Rome has been continuously inhabited for 2,800 years — and each layer is BURIED beneath the next. Every church sits on an earlier church. Every palazzo sits on a Roman house. And you can GO DOWN: San Clemente descends 3 levels to a 1st-century Mithras temple. Catacombs extend 600km. The Stadium of Domitian hides beneath Piazza Navona. Underground Rome is a parallel city.
1. Basilica di San Clemente (Via Labicana): THE definitive underground experience — 12th-century church → 4th-century church → 1st-century Roman apartment → Mithras temple with altar STILL IN PLACE. €10. 3 layers of Rome in 30 minutes. 2. Catacombe di Domitilla: 17km tunnels on 4 levels — the deepest you can go. €10. 3. Domus Aurea (Nero's Palace): 300 rooms underground, hard hat required. €16. Weekends only. 4. Sotterranei di Piazza Navona: Stadium of Domitian (1st century) beneath the piazza. €8.
5. Case Romane del Celio (Clivo di Scauro): Roman houses beneath the church of SS. Giovanni e Paolo — frescoed rooms (2nd-4th century) including a room with cupids and winged figures. €8. 6. Basilica Sotterranea di Porta Maggiore: A MYSTERY — 1st-century underground basilica discovered in 1917. Stucco ceilings with mythological scenes. Purpose unknown (Neopythagorean temple?). By appointment: soprintendenzaspecialeroma.it. 7. Ipogeo degli Aureli (Via Manzoni): 3rd-century painted tomb — rare Gnostic Christian frescoes. By appointment.
8. Mitreo di Santa Prisca (Aventino): Mithras temple beneath the church — frescoed cult scenes. By appointment. 9. Mitreo del Circo Massimo: Mithras temple + Roman apartment discovered during metro construction. €5. 10. Bunker di Mussolini (Villa Torlonia): WWII anti-air-raid bunker beneath Mussolini's residence — gas-proof doors, concrete walls. Rarely open — check Roma Sotterranea tours. 11-15: Colombario di Pomponio Hylas (Via Appia). Basilica di San Nicola in Carcere (3 Republican temples beneath). Crypta Balbi (medieval on Roman theatre). Mausoleo di Augusto (underground circular tomb). Insula dell'Ara Coeli (Roman apartment block visible from street level).
Guided tours: Roma Sotterranea (romasotterranea.it) — specialist underground tours. Through Eternity — underground-focused walking tours. Many underground sites require ADVANCE BOOKING — they have limited capacity.