The Leaning Tower of Pisa, or simply the Tower of Pisa, is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its nearly four-degree lean, the result of an unstable foundation. The tower is situated behind the Pisa Cathedral and is the third-oldest structure in the city's Cathedral Square (Piazza del Duomo), after the cathedral and the Pisa Baptistry.