Since the beginning of the 1900s, high waters have become ever more frequent as the level of the land has dropped with respect to the sea. The floods cause inconvenience to inhabitants and damage to architecture and buildings. There is also an ever present risk of a catastrophic event such as the 4 November 1966 flood when Venice, Chioggia and other built up areas in the lagoon were completely submerged under more than a metre of water.