As just about any history buff knows, just prior to World War II France set up a series of underground military bunkers on their eastern border, to prevent an invasion of the Germans.
Did they work? Many people say: they did what they were supposed to do! The Germans successfully invaded France, but by driving around the Maginot Line and coming down from the north, through Belgium.
Just about all these bunkers are still in existence, and some of them are now museums that you can visit:
They look rather boring from up on top, just a few gun turrets sticking out of the ground:
But underground, some of them are truly massive, as this sketch shows: