The Leaning Tower of Pisa, or simply the Tower of Pisa, is the campanile, or freestanding bell tower, of Pisa Cathedral. It is known for its nearly four-degree lean, the result of an unstable foundation. The tower is one of three structures in Pisa's Cathedral Square, which includes the cathedral and Pisa Baptistry. The height of the tower is 55.86 metres from the ground on the low side and 56.67 m on the high side. The width of the walls at the base is 2.44 m. Its weight is estimated at 14,500 tonnes.