Geocaching is the use of 24 to 32 GPS satellites (and mobile phone masts) to find 'treasure' - typically Tupperware boxes of 35mm film tubes filled with tat & a log book.
Once you find one you sign the log book and take some tat and leave some tat.
I bought the official geocaching app (£5.99! Thats almost 800 Microsoft points!) and it's ace. It lets you look up ones near you and has it has all the facilities of
the web site as far as I can tell (read descriptions, hints, upload and browse photos etc, post in the online log book etc).
But best of all it has satellite maps to help you find your geocache and it's own compass that points in its direction.
It's staggeringly impressive to have your phone lead you to a hidden tiny box in the corner of a hidden park you new nothing about.
Or in other words fucking hell WE ARE LIVING IN THE FUTURE!