The city is 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. This created an ever-deeper lagoon environment.
In 1604, to defray the cost of flood relief Venice introduced what could be considered the first example of a 'stamp tax'. When the revenue fell short of expectations in 1608, Venice introduced paper with the superscription 'AQ' and imprinted instructions, which was to be used for 'letters to officials'. At first, this was to be a temporary tax, but it remained in effect until the fall of the Republic in 1797. Shortly after the introduction of the tax, Spain produced similar paper for general taxation purposes, and the practice spread to other countries.
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. It was realised that extraction of water from the aquifer was the cause. The sinking has slowed markedly since artesian wells were banned in the 1960s. However, the city is still threatened by more frequent low-level floods (called Acqua alta, "high water") that creep to a height of several centimetres over its quays, regularly following certain tides. In many old houses the former staircases used to unload goods are now flooded, rendering the former ground floor uninhabitable.
Some recent studies have suggested that the city is no longer sinking, but this is not yet certain; therefore, a state of alert has not been revoked. In May 2003 the Italian Prime Minister Silvio Berlusconi inaugurated the MOSE project (Modulo Sperimentale Elettromeccanico), an experimental model for evaluating the performance of hollow floatable gates; the idea is to fix a series of 78 hollow pontoons to the sea bed
