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.
For the answer to the question above, I would say that the Netherlands
was one good example of what are the dangers when creating polders that
are below the sea level. The water always goes where there is a lower
level of the land. That's when the flooding occurs. You can't have a
flood on the mountain. Water will always find its way to lower areas of
land.
