In 1294 Florence, jealous of the cathedrals of Siena and Pisa, approved Arnolfo di Cambio’s design for Santa Maria del Fiore to replace the crumbling Santa Reparata. The first stone was laid in 1296, di Cambio died, the Black Death struck in 1348, and the Wool Merchants’ Guild pushed the nave to completion around 1380. In 1367 the city rejected flying buttresses as ugly and northern, committing to a 44 meter dome starting 52 meters up that no one knew how to build. When the 1418 competition came, goldsmith Filippo Brunelleschi, who had lost the 1401 Baptistery doors contest to Lorenzo Ghiberti and spent years in Rome studying the Pantheon, proposed a self-supporting double shell and hid its secrets in his model.

Named co-supervisor with Ghiberti, Brunelleschi allegedly faked an illness, let his rival botch the masonry, and took sole control in 1423. The dome was consecrated in 1436 by Pope Eugene IV, and Brunelleschi won the lantern competition too before dying in 1446. Inside came Uccello’s clock, Vasari’s Last Judgment, and Toscanelli’s solar meridian, while the marble facade only arrived in 1887. Today over 490 cracks are watched by more than 160 sensors.

• Hidden chains of sandstone and iron, plus one of chestnut wood, that hold the 37,000 ton dome together against hoop stress
• Over 4 million bricks laid in a herringbone pattern that stopped wet courses from sliding without wooden centering
• Reversible ox hoists and lewis clamps that historians rank among his chief contributions
• Young Leonardo da Vinci, apprenticed to Verrocchio, helping with the copper ball in 1469 and sketching Brunelleschi’s machines
• The 1985 alarm over concrete plugs, and finite element analysis showing gravity widens the Type A cracks about three millimeters a century