AviationFreak steeds perfekt en nog mijn derde taal in de huidige rangschikking... for a little over 7 years I felt a submarine, living in Lisse and working in the Ship's Hole.
I did not argue about the principle, this is cast in iron since NETHERlands was a few centuries back mostly wetlands and marshlands.
The map however does not show solely areas which are (under normal circumstances) below sea level, but also (and mainly) areas which would be flooded in some specific and unfortunate circumstances, IF
there were no dikes/dykes.
It also includes nearly 8,000 sqkm of land reclaimed recently from the sea.
My family originates (15th century) from a place (which also determined our family name) which is shown below sea level on your map.
I can assure you that there was no dike, nor is there one now, and still, at last check I could not find skin between my toes...
The total area (naturally) below sea level ranges from 25 to 70% depending on which website one consults.