The jar seen here is typical of the size of a geocache.  Although this one was hidden inside a hollowed log, they can also simply be hidden under things like leaves.  Note the international geocaching logo on the cache jar.  Inside, a log sheet is there for the finder to sign.  Find enough geocaches and redeem goods and activities available during your stay.  (photo from Jim and Sarah's Flickr page: http://www.flickr.com/photos/jimsarahmiller/3411142408/)
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