Richard Nixon, then an ambitious young Congressman from California, revealed to the public on this date back in 1948 the existence of the so-called 'Pumpkin Papers,' secret State Department documents that had been stolen by employees there who were also communist sympathizers spying for Russia. These documents (mostly photos of documents on microfilm, actually) came to be called the 'Pumpkin Papers' because the reformed former communist who led authorities to them, Whitaker Chambers, had once hidden them in a pumpkin on his family farm.Contrary to popular belief at the time, however, the papers were not still hidden in that pumpkin when they were handed over in 1948.
The Pumpkin Papers played a central role in the later conviction of Alger Hiss on perjury charges in 1950, charges related to his alleged spying. An urbane, well educated man, Hiss personified the East Coast Establishment that Nixon was reputed to have so loathed. Hiss denied having been a spy, and in later years became somewhat of a martyr figure to left leaning intellectuals during the Cold War, who thought Hiss had been railroaded by grasping neanderthals on the House Un-American Activities Committee (HUAC).
In 1985, a very high level KGB defector named Oleg Gordievsky confirmed that Hiss had indeed been a spy. And after the end of the Cold War, when KGB files were opened, further documentary evidence was found confirming that Hiss had been a well-established Soviet spy.
That being said, Wikipedia has a great epilogue to this story. It reads, "The five rolls of 35 mm film known as the 'pumpkin papers' were thought until late 1974 to be locked in HUAC files... On July 31, 1975, as a result of ... follow-on suits filed by Peter Irons and by Alger Hiss and William Reuben, the Justice Department released copies of the 'pumpkin papers' that had been used to implicate Hiss. One roll of film turned out to be totally blank due to overexposure, two others are faintly legible copies of nonclassified Navy Department documents relating to such subjects as life rafts and fire extinguishers, and the remaining two are photographs of the State Department documents introduced by the prosecution at the two Hiss trials, relating to U.S./German relations in the late 1930s."