Here’s a photo of one of my evil geocaches in the wild. If you look closely enough, you can see it. It went live last week and has already been found several times.
Wikipedia has this to say about geocaches:
For the traditional geocache, a geocacher will place a waterproof container containing a log book (with pen or pencil) and trade items then record the cache’s coordinates. These coordinates, along with other details of the location, are posted on a listing site (see list of some sites below). Other geocachers obtain the coordinates from that listing site and seek out the cache using their GPS handheld receivers. The finding geocachers record their exploits in the logbook and online. Geocachers are free to take objects (except the logbook, pencil, or stamp) from the cache in exchange for leaving something of similar or higher value.
Typical cache “treasures” are not high in monetary value but may hold personal value to the finder. Aside from the logbook, common cache contents are unusual coins or currency, small toys, ornamental buttons, CDs, or books. Also common are objects that are moved from cache to cache called “hitchhikers”, such as Travel Bugs or Geocoins, whose travels may be logged and followed online. Cachers who initially place a Travel Bug or Geocoins often assign specific goals for their trackable items. Examples of goals are to be placed in a certain cache a long distance from home, or to travel to a certain country, or to travel faster and farther than other hitchhikers in a race. Higher value items are occasionally included in geocaches as a reward for the First to Find (called “FTF”), or in locations which are harder to reach. Dangerous or illegal items, weapons, food and pornography are generally not allowed and are specifically against the rules of most geocache listing sites.