Geocaching is a GPS-based game that can be played all around the world, whereby a cache is hidden somewhere, and the coordinates and info uploaded to a website for people ('cachers') to try and find it with. All that is needed is some means of pinpointing a location from coordinates, whether it be a GPS unit, smartphone or just a map, and the time to go and hunt the cache down.
Caches come in many forms, from tiny 'nano' caches, through tupperware tubs and ammo boxes, all the way up to crates or barrels. If it's large enough, cachers can find items in the cache called swag, which can be swapped for other items (on a trade-equal-or-trade-up basis), but the main joy of caching is the hunt and the find, with every cache containing a logbook to record the visit. It can also be logged online, with room to details stories about the search.
The treasure hunt aspect can be expanded on, with some caches requiring the solving of clues, or the finding of multiple pre-caches to track down the final hide. There are many other variations, including using webcams to prove your presence, and the photographing of locations or landmarks.
Trackable items can be made which can be taken from one cache and moved to another, often given a specific target location to reach, with people monitoring progress online.
The main official website for Geocaching can be found here
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