Casque

Each of the twelve puzzles in The Secret are intended to lead the seeker to the location of what Byron Preiss called a casque, an unfired clay box designed by JoEllen Trilling and containing a clay key. Preiss’ use of “casque” is somewhat unusual, as the term is usually used to refer to a metal helmet of the sort worn by conquistadors.

Although produced from a single mold, each casque is uniquely decorated, in three fashions.

The inside lid displays a specific time reading on a clock face,
a single figure on the exterior is colored,
and each key contained therein is painted a different color.

These customizations make it possible for genuine casques to be differentiated from hoaxes, as the decoration choices made for each casque remain confidential. As protection against the elements while being buried,

As of 2025, three of the treasure boxes have been recovered. The first was found in Chicago, Illinois; the second in Cleveland, Ohio; and the most recent treasure box was found in Boston, Massachusetts. The remaining nine treasure boxes have not yet been recovered. The Boston treasure box's recovery was filmed for Discovery Channel's television show Expedition Unknown and aired on Wednesday, October 30, 2019, a recovery which only occurred due to the puzzle being featured in two previous episodes of the series.

Over time, some of the cached casques may have been destroyed, or been built over, as was happening with the Boston casque, which was buried in Langone Park, in Boston’s North End which was undergoing renovation, resulting in the casque being dug up by an excavator. Clues would no longer match in cases where the surrounding area has changed since the book was originally published. Similarly, boxes in like New Orleans (which faced mass destruction due to Hurricane Katrina in 2005) and New York City (which has undergone major renovations since the 1980s) may have become extremely difficult to locate.