It is described in the first paragraph of the feature as "an obscure and eccentric form of orienteering". Google "letterboxing" for many sites devoted to this hobby.
There is a similar hobby called geocaching where GPS is used.
Questions:
- is this kind of mention valuable to orienteering or not?
- can we learn anything from what interests and motivates letterbox enthusiasts?
- are there other even more eccentric forms of orienteering? (apart from Night O
)