Italy has more active volcanoes than any country in continental Europe. Etna erupts every 2-3 years (you can hike to the summit craters). Stromboli erupts every 15-20 MINUTES (you can watch from a boat at night). Vesuvius last erupted in 1944 and could erupt again (3 million people live in the danger zone). Campi Flegrei is a supervolcano with the ground rising 2cm/year. All of them are visitable. Some of them are currently erupting.
1. Etna (3,357m, Sicily): Europe's tallest active volcano. Erupts every 2-3 years (sometimes continuously). Hike: Cable car from Rifugio Sapienza (south, €30 one-way) + 4x4 jeep to 2,900m + guided hike to summit craters (3,300m, €70-100 with mandatory guide). Or: Easier north side from Piano Provenzana. The lava flows are visible from Taormina's Greek Theatre. 2. Stromboli (Aeolian Islands): Erupts every 15-20 minutes. The "Lighthouse of the Mediterranean." Night boat tours (€25-35 from Lipari/Milazzo) let you watch red explosions from the Sciara del Fuoco (the lava slide) from the sea. Summit hike (924m, 3h up, mandatory guide, €28, evening departure for eruption viewing at sunset — one of the most extraordinary natural spectacles in Europe).
3. Vesuvius (1,281m, Naples): Last erupted 1944. Hike to the crater rim: Bus/car to 1,000m parking → 30 min walk to the crater (€10 entry). Look into the crater — steaming fumaroles, the smell of sulfur, the knowledge that 3 million Neapolitans live on the slopes. 4. Campi Flegrei (Naples): A 13km-wide caldera = a SUPERVOLCANO. The ground rises 2cm/year (bradyseism). Solfatara crater (currently closed to public after a fatal accident — view from outside). Rione Terra underground + Amphitheatre + Sibyl's Cave. 5. Vulcano (Aeolian Islands): Last erupted 1890. Hike to the Gran Cratere (391m, 1h, sulfurous fumaroles at the rim — the smell is INTENSE). Mud baths: Natural volcanic mud pools on the beach (free — cover yourself in mud, let it dry, wash in the sea). 6. Ischia: Volcanic island in the Bay of Naples — thermal hot springs powered by volcanic heat (Negombo Gardens, Poseidon Gardens, Castiglione — €20-35, day-long bathing in volcanic pools overlooking the sea).