In terms of elevation, the Piazza San Marco is one of Venice's lowest points: The Piazza is almost at the same level as the Venetian Lagoon.

Storm sewers in the Piazza empty into St. Mark's Basin (near the entrance to the Grand Canal), and during high tides, it isn't unusual for water to back up through the drains and into the square.

Such flooding is most dramatic during acqua alta (flood tides that occur mostly from October through April), but it can happen at any time of year--even on warm spring and summer days, when children and tourists are fascinated by puddles on the Piazza: