The white skeleton of the Maeslantkering, the massive floating water barrier protecting Rotterdam from high seas, dazzles in the sunshine. This engineering marvel is one of the largest moving structures in the world. Each arm is as large as the Eiffel Tower and weighs twice as much. If a storm surge above three meters is anticipated in the port of Rotterdam and its hinterland of 1.5 million people, the Maeslantkering automatically starts to close, flooding the two arms which move the barrier itself into place and dropping it into the waterway to form a watertight seal. The yearly test closing of the barrier, which seals the entrance to Europe’s busiest port, costs up to 30 million euros ($40 million).