The Leaning Tower of Pisa is the freestanding bell tower of the Cathedral of Pisa, Italy. The third oldest structure in the city's Square of Miracles, it is known worldwide for its unintended tilt. The tower's tilt began during construction in the 12th century, caused by an inadequate foundation on ground too soft on one side to properly support the structure's weight. The tilt gradually increased until the tower was stabilized (and the tilt partially corrected) by efforts in the late 20th and early 21st centuries. The tower, which measures 55.86 metres (183.27 feet) from the ground on the low side and 56.67 metres (185.93 feet) on the high side, has been listed as a UNESCO World Heritage Site since 1987.