Bologna's hulking Gothic basilica is Europe's sixth-largest church, measuring 132m by 66m by 47m. Work began on it in 1390, but it was never finished, and still today its main facade remains incomplete. Inside, look for the huge sundial that stretches 67.7m down the eastern aisle. Designed in 1656 by Gian Cassini and Domenico Guglielmi, this was instrumental in discovering the anomalies of the Julian calendar and led to the creation of the leap year.
Several planned terrorist attacks to destroy the cathedral have been thwarted by police over the years, and indeed a police car and armoured military vehicle are permanently stationed in front. The culprit? Giovanni da Modena's 15th-century fresco, housed inside the Cappella dei Re Magi (€3), depicting Muhammad being devoured by demons in Hell. Original plans called for the basilica to be larger than Rome's St Peter's, but in 1561 Pope Pius IV blocked construction by commissioning a new university on the basilica's eastern flank. If you walk along Via dell'Archiginnasio, you can still see semi-constructed apses poking out oddly. Until early 2020, the Terrazza Panoramica, entered from the back, offers a nicely framed perspective on the city's famed Le Due Torri.