Including an exploration of the claustrophobic prison cells of Devil's Island; Vemork, a vast mountainside complex that held the power to change the world; Salton Sea, where a string of crumbling towns are linked by a toxic secret; and Ralsko, a sprawling secretive base that's been left standing on deadly ground.

Including an exploration of a marooned iron-clad structure in Portland, off the coast of England, that hides a dramatic story of war and resilience; Pratt Cotton Gin Mill, a vast facility in the heart of America's deep south that helped tear apart a nation; and a Mediterranean island fortress at Spinalonga, Greece, that became the scene of a dark conspiracy.