During the early 1980s, the Swedish navy detected a recurring mysterious sound deep underwater off their coast. This sound was repeatedly heard near secret military bases, harbors, and along the entire coastline. After analysis, the navy concluded the sound indicated an invasion and a potential act of war. Annie McEwen investigates this deep-sea mystery, filled with international intrigue. The situation prompts reflection on how deeply held beliefs can be challenged. The podcast episode is reported by Annie McEwin with assistance from Magnus Ormstadt. Production credits go to Annie McEwin, Matt Kielty, and Sarah Qari. Original music and sound design were created by Jeremy Bloom. The episode draws attention to external resources like "Atomic Artifacts" and "Neither Confirm Nor Deny." Support for Radiolab comes from listeners, the Simons Foundation, the John Templeton Foundation, and the Alfred P. Sloan Foundation.