It took a long time to design and approve MOSE. Construction is taking a long time, too. That's because MOSE is currently the most massive public works project in the entire world. Its basic premise -- to stop high water from entering the lagoon -- is more complicated than it might sound.
There are three primary routes for water flowing in and out of the lagoon: the Lido, Malamocco and Chioggia inlets. When complete, MOSE will block waters entering through those inlets, to levels more than 2 meters (6.5 feet) above those on the city side of the dam.
MOSE is a series of mobile steel gates that are raised and lowered on command. When the tide is out and waters low, these hollow gates fill with water and lie flush with the seafloor, folded away in trenches. As waters rise, engineers use air compressors to blow out seawater and fill the gates with air.
Then, natural buoyancy causes the gates to swing upwards on hinges attached to the seafloor. The top of the gates jut out over the waves, plugging the inlets and stopping rising water from entering the lagoon, and in theory, protecting Venice and surrounding areas from flooding. Because the gates swing to and fro on hinges, they allow for some give, or oscillation, during periods of rough waves and powerful storms. They're also angled so that surges of water toward the inlets won't slam them shut and defeat the project's purpose.
To span the full width of all three inlets, there will be a total of nearly 80 gates, with each individual gate spanning up to 66 feet (20.1 meters) across. At 874 yards (800 meters) wide, the Lido -- the northernmost inlet -- is the widest. At wider points like those found in this location, there will be multiple rows of gates to ensure that as much water as possible is stopped.
The projected completion date is scheduled for some time in 2016. Until then, construction workers are working quickly, in the hopes that they can beat the next big weather event.