Basilica di San Petronio (Basilica of St. Petronius), Bologna (must see)

Dedicated to Petronius, the city's patron saint, this 14th-century edifice stands as Bologna's principal church, containing an impressive 22 side chapels and featuring the Four Crosses, one of the oldest Christian symbols in the city. As one of Italy's most monumental Gothic basilicas, capable of housing up to 28,000 people, it was on track to surpass Saint Peter's in Rome until Pope Pius IV diverted funds to establish a new university, the Archiginnasio. Consequently, the façade, designed with cosmological and esoteric "diagramming", was left incomplete, with only partial cladding in pink Verona marble and a truncated transept visible down the alley to the right of the basilica. Nonetheless, San Petronio remains monumental, symbolizing civic will rather than religious authority. Built atop Roman foundations, the vaulted church reveals columns repurposed from the Augustan era, seamlessly integrated with medieval additions.

The Porta Magna, or central portal, features highly expressive reliefs, the final masterpiece of Sienese sculptor Jacopo della Quercia, which occupied the last 13 years of his life (1425−38). Adorning the architrave are scenes from the New Testament, while dramatic reliefs on the two pilasters flanking the door depict scenes from the Old Testament. Michelangelo, during his visit to Bologna in 1494, admired these reliefs and incorporated several motifs – such as "The Creation of Adam" – into the Sistine Ceiling. The lunette above the portal showcases the Madonna and Child between Saint Petronius and Saint Ambrose.

Also inside lies the world's longest indoor meridian line, inlaid into the pavement of the left aisle in 1655. Stretching 66.8 meters (219.16 feet), it was calculated and designed by astronomer Giovanni Cassini, who taught astronomy at the University. A sunray enters from a hole in the vault 27 meters high and precisely strikes the line, enabling accurate time measurement.

Tips:
Admission is free, but there's a small fee for photography. Visitors should adhere to the dress code, ensuring that shoulders and knees are covered. From the newly renovated panoramic terrace (reachable from Piazza Galvani via elevator and a short flight of stairs), one can enjoy magnificent views of the cityscape and the Apennine hills to the south.