The city's most extensive catacombs lie beneath the Basilica di San Giovanni, itself a pretty, truncated church that served as the city's cathedral in the 17th century. It is dedicated to the city's first bishop, St Marcian, who was tied to one of its pillars and flogged to death in 254. The church and eerie catacombs are only accessible on 30- to 40-minute guided tours (available in English), which depart regularly from the site's ticket office.

In the catacombs, thousands of little niches line the walls, with tunnels leading off from the main chamber (decumanus maximus) into rotonde, round chambers used by the faithful for praying. All of the treasures that accompanied the dead on their spiritual journey fell victim to tomb robbers over the centuries bar one: a sarcophagus unearthed in 1872 and now on exhibition in the Museo Archeologico Paolo Orsi.