The catastrophic floods of New York, Bangkok and New-Orleans have shed light on the extreme vulnerability of more than 130 coastal cities faced with the violence of the sea. The mega-cities are threatened by a series of unusual phenomena: a surprising subsidence of soils, an unexplained rising of the level of the sea in some parts of the world, an increase in the frequency of extreme climatic events and exponential urbanization. Considering that science is capable of anticipating dangers and of suggesting protections, why do such disasters occur so often? Can they be avoided?