MOSE is a major civil engineering project designed to protect Venice and its lagoon from high water. Construction started in 2003, and is expected to finish next year. The project involves installing four barriers at three inlets to the Venetian lagoon. The barriers comprise 78 independently operated gates of varying sizes. When not in use, the gates are filled with water and lie flat on the seabed. When they are needed, compressed air is pumped in, the ballast water flows out, and the gates rise into position under their own buoyancy. Although the basic principle is quite simple, engineering and construction presented a number of challenges.