The Leaning Tower of Pisa is the campanile, or freestanding bell tower, of the cathedral in the Italian city of Pisa. It is world-famous for its unintended tilt to one side. Construction began in 1173, and the tower began to lean almost immediately because the foundation was built on soft ground composed of clay, fine sand, and shells. Paradoxically, the same soft soil that caused the lean has helped the tower survive at least four strong earthquakes since 1280; the softness of the ground prevents the tower from resonating with the earth's movement. In the late 20th and early 21st centuries, engineers performed a massive stabilization project by removing soil from underneath the higher side, successfully reducing the tilt by 44 centimeters and ensuring the tower's stability for another 200 years.