The Leaning Tower of Pisa began to lean while it was still being built, on its first floors.
Why it is true
In 1178 the soft clay and sand underneath gave way on one side. Work stopped for almost a hundred years, and later builders made the upper floors slightly crooked to compensate. Between 1990 and 2001 the lean was reduced by about 44 centimetres.
Source: Opera della Primaziale Pisana
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