Acqua alta literally means “high water”—and that's what it is! Because of Venice's unique position as an island criss-crossed with canals, the city is, of course, very sensitive to water levels. So whenever water levels rise in the Venetian lagoon… the island of Venice is at an increased chance of flooding.

Venice has "a lot of water" because it was intentionally built in the middle of a marshy lagoon for defensive reasons. In the 5th century AD, people fleeing Germanic and Hun invasions from mainland Italy sought refuge in the shallow, difficult-to-navigate waters of the Adriatic Sea. The city is actually an archipelago of 118 small islands connected by canals and over 400 bridges. To create a stable foundation on the soft mud, early Venetians drove millions of wooden piles (made from larch and oak) deep into a hard layer of compressed clay called "caranto." Because these piles are completely submerged in oxygen-poor water and silt, they do not rot; instead, they have mineralized over centuries into a stone-like state. Today, the abundance of water is a dual-edged sword: while it is the source of the city's unique charm and historical wealth, it also presents a modern existential threat. A combination of "subsidence" (the city sinking) and "eustasy" (rising sea levels) causes frequent "Acqua Alta" or high water events, which the city now combats using the MOSE mobile barrier system.