The Zodiac was a serial killer who terrorized Northern California in the late 1960's and early 70's. Although he claimed to have murdered 37 people, only 7 attacks are confirmed to be the work of the same killer. Several Bay Area newspapers began receiving correspondence from people claiming to be the Zodiac. Many of these letters are assumed to be hoaxes, but a few contained information that was not publicly available, leading investigators to believe these to be genuinely penned by the Zodiac Killer himself.

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In 1969, he devised a 408-character cryptogram and sent portions to 3 separate newspapers. The cryptogram was deciphered by a pair of local schoolteachers within a week of publication. The next coded message received from the Zodiac was the "Z340" cipher (pictured above) which has never been decoded, despite the best efforts of cryptography and linguistic experts. It is entirely possible that it was designed to be intentionally uncrackable, in order to foil any efforts by those who would seek to learn the Zodiac's true identity.

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Source: Wikipedia