Piazza San Marco floods on high tides:
Wikipedia-When you first walk into St Marks square and see the elevated platforms you think that there must be a concert or something. Then you realize that they are actually platforms so you can stand in line when the tide rises. The Piazza San Marco is not far above sea level and during the Acqua Alta, the “high water” from storm surges from the Adriatic or heavy rain, it is quick to flood. Water pouring into the drains in the Piazza runs directly into the Grand Canal. This normally works well but, when the sea is high, it has the reverse effect, with water from the lagoon surging up into the Square.