Construction commenced on what would become the Tower of Pisa, intended as a freestanding campanile for the city's cathedral. Built on unstable, weak subsoil, the tower began leaning noticeably after only three of its eight stories were completed, yet builders continued for nearly two centuries, ultimately producing one of the world's most recognizable architectural mistakes.
The tower's engineers, likely Bonanno Pisano among them, chose a site near the Arno's alluvial plain, ground far too soft to support a marble structure of the tower's weight. By the time the third story was completed around 1178, the uneven settling of the foundation had already produced a visible tilt, and construction abruptly halted, possibly because Pisa became embroiled in warfare with rival city-states.
The pause, lasting almost a century, likely saved the tower from collapse, allowing the soil beneath to stabilize somewhat before construction resumed. Later builders attempted to compensate for the lean by making upper stories slightly taller on one side, giving the tower a subtle banana-like curve rather than a straight tilt.
The tower was finally completed around 1372, roughly two hundred years after the first stone was laid. It continued tilting for centuries afterward, reaching a lean of over five degrees before a major stabilization project in the 1990s and 2000s used soil extraction techniques to reduce the angle and secure its future, without ever making it straight, since a straight tower would no longer be the tower the world knows.
Key people: Bonanno Pisano