Seiano Cave, or Grotta di Seiano, is a Roman tunnel running for roughly 770 metres through the tuff rock of Naples’ Posillipo hill. More an ancient engineering work than a natural cave, it offers an atmospheric route into the Pausilypon Archaeological Park.
The passage dates to the late 1st century BC and is usually attributed to the Roman engineer Lucius Cocceius Auctus. It connected the Phlegraean coast with the villa of Pausilypon, the seaside estate of the wealthy Vedius Pollio, who left it to Augustus. Openings along the tunnel brought in air and light, revealing how Roman builders adapted to the hillside.
The walk is memorable for the contrast between the cool, dim passage and the landscape beyond it. At the archaeological park, the remains of a Roman theatre and odeon sit among greenery above the sea, with views towards Nisida and the Gulf of Pozzuoli.
Access is generally through a guided visit to the park, so check current opening days and book ahead. Wear sturdy shoes for uneven or damp ground and bring a light layer: the tunnel stays cool even in summer. The passage is long and enclosed, which is worth considering if you are uncomfortable in confined spaces.