Hollow Nickel Case
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The Hollow Nickel Case, refers to the method that the famous Soviet spy Rudolph Abel (aka Vilyam Genrikhovich Fisher) used to exchange information between himself and his contacts.
The method involved transporting microfilm inside a hollow piece of metal which was engineered to appear like a nickel coin. Ultimately the method led to Abel's capture when the FBI got a hold of one of his hollow nickels that had broken, thereby exposing the enclosed microfilm
