The Secret is a puzzle book published in 1982 by Byron Preiss, containing 12 short verses and 12 elaborate fantasy paintings. Readers were expected to pair each painting with a verse in a way that would provide clues to finding one of 12 plexiglass casques buried in various parks around North America. Each box contained a ceramic key that could be redeemed for a jewel worth $1,000. Reportedly, Preiss thought the puzzles would be quite simple to solve, and all casques would be quickly located and unearthed. One of the boxes was found in Chicago in 1983, and another in Cleveland in 2004, but the remaining 10 boxes have never been found.

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On July 9, 2005, the author died in a traffic accident in New York, leaving no further hints as to the location of the remaining casques. Discussion about the book remains lively to this day, though changes in landscaping and property ownership may have rendered several boxes impossible to retrieve.

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The above image and poem are suspected to be hints to the location of a casque buried in San Francisco.

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Source: Wikipedia