What potential dangers do you see with creating polders that are below sea level

The biggest problem with the creation of the polders that are below the sea level is the flooding of the same.

Even though the polders are made very carefully, everything is well planned, and all the required measures of long term safety of them are put in action, still there's always the concern of flooding hanging around. Because the polders are bellow the sea level, if by some chance the sea manages to penetrate in them, probably after some natural disaster, than the whole polder will be filled with water and become one huge pool, and because of this water everything in it will be destroyed in just a matter of minutes, thus lots of human lives will be lost, and a huge economic damage will be made.