The Dutch Water Line
Today I've been reading about The Dutch Water Line - interesting to see that effectively the Dutch managed, through massive engineering, to use their system of dykes to turn almost their entire country into a heavily fortified citadel. Also interesting to note that in WW2 the Germans basically then just flew over the water and bombed them anyway, forcing surrender. But still the Dutch held onto the idea and rebuilt a new version to ward off the Russians, who, luckily for the Dutch, never decided to try it out.