Acqua alta means "high water" in Italian, and the phrase is what Venetians use to describe high tides that flood the city. In the last century, the frequency of acqua alta has increased from fewer than 10 times a year to more than 60 times a year, leaving many to wonder how much longer Venice can stay above water. A flood in 1966 left more than three feet of water on the city's streets, and during a severe flood in 2009, some parts of Venice were under four feet of water. That's waist-high! Rising sea water isn't the only problem Venice faces. In the 20th century, excessive groundwater pumping led the city to "sink" about 2 millimeters a year, though that number has slowed to about 0.08 inches each year, according to a 2012 report.