Ostia Antica is Rome's port city — population 50,000 at its peak, abandoned in the 5th century, preserved under river silt. 34 hectares of excavated streets, apartments (insulae), baths, theatre, fast-food bars (thermopolia), a fire station, and the Piazzale delle Corporazioni with mosaic advertisements. Metro-train from Piramide station, 25 min, €1.50. €12 entry. 1/10 the visitors of Pompeii.
Must-see buildings: Theatre (4,000 seats, Agrippa 12 BC, summer performances). Thermopolium (painted menu on wall, L-shaped counter). Terme di Nettuno (climb terrace for bird's-eye mosaic view). Forum (Capitolium + Basilica). Fullonica (laundry — Romans used urine to clean clothes, the vats survive). Insula (4-story apartment, 3 floors standing). The mosaics: Piazzale delle Corporazioni — each company's floor mosaic advertises their trade (ships, elephants, grain, dolphins).
Practical: €12 (free first Sunday). Metro B→Piramide→Ostia Lido train, stop Ostia Antica (25 min, €1.50). Open Tue-Sun 8:30am-sunset. Duration: 3-4h. Bring water + hat — zero shade. Combine with Ostia Lido beach (2 stops further on same train).