Basilica di San Petronio, Bologna

Basilica di San Petronio dominates the west side of Piazza Maggiore in central Bologna and ranks among the largest churches ever built, though it remains permanently unfinished. Begun in 1390 on the commission of the city's governing council — dedicated to Bologna's patron saint Petronius — the building was intended to surpass even St Peter's Basilica in Rome, but papal intervention redirected funds toward the University of Bologna and work halted in 1479 with the upper portion of the facade still in raw brick. The partially-clad facade reveals the project's ambition: the lower half in white and pink marble with three elaborately carved portals, including the central portal by Jacopo della Quercia (executed 1425–1438), one of the great works of pre-Renaissance Italian sculpture. The upper half remains in unclad brick, creating a frank architectural statement about unrealised medieval ambition. The interior measures 132 metres in length, 66 metres wide, and 45 metres to the vault — making it, even unfinished, the sixth largest church in the world by volume. Twenty-two side chapels line the aisles, containing significant artworks including the Bolognini Chapel (5th chapel on the left) with Giovanni da Modena's famous fresco cycle of the Last Judgement featuring a controversial depiction of the Prophet Muhammad in Hell, based on Dante's Inferno. The most remarkable scientific object in the church is Cassini's Meridian Line (1655): a brass strip inlaid in the nave floor through which a pinhole in the south wall at a height of 27 metres projects a solar image at noon precisely, creating a functioning solar calendar and astronomical instrument. Entry to the basilica is free; modest dress is required.