The ruins of a mysterious castle on a New York island reveal a hidden arsenal tied to one of America's largest weapons empires. Massive stone walls uncovered along a remote Irish causeway point to a forgotten Bronze Age fortress. A colossal star-shaped mound hidden in the jungles of Samoa raises questions about lost civilizations and ancient rituals. And a ghostly island that appears and disappears in the Caspian Sea reveals powerful forces shaping land in real time.