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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.
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.