What were once among the most-advanced engineering projects ever undertaken now lay in abandoned ruins around the world. Experts explore why some of Earth's biggest and most-mysterious constructions were built and what brought about their downfall. 　　S7 E1 · Hollywood Heroes 　　S7 E2 · Stalin's Subway 　　S7 E3 · Disaster at the Maya Hotel 　　S7 E4 · Murder Underground 　　S7 E5 · El Dorado Ca...