In 1909, beneath the fourteenth-century floor, a sea of mosaic resurfaced: 760 square metres laid shortly after 313 AD, when Christianity had just become lawful — the largest early-Christian carpet in the West. The whole story of Jonah swims across it, among fish, octopuses and ancient art's most likeable sea monster; the Good Shepherd watches from the centre.
The frescoed crypt (twelfth century) and Popone's bell tower complete a UNESCO monument that alone justifies the detour.
Our tip: visit the excavations crypt too, where raised walkways run above further mosaics: the combined ticket costs little and doubles the wonder.