Satellite images capture dozens of perfectly straight parallel lines hidden in the dense Polish woodlands of Silesia. Experts investigate and uncover a massive WWII military complex once used as a notorious and deadly Nazi weapons training facility for developing the MP44 Sturmgewehr machine gun and training the 13th Waffen SS Mountain Division.

Satellite images capture monumental ruins surrounded by bizarre feather-shaped structures on a Welsh mountaintop; experts investigate a hidden tunnel containing the remains of an abandoned slate mine with an exploitative and deadly past.