Geocaching is a sort of global treasure hunt based on GPS technology. On May 1, 2000 the GPS system turned off its Selective Availability "feature" -- a system that intentionally introduced errors into civilian GPS access, in order to confound potential attackers. When that happened, the GPS system suddenly became reliable and accurate enough for non-military personnel to pinpoint specific locations.
A day or two later (sources disagree), Oregonian Dave Ulmer placed the first geocache (then called a "GPS stash") and posted its coordinates online:
N 45 17.460 W122 24.800
"Lots of goodies for the finders. Look for a black plastic bucket buried most of the way in the ground. Take some stuff, leave some stuff! Record it all in the log book. Have Fun!
"Stash contains: Delorme Topo USA software, videos, books, food, money, and a slingshot!"
By May 6, 2000 Ulmer's geocache had been found by two searchers, and one had logged the find in the included logbook. This event set in motion a new worldwide hobby/game in which geocaching enthusiasts hide caches (typically in waterproof plastic containers), post the location online, and wait for others to find them.
In the years since 2000, the hobby has evolved an organized online community, prompting users worldwide to strap on their backpacks and leave the house, GPS in hand. Some objects, including Geocoins and Travel Bugs, travel between caches -- seekers pick them up and move them from place to place, (usually) logging their activity in the provided logbook. Similarly, Geotokens are objects that can be traded for other items or kept for sentimental value.
Geocaching has its own set of ethics and policies, many of which are documented at GeocachingPolicy.org. Policies range from specific guidelines about retrieving and hiding items to specifics about environmental impact. Seekers should also be careful not to run afoul of Johnny Law when finding a cache -- according to Wikipedia's excellent Geocaching page:
When geocaching in busy locations, searching for a cache can require tact and craftiness to avoid the attention of the general public (also known by geocachers as "muggles"). The person hiding a geocache frequently takes this into account so that the hider and those looking for caches will not cause undue alarm. When care is not taken in hiding or finding a geocache, cachers have been approached by police and questioned when they were seen as acting suspiciously. Other times, investigation of a cache location after suspicious activity was reported has resulted in police and bomb squad discovery of the geocache. A number of caches have been destroyed by bomb squads.
Readers: have you participated in the hobby? Share your geocaching stories in the comments!