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The concept is really quite simple: someone hides a ``stash'' - usually a large Tupperware container filled with assorted goodies - in an interesting, out-of-the-way place, and records the exact coordinates with a GPS device. Those coordinates, along with a few helpful hints, are posted to the GeoCaching web site. The stash seekers then use their GPS systems to find the treasure. Each person who locates the stash adds an entry to the included log book, takes one of those goodies, replaces it with one of their own, and then re-hides the container.
Please see http://www.geocaching.com/ for details about this exciting pastime, which is also known as GPS stash hunting.
See also: letterboxing