Geocaching is and GPS based treasure hunt, with caches all over the world. Several websites list coordinates to hidden containers containing some sort of surprise. Using a smartphone, GPS devices, or just your sense of spatial location will lead you to the general location of the cache. Descriptions, clues, or shear scavenging willpower help narrow down the exact location.
Geocachers may take the contents of the contain, if they replace it with something of higher value in return.
See also: Dead Drops