Ostia Antica

Rome's vast ancient port city, abandoned in late antiquity when the Tiber silted up and malaria spread — and remarkably preserved as a result. You walk paved streets past intact apartment blocks (insulae), bakeries with stone ovens, taverns with marble counters, public baths with mosaic floors, and a still-functional 4,000-seat theatre. It's essentially Pompeii without the crowds, and it's reached in 30 minutes by train from Roma Porta San Paolo (covered by a standard Metro ticket). Allow at least half a day.