It was the early 80s, the height of the Cold War, when something strange began happening off the coast of Sweden. The navy reported a mysterious sound deep below the surface of the ocean. Again, and again, and again they would hear it near their secret military bases, in their harbors, and up and down the Swedish coastline.  After thorough analysis the navy was certain. The sound was an invasion into their waters, an act of war, the opening salvos of a possible nuclear annihilation.  Or was it?  Today, Annie McEwen pulls us down into a deep-sea mystery, one of international intrigue that asks you to consider the possibility that maybe, just maybe, your deepest beliefs could be as solid as...air. This episode was reported by Annie McEwen and produced by Annie McEwen, Matt Kielty, and Sarah Qari, with sound design by Jeremy Bloom.  Special thanks to Bosse Lindquist. Support Radiolab by becoming a member today at Radiolab.org/donate.