During the 1960s the government of Italy requested aid in preventing the leaning tower from toppling. A multinational task force of engineers, mathematicians, and historians discuss stabilisation methods. In 1990, after over two decades of stabilisation studies, they decided to slightly straighten the tower to a safer angle, by removing 38 cubic metres (1,342 cubic feet) of soil from underneath the raised end. The tower was straightened by 45 centimetres, returning to its 1838 position and was declared stable for at least another 300 years (wikipedia).
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