs that run for miles underground.

BBC: All roads lead to Rome

By 191 BC, the Via Appia was complete, reaching as far as Brindisi in present-day Puglia.

BBC: All roads lead to Rome

Another, Appia.com, says it has 140, 000 paid and free apps drawn from 32, 000 developers.

FORBES: How Many Kinds of Enterprise App Stores are There Today?

Besides having enormous military and political importance, the Via Appia Antica was also the road of tombs.

BBC: All roads lead to Rome

It is tucked away off Via Appia Antica, the ancient road where 6, 000 of Spartacus' slave army were crucified in 71 BC.

BBC: From gladiators to pasta: Top Roman courses to take

Via Appia Antica stretches out south of Rome, as straight as a die, its steppingstone cobblestones disappearing into the distance.

BBC: All roads lead to Rome

Like the billboard-lined highways of today, Via Appia was a place to advertise: here, the aristocracy showed their wealth via their tombs, and the Roman Empire showed their might through engineering and punishment.

BBC: All roads lead to Rome

Sacred catacombs open to the public include the Catacombs of Priscilla (Via Salaria, 430), the Catacombs of St Callixtus (Via Appia Antica, 110-126) and the Catacombs of St Agnes (Via Nomentana, 349).

BBC: Exploring the history of catacombs