Germany's preparations for WW2 began years before 1939. Under various guises, Germany began work on what would become their most technologically advanced front.

In Poland lies a forgotten Nazi complex which could have sheltered up to 27,244 people from Allied bombs, the purpose of which largely remains a mystery.

Hidden in a remote mountainous forest is a structure nicknamed the Fly Trap or Hitler's Stonehenge. Researchers still have no definitive idea what it is.

Outside the town of Politz lie the ruins of what was once one of Germany's most important assets - a place that kept the German war machine moving.

Secrets of Hitler's Castle: When the Nazis took Ksiaz Castle, it wasn't for the view. Secret staircases and lifts were built, while underneath, a vast network of tunnels was taking form. S2 Ep6

Built in 1934, Ordensburg Vogelsang, a virtual Nazi Castle was constructed with the intention of building the New German; elite future leaders of the Reich.

In 1933, Heinrich Himmler signed a 100-year lease for Wewelsburg castle in Büren, Germany. The space became a clandestine training space for the SS.