Because the Netherlands sits at a low elevation, the Dutch have built walls or barriers to hold back the waters of the North Sea called:

the correct answer is - end permian.

the end of the permian was marked by the biggest mass extinction out planet had ever witnessed, so no wander that this period has been given the nickname of ''the great dying''. around 96% of the species died out, and it was caused by multiple natural catastrophes, including massive super volcano explosions and lava flows, huge amounts of co2 released into the atmosphere, as well as methane from the algae, and that contributed to acidified and stagnant oceans. it is thought that it set life on earth backwards for about 300 million years.