Complex sentence: The Crypta Napoletana is an old Roman tunnel in Naples passing beneath the Posillipo hill and connecting Naples with the so-called Phlegrean Fields and, then, the town of Pozzuoli along the road known as the "via Domiziana".
Split into simpler sentences: The Crypta Napoletana is an old Roman tunnel in Naples. It passing beneath the Posillipo hill and connecting Naples with the so-called Phlegrean Fields and, then, the town of Pozzuoli along the road known as the "via Domiziana".