According to Wikipedia, geocaching is an “outdoor activity in which the participants use GPS or other navigational techniques to hide and seek containers (called “geocaches” or “caches”). I learned of
Geocaching is a high-tech treasure hunting game played throughout the world by adventure seekers equipped with GPS devices. The basic idea is to locate hidden containers, called geocaches, outdoors and then share your experiences online. Geocaching is enjoyed by people from all age groups, with a strong sense of community and support for the environment.