I just watched a fascinating show on PBS about the sinking of Venice. The most startling revelation (to me) was that the city has no public sewer system. So most of the sewage just gets dumped directly into the canals!
But the show was also gave a really interesting explanation of why the city is sinking, what they are talking about doing about it, the potential effects of climate change, etc. The city is sinking more slowly now that nearby cities have stopped pumping water out of the aquifier underneath it, but it is still heading down as the sediments are compacted. In the old days, when the water got too high, Venetians would just lay down more stone and raise the height of the streets and building floors. Or they would demolish buildings and rebuild them slightly higher. But for the last 200 years or so, no one has wanted to destroy or change the beautiful archictecture of the city. So things have stayed at the same level despite the fact that the city is now something like 8 inches lower than it was 100 years ago. The result is lots of flooding -- in 1900, St. Mark's square flooded 7 times, and in 1996 it flooded 99 times.
The TV show discussed a plan to build giant gates that would rise across the entrances to the lagoon when high water is expected. That's what the sign we saw in a square in Venice was talking about. After watching the show, though, I think that whover put it up was a bit of a crackpot.
That said, there are some interesting issues associated with the gates. For one thing, if global warming causes significant changes in weather patterns or sea level, they could be entirely ineffective well before the end of their projected 100-year life. And there is concern that if the water level continues to get higher, they might end up being closed so often that pollution in the lagoon would become intolerable. (Remember what I said about the lack of sewers!)
All of this relates to another problem -- the city of Venice is becoming a museum rather than a living city. Millions of tourists visit the city each year, and the number of permanent inhabitants has dwindled to 70,000, driven out by the high prices and flooding.
Definitely a beautiful and fascinating city, but one with its share of problems.