This is another one of my favourite photographs taken in Venice in November 2004:
We were in Venice at the end of October when they have their lunar tides which are associated round the time of the Full moon. They warn the residents by means of an air raid siren a couple of hours before it is due to flood. It is really an experience being there when this happens, I quite randomly took this photo of people going about their normal day as if nothing was wrong and afterwards realised how stunning it was (well at least I think so anyway).
I just read this on the internet which I think is quite sad that they have done away with the sirens:
"Until late 2007 Venice still used sirens dating to the Second World War to warn its residents of floods. Now the historic sirens have been replaced by a new, more complicated system. When the tide is predicted to reach +110cm a recording of the old-style sirens will still be sounded to warn of acqua alta. But this will be followed by another signal which will alert Venetians to the level of flooding expected: the more tones sounded the higher the water, from one tone predicting 110cm to four rising notes indicating serious flooding of 140cm. The speakers are concealed inside belltowers and public buildings."
It is worth reading the information on this link if you are planning a trip to Venice. http://www.italyheaven.co.uk/veneto/venice/acqua-alta.html