The lowlands of the northern area of France do not posses karstification or natural caves. French learn at school that there is mining in northern France, the border area to Belgium and Luxemburg has coal mines and to the east some iron ore mines. One of those coal mines at Lewarde is open to the public.
Rather unknown is the fact that the limestone of the area was mined for building and other purposes. This created numerous underground limestone quarries, not as vast as in the Loire area, but still of some size. None of those quarries are open to the public.
During World War II this part of France was most imoprtant for the German Third Reich. It was of enormous strategic importance because of its adjacency to England, which was one of the Allied Forces. They fortified the northern coast of France with the bunker system Atlantikwall and built weapons to raid Enland and especially London from here.
The most important and interesting structures were built in the last two years of World War II. The German military strategists knew they would loose the war without some kind of superweapon. As a result the socalled V-weapons were constructed: Vergeltungswaffe means retaliatory weapon, reprisal weapon, or vengeance weapon. There were actually three of them, V1 was a flying bomb, V2 was the first rocket, and V3 was an oversized ballistic cannon. Beginning in 1943 numerous launching sites were built, most of them in Pas de Calais, to raid London. None of them was completed, as the Allies were able to destroy them. But most of the structures were so compact, their ruins are still there and still impressive. Numerous of them are now exhibition sites or museums and open to the public.