The buildings of Venice are built on closely spaced wood piles. Since the wood is underwater where there is no oxygen, the piles are mostly still in tact, even after centuries of submersion. This is because wood can't rot without oxygen. Still, the foundations of the buildings and houses that rest on the piles are heavy and now the whole weight of the city is gradually sinking the islands on which Venice is built. The buildings are often threatened by flood tides that push in from the Adriatic Sea between autumn and early spring. Also, air pollution from nearby factories on the mainland is wearing away the ancient buildings.

During the 20th century, when many artesian wells were used to draw water for local industry, Venice began to sink. When people realized that it was the wells that caused the sinking, they were banned in the 1960s. But even though the sinking has slowed down, the city is still threatened by more frequent, low-level floods that regularly occur after certain tides. In many old houses the staircases are now flooded, so in those houses no one can live on the lower floor anymore. Nowadays many Venetians have simply moved to the upper floors and continue on living their lives in the same house with one less floor.

Some recent studies have suggested that the city is no longer sinking, but this is not yet certain. There is now a project that blocks the incoming water from the Adriatic Sea, a project that should be completed by 2011. But sea levels are still rising and in fact, the whole east coast of Italy is very slowly sinking. Some experts say that the best way to protect Venice is to physically lift the city to a greater height above sea level. This could be done by pumping water into the soil underneath the city. This way, some people hope it could rise above sea level, and be protected for hundreds of years to come.