The “Insulinde” barely had taken a southerly course when an enormous, entirely unexpected ground sea rose up from the depths. It broke with thunderous violence over the ship and seemed to engulf everything. Not much of the “Insulinde,” listing some 70 degrees at least, remained above water…’ This single short sentence from a rescue report presents us with the main danger of the Dutch coast: a ground sea. A ground sea comes on howling at a place and a time that no one suspects it. Every seaman there knows that on the banks and grounds he may well expect such a deadly breaker, and he is on the alert for it, even though there is little he can do against the immense power of the water.
Piet Bakker, Storm op de kust. Reddersvolk tussen de grondzeeën (Amsterdam: Van Holkema & Warendorf, 1942), 36. My translation.