Depth in Venice How will the city's underwater dam work? By Jacob Leibenluft Venice experienced its biggest flood in 22 years earlier this week. But the city is already building an underwater dam to prevent future floods. How can an underwater system stop Venice from flooding? With giant steel barriers, or "gates," that rise from the sea floor. When the city's water level is normal, the 78 hinged gates—generally about 20 meters wide, 4 meters thick, and up to 30 meters long—will rest horizontally at the bottom of the ocean, filled with water. But when flooding is expected, the gates will fill with compressed air. This causes them to rise—picture half of an opening drawbridge—until they break the surface of the water. The gates don't necessarily come to a 90-degree angle with the seabed, but they're still high enough to keep out rising waters. (The gates—set up in rows at three different inlets—are close enough to one another to prevent much water from passing between them.) It should take about 30 minutes for the gates to rise and another 15 minutes for them to go back down to the sea floor. (To see graphics and videos showing how the system will work, click here.) Venice first began planning a barrier to protect against waters after a massive 1966 flood that stranded residents, damaged several historic buildings, and left 5,000 people homeless. But the project faced several challenges. First, the barriers couldn't block the flow of water between the Adriatic Sea and Venice's lagoon under normal conditions—particularly crucial because the city's canals double as its sewer system. Additionally, aesthetic concerns dictated that the barriers could not be visible from the city. Although an underwater system was first proposed in the early 1970s, it wasn't until 2003 that the eventual design—dubbed MOSE, an acronym that doubles as the Italian for Moses—got the final go-ahead. It's now set to start operating in 2014. The project has been surrounded with controversy since it was first conceived. Environmentalists have argued that the gates will need to be up too often, disturbing a fragile ecosystem that relies on tidal flows. Some critics also estimate that global warming will push sea levels so high that MOSE won't be effective. As a result, one team of engineers has proposed another ambitious project: pumping sea water into aquifers below the city to raise its ground level several inches. When MOSE is finished, Venice will not be the only city with moving flood barriers—similar systems now operate to protect London and several Dutch cities, albeit with gates that are far more visible.