When the tower was built, the hill had been flattened and the channel which flowed near by had silted up but the silt was not as firm as the core of what had been the hill. 'When they built the tower they put it, without knowing it, on a hillside,' said Mr Pierotti. 'At that time the hill was invisible because the actual surface of the land was flat . . . If we do not do something . . . the whole base will continue to slide down the hillside until the tower collapses.'
Rome - Scientists believe excavations around the Tower of Pisa have solved the mystery of why it leans, writes Patricia Clough. The archaeologists have discovered that in ancient times the ground under the tower was a small hill which rose as an island from among channels created by the two rivers that flow through Pisa, the Arno and the Serchio, according to Piero Pierotti, lecturer in the history of town planning at Pisa University.