Leaning Tower of Pisa

Construction of the Leaning Tower of Pisa began in 1173 as a freestanding bell tower for the neighboring cathedral. Almost immediately after the third story was completed in 1178, the structure began to sink on its southern side. This tilt was caused by an inadequate three-meter foundation set in weak, unstable subsoil consisting of clay and sand. Work was halted for nearly a century due to ongoing wars, which ironically allowed the soil to settle and prevented the tower's collapse. Construction resumed in 1272, with engineers attempting to compensate for the lean by building upper floors with one side taller than the other, resulting in a slightly curved shape. The seventh floor was finished in 1319, and the bell-chamber was added in 1372. By the late 20th century, the lean reached a dangerous 5.5 degrees. A massive restoration project between 1990 and 2001 removed soil from the high side, successfully reducing the tilt to about 4 degrees and stabilizing the landmark for centuries to come.