Catacombs of San Callisto

The largest of Rome's underground Christian burial complexes — 20 kilometres of tunnels on four levels, holding around half a million graves including those of 16 popes from the 3rd century and Saint Cecilia, patron saint of musicians. The site became Christianity's official cemetery under Pope Callixtus I in 217 AD. Visits are by guided tour only (multiple languages), photography is prohibited, and the temperature stays around 15°C year-round — bring a layer.