Uh, it warn't "below sea level" when it was started. The current depth is the result of several centuries of subsidence , silt deposition, and a number of other factors.
Good point, but if NOLA has been sinking all that time then it was not too smart to continue building in a sinking city. You might call this short-sighted real estate development with no long term plan, similar to home building in the western states in areas with intense fire danger.