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. S4 E1 · The World's Strangest Ghost Town S4 E2 · Palace of Death (Aerotrain) S4 E3 · Guernsey Nazi Towers S4 E4 · Bodie USA S4 E5 ·...