Flooding
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.
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