Pisa’s bell tower started with a widow, Donna Berta di Bernardo, who left 60 soldi in her will in 1172 for its first stones. Construction began in August 1173 on a foundation just three meters deep, set in soft clay, sand, and shells, and by 1178 the tower was sinking at the second floor. Wars with Genoa, Lucca, and Florence then halted work for nearly a century, which gave the soil time to compress. In 1272 Giovanni di Simone built the new floors taller on the low side to correct the tilt, adding weight where it was already sinking and leaving the tower curved like a banana. It took 199 years to finish, with Tommaso di Andrea Pisano adding the bell chamber, and the original architect is still debated.

The finished tower held seven bells, including one from 1262 that rang for executions such as that of Count Ugolino in 1289. The Galileo cannonball story rests only on a biography by his pupil Vincenzo Viviani. In World War II US Army Sergeant Leon Weckstein, sent to see whether Germans were using it as a lookout, was so struck by the square that he never called in artillery. By 1990 the lean hit 5.5 degrees, and after a deadly tower collapse in Pavia the year before, Pisa closed it on January 7, 1990. Engineers removed about 38 cubic meters of soil from under the high side, bringing the tilt back to its 1838 position, and in 2008 declared that it had stopped moving.

• A bronze fragment bearing Bonanno Pisano’s name, found in 1820, that may be debris from cathedral doors burned in 1595
• A 2001 study pointing to Diotisalvi, builder of the San Nicola bell tower and the Pisa Baptistery
• The government’s mandate for the 1990s rescue: save the tower but keep the lean
• Steel cables, 870 tons of lead counterweights, and slow soil extraction instead of jacking up the low side
• The 2018 finding that the soft soil keeps the tower from resonating with earthquakes, protecting it from at least four since 1280