Tourists wading across the Plaza San Marco -- a common event in Venice, the European symbol of both rising seas and the difficulty of preventing damage to irreplaceable coastal cities. Venice has been sinking for hundreds of years, perched as it is on unstable sediments of the Lagoon. But the rising Adriatic Sea is rapidly exacerbating the problem. At the current rate the sea will rise a foot in this century. Italian officials made the decision to construct elaborate tide dams at Lagoon entrances. Environmental groups and some scientists warn that higher tide and storm levels will soon overcome these defenses, while the dams may isolate the Lagoon from the natural flushing it needs to remain a viable ecosystem.