Campos Flégreos
The extraordinary 770 metre-long "Grotta di Seiano" or "cave of Sejanus" passes beneath the Posillipo hill and connected the imperial villa and other patrician villas nearby with the Phlegraean Fields and the towns and ports of Puteoli (Pozzuoli) and Cumae. It owes its name to Lucius Aelius Seianus (Sejanus), prefect of the emperor Tiberius, who according to tradition commissioned its enlargement in the first century AD; the first tunnel was built 50 years earlier by architect Cocceius Auctus at the behest of Agrippa. The eastern entrance is cut into the rock cliff within the archaeological park while the western entrance was a monumental arch with "opus reticulatum" lining the cliff sides, and both ends being of about 14 m height.