The entire floor of the Latin cross-shaped basilica, rebuilt after an earthquake in 1348, is covered with one of the largest and most spectacular Roman-era mosaics in the world. The 760-sq-metre floor of the basilica's 4th-century predecessor is protected by glass walkways, allowing visitors to wander above the long-hidden tile work, which includes astonishingly vivid episodes from the story of Jonah and the whale, the Good Shepherd, exacting depictions of various lagoon wildlife, and portraits of wealthy Roman patrons and their quotidian business interests.
Treasures also fill the basilica's two crypts. The 9th-century Cripta degli Affreschi (Crypt of Frescoes) is adorned with faded 12th-century frescoes depicting the trials and tribulations of saints, while the Cripta degli Scavi (Excavations Crypt) reveals more mosaic floors in varying states of preservation. Some images were destroyed or badly damaged by the erection of the basilica's 73m-high bell tower, built in 1030 with stones from the Roman amphitheatre. The FVG Card offers free admission.