The Leaning Tower of Pisa is leaning because it was built on soft, unstable alluvial soil composed mainly of sand, clay, and shells, which could not properly support the heavy weight of the tower. The tower's foundation is shallow, about 3 meters deep, and as construction progressed, the uneven settling of the soil caused the tower to tilt. This leaning began to be noticeable as early as the construction of the second floor in 1178. Despite attempts to correct the tilt by making upper floors one side taller, the uneven foundation continued to cause the inclination. Additionally, construction delays due to wars allowed the soil to settle somewhat, preventing a premature collapse. Over the centuries, engineers have worked to stabilize the tower to prevent it from falling, including efforts in the late 20th century to reduce the tilt from about 5.5 degrees to around 4 degrees.
