The Maginot Line, France
In preparation for the German assault, the French military built a series of bunkers along the French-German line. The defensive Maginot Line was built to withstand infantry, bombs, and tanks from the German Blitzkrieg.
To avoid the heavy defenses Germany chose to invade through Belgium instead, entirely bypassing the Maginot Line. Its tunnels, shelters, and forts stretch 450 miles still stand today without a single soul inside.