The Campi Flegrei (Phlegraean Fields) is an active volcanic caldera west of Naples — 13km diameter, 24 volcanic craters, the ground rising 2cm/year from magmatic pressure. The Romans called it the entrance to the Underworld: Virgil set the Aeneid's descent to Hell here. The Sibyl of Cumae prophesied from a 130m tunnel carved through the volcanic rock. And the archaeological remains are staggering: an entire underground Roman city (Rione Terra, Pozzuoli), the best-preserved amphitheatre underground in the Roman world, and a SUBMERGED city at Baia — Roman villas, mosaics, and statues visible by snorkeling or glass-bottom boat, sunk by volcanic subsidence. Campania archaeology →
1. Rione Terra — Underground Pozzuoli (€5, guided tours). Walk through the streets of the Roman colony of Puteoli — shops, taverns, bakeries, a port warehouse — all UNDERGROUND, sealed beneath the medieval town above. The most complete underground Roman urban excavation after Herculaneum.
2. Flavian Amphitheatre — Pozzuoli (€5 or combo €10). The 3rd largest Roman amphitheatre (40,000 capacity). The underground is the best-preserved of ANY Roman amphitheatre — the machinery for lifting animals and scenery to the arena floor is still visible: channels, pulleys, trapdoors. You walk WHERE the gladiators and wild animals waited. St. Gennaro was beheaded here in 305 AD (patron saint of Naples — his blood "liquefies" 3 times/year at the Duomo).
3. Parco Archeologico di Baia — Underwater City (glass-bottom boat €12-15, or snorkeling/diving €30-50). Roman villas, mosaics, and marble columns SUBMERGED in 5-10m of water — the coastline sank due to volcanic bradyseism (slow ground movement). You look down through clear water at a 2,000-year-old city. The Nymphaeum of Emperor Claudius (statues visible underwater). The most surreal archaeological experience in Italy.
4. Antro della Sibilla — Cumae (€4). A 130m trapezoid-section tunnel carved through volcanic rock — the Cumaean Sibyl (priestess of Apollo) prophesied from the chamber at the end. Virgil described it in the Aeneid (Book VI): Aeneas enters here to descend to the Underworld. The tunnel is atmospheric, lit by side openings, your footsteps echoing. 5. Solfatara (currently closed for safety — check status). A shallow volcanic crater with fumaroles (steam vents), bubbling mud, sulfur deposits. The ground is warm underfoot. The smell is sulfur. The sensation is standing on top of an active volcano.
From Naples: Metro Line 2 to Pozzuoli (20 min, €1.50) or Cumana railway (30 min). Combo ticket: €10 (Rione Terra + Amphitheatre + Museo Archeologico). Baia boat tours: depart from Baia port (bus from Pozzuoli 15 min). The day plan: Morning Rione Terra + Amphitheatre (Pozzuoli) → bus to Baia → glass-bottom boat → bus to Cumae → Sibyl's Cave → return to Naples for pizza. The most archaeologically dense day trip from Naples.