Geocaching is a popular outdoor activity that centers around containers secreted away in both urban and remote locations and known only by the coordinates provided by Global Positioning Satellites (GPS).
The sport is a young one, as prior to 2000 the GPS coordinates provided by commercial GPS units were obfuscated. The less-than-precise readings might have been fine for navigating freighters or driving from Chicago to LA, but if you wanted to find something as small as a shoebox-sized container in a national forest, you were out of luck. When the Department of Defense turned off the system (Selective Availability) that introduced random errors into readings made by commercial devices, it suddenly became possible to stash something under a specific tree in Central Park and have others be able to locate it via handheld GPS devices.
So what exactly goes into these containers? While the contents and rules vary, the waterproof containers generally house a log book with a pencil or pen, and a handful of miscellaneous items placed there both by the creator of the cache and its visitors. Common practice is that visitors will either not take anything from the cache or will take something and replace it with something of equal or greater value in order to keep the cache stocked. Caches range from simple one-stop caches to elaborate multi-stage caches that require extensive travel to complete. Anyone interested in getting started with Geocaching would be well served to check out Geocaching.com, a mecca of information about the sport.