Venice is sinking...   

Venice is a warren of canals. Where most cities have roads, Venice has canals. Gondolas and water taxis transport people to and from destinations. With water levels rising, flooding has become a serious problem and is contributing to Venice sinking. During the high tides in autumn and winter, the Piazza San Marco, the lowest area of the island, becomes totally flooded with water.

When the high tide season arrives, the streets of Venice become blocked with water. Wooden walkways must be erected in order for pedestrians to walk safely. The high water level is causing damage to Venice and disruption to its inhabitants. It has now reached the point at which city governors see the problem as critical.

Venice has always been slowly sinking. Over the last 1,000 years, it has sunk by around seven centimeters for every century, but recent reports have stated that in the last century alone, the city of Venice has lowered by around 24 centimeters. This may have more to do with global warming and the melting polar ice caps than with Venice sinking into its own foundations.

Source: http://www.wisegeek.com/is-venice-sinking.htm

I believe global warming has a lot to do with it - it is a beautiful city that I was lucky enough to visit this December just gone and I would highly recommend a trip there before it is too late!