The "battle" of Verdun lasted 10 months; it was less of a battle in the usual sense of a single let's-fight-and-loser-runs-away event, and more like an open-air industrial blender made of shrapnel and bullets into which a continuous stream of mostly innocent people were ordered to walk over a long, long period of time.
The Somme was similar, but worse because it was bigger. All war is hell, but World War 1 was much, much worse.
It was mostly abandoned when the French realised how little resistance similar structures in Belgium had offered to German heavy artillery. The Germans took it without much of a fight, then the French took it back a few months later much the same way.
Fort Douaumont was one fort that fell along with Fort Vaux which saw some hellacious fighting in it's corridors. Just made a post about it after seeing this one:
https://lemmy.world/post/11304986