The Leaning Tower of Pisa (Torre Pendente di Pisa in Italian) is one of the great icons of Europe. Begun in 1173, the bell tower of Pisa Cathedral is famous for the shifting of its sandy foundations that has led to a significant lean of 5.5 degrees.

The Tower of Pisa (La Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa. It is situated behind the Cathedral and is the third oldest structure in Pisa’s Cathedral Square (Piazza del Duomo) after the Cathedral and the Baptistry.

Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 due to a poorly laid foundation and loose substrate that has allowed the foundation to shift direction. The tower presently leans to the southwest.

The campanile was begun in 1173 as the final structure of the magnificent cathedral complex on the Campo dei Miracoli in Pisa. The settling of its foundations and resulting lean became apparent before it was even finished – after only three stories were completed. The engineer, belived to be Bonnano Pisano, tried to compensate by making the new stories a little taller on one side. However, the extra materials caused the tower to sink even more.

Work was suspended several times as engineers worked to find a solution, but the tower was still leaning when it was completed in 1350. The architectural design remained unchanged throughout, as later builders stayed faithful to the original Romanesque designs.

Over the years various attempts have made to straighten the tower, including the injection of cement into the foundations and various types of bracing, but in the late 20th century the structure was still subsiding at the rate of 0.05 inches (1.2 mm) per year and in serious danger of collapse.

In 1990, the Leaning Tower was closed and the bells were silenced as engineers undertook a major straightening project. The main technique was to siphon more than 70 tons of earth from underneath the foundations while supporting the tower with steel cables and lead weights. The work was completed in May 2001 and has decreased the lean by 17 inches (44 cm) to 13.5 feet (4.1 metres).

Made of gleaming white and pastel marbles, the Leaning Tower has a diameter of 52 feet (16 m) at the base and would stand 185 feet (56 metres) high if it were straight. It currently leans 5.5 degrees, which amounts to about 15 feet or 4.5 metres from vertical.

The famous lean of the bell tower often overshadows its magnificent architecture, which is an exceptional example of the Romanesque style. The round tower is made of fine multicolored marble and has eight stories in all, each surrounded by an arcaded gallery. The repeating registers of arches give the tower an exceptionally harmonious and rhythmic appearance.

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