Venice is built on a set of marshy islands and has always been sinking. The rate accelerated in the Twentieth Century, and the city is now more susceptible to the "aqua alta" or "high water." This phenomenon becomes a problem in the fall and winter.
Piazza San Marco is the lowest point of the community and gets flooded first. I expected the water to flow in from the Grand Canal, but that's not how it works.
The water just bubbles up through the drains or even cracks between the stones. It can be a curious sensation to see water trickling out from under your shoes! Unfortunately all this liquid in addition to to inundating the square leaves a slimy film that makes the pavement treacherous.
The tide rises to several inches. Platforms are set up so people can get around.
Even the Basilica of San Marco needs walkways to get people in and out. The vestibule gets completely flooded.