Approximately two dozen American diplomats in Havana, Cuba, first heard piercing, metallic hissing in 2016. They began experiencing nausea, vertigo, headaches, and other cognitive ailments, but doctors haven't been able to figure out what could be making them sick.

Theories include the use of sonic weapons launched by hostile foreign actors like Russia. But the CIA has dismissed that idea, saying most cases of "Havana Syndrome" actually stem from other pre-existing medical conditions. Still, they acknowledge that some cases are unexplainable.

The new investigative podcast "The Sound: Mystery of Havana Syndrome" explores what we know about this mysterious illness – and what questions remain.

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