Many people know that Venice is slowly sinking. A study published in the year 2000 by Albert Ammerman says that the city has "sunk" 24 centimeters in the past 100 years. (For all you metrically challenged people - that's 9.5 inches). But is Venice sinking or the water rising? The answer is complex, but in simple terms - the answer to both questions is yes. Every year Venice floods (as often as 50-100 times a year) as a result of high tides and heavy rainfall. Studies show that global warming and changes in the winds and weather in the Adriatic sea are causing the sea level to rise as well.
We visited Venice in March, and while we didn't see a full-fledged flooding of the city, after a day and a half of light rain the waters of the canal and the lagoon were lapping over the edges of the walk near Piazza San Marco. At true "acqua alta" (high water) the entirety of Piazza San Marco is fully immersed under water. Some Venetians keep knee high (or higher) boots at both their home and place of business so they are prepared when the flood warning bell sounds.
In 1966, the city suffered the most significant flood. At that time, the city was covered in almost 1-2 meters (roughly 3-6 feet) of water. The flood almost destroyed the city, both historically and economically. When the water level rises too high, the vaporettos (the boat "buses" that serve the city) are not able to run because they can't fit under the bridges. When the water level reaches 4 feet, city workers are no longer allowed to lay down the network of raised wooden walkways that serve as makeshift sidewalks during times of high water, for fear that the walkways could be swept away by the high water.
These wooden walkways serve as a visable reminder of the battle between Venice and the sea. On our visit, it took us a while to figure out what all the low wooden tables piled up around Piazza San Marco, Lista di Spagna and other main pedestrian areas were. Reading in our guidebooks, we discovered they were not tables, they were the walkways. When not in use, especially during flood season, the walkways lay stacked in piles around the various tourist destinations.
Solutions to the problem are controversial and complex. The idea most often discussed is a project to build mobile floodgates at key inlets of the lagoon where the Adriatic pours in. Supporters say this will stop the devestating high tides that eat away at the foundations of the city, while opponents say the gates will threaten the ecological system of the lagoon and prevent the cleansing waters of the Adriatic from entering the lagoon. Another piece of the solution that has been discussed to raise the foundations of key areas of the city up to 100 centimeters (40 inches).
No matter how you look at it, Venice is in danger. The problem is centuries old, and the solutions are far from simple.
See links below for articles related to the flooding, including pictures of recent floods.
Tidal floods a part of life in watery Venice - interesting article about the flooding including several good pictures (one of Piazza San Marco underwater)
Artwork Documents Sinking Venice - how paintings from the 18th century can help document flooding
Can Venice Be Saved? - an article from TIME for Kids, but it is very well written, and has a picture of Piazza San Marco underwater.