A forgotten city in the cliffs of Colorado and a desolate complex of windowless buildings in the Rust Belt now stand in ruins; experts investigate the secrets of these deserted American mysteries to uncover how they were built and why they were abandoned. This episode is a mashup of S07E08 "Ruins of the Rust Belt," S05E06 "Secrets of the Doomsday Caves," and S04E07 "Alien Hell Portal."