At the bottom of a long, steep and exhausting descent, the Grotta di Seiano isn't actually a cave but a 1st-century tunnel linking the Roman Villa Pausilypon to Pozzuoli. When the villa's owner Publio Vedio Pollione died in 15 BC, he bequeathed his clifftop pad to his friend Emperor Augustus. Phone reservations are required to enter the site, with or without a guided tour. If you do fancy the latter, book one directly by calling cultural association CSI Gaiola (www.gaiola.org).
Ventilated by three air ducts opening onto the sea, the tunnel was dug out of the tufa by Cocceius, the same Roman engineer who built the Crypta Neapolitana in the Parco Vergiliano.
