Some of the world's most nightmarish places are abandoned buildings and failed engineering projects; and using cutting-edge tech, experts make new discoveries to reveal the secrets of how they were built and why they were abandoned. This two-hour episode uses segments from various past episodes with cold weather as a common trait.

Underground bunkers and subterranean labyrinths entomb dark secrets of the past waiting to be discovered; experts investigate the mysteries of these astonishing feats of engineering to uncover how they were constructed and why they were abandoned.