The Maeslant Storm Surge Barrier, which first began operation in 1997, consists of "2 incredible movable gates which automatically sweep across the water, meeting in the middle of the waterway to form a temporary wall, each of the gates rotating on 680-ton steel ball joints." Video after the break. Click here for first picture in gallery.
...with a diameter of 35ft, are also the largest in the world. the barrier's movable design was chosen as the waterway it guards is far too valuable in terms of trade to shut off permanently: this single barrier cost $700m and has absolutely no effect on ship traffic[via Deputy-dog]