What did the Germans hide inside this WWII hill Secret project or just hidden in plain sight

What did the Germans hide inside this WW2 hill Why so much effort to defend this very special location. . Our unique videos takes a ton of time to create so If you like to support our work : My Email : Check out our monthly WW2 diorama giveaway videos in the beginning of each month. Beautiful WW2 dioramas for my supporters. Check out these incredible Robotics skills : My underwater exploration gear : QYSea Fifish V6S V6 and Fifish V6 Expert. With a gripper to recover artifacts from the deep Omni directional and totally awesome for underwater exploration Our flashlight gear :