... Engineers developed magnificent gates that allowed rivers to flow through the dikes and that could be close with high reliability to fend off an approaching surge. Large parts of cities, like in London, were developed below sea level. One-third of the Netherlands are today below sea level, protected by dikes, barriers and pumps, and two-third of the Gross National Product of the country are earned in the area below sea level, indicating that much of the country's assets are in the area exposed to the hazard of being flooded if the protections fail.

Upper left: The Thames Barrier; taken from .... Lower right: the Rotterdam barrier; taken from http://www.deltawerken.com.