The buildings of Venice are constructed on closely spaced wood piles which penetrate alternating layers of clay and sand. The buildings are often threatened by flood tides pushing in from the Adriatic between autumn and early spring. Six hundred years ago, Venetians protected themselves from land-based attacks by diverting all the major rivers flowing into the lagoon and thus preventing sediment from filling the area around the city.
During the 20th century, when many artesian wells were sunk into the periphery of the lagoon to draw water for local industry, Venice began to subside. This sinking process has slowed markedly since artesian wells were banned in the 1960s. However, the city is still threatened by more frequent low-level floods (so-called Acqua alta, "high water"). In many old houses the ground floor is unoccupied due to the periodic floods, but people continue to live and work in the upper stories.
Venice is also famous world-wide for its unique Carnival, and with its lagoon is a UNESCO World Heritage Site.