Letterboxing is an outdoor pursuit with similarities to orienteering.
A small pot (the letterbox) containing a stamp and visitors' book is hidden on the moor, and a clue is written to lead others to its position. Clues may be as simple as a map reference and list of compass bearings, or may be more cryptic.
When a letterbox is found, the letterboxer takes a copy of the stamp, as well as leaving their own personal print in the visitors' book.
Letterboxing began on Dartmoor but is now popular in areas all over the world.
Dartmoor Letterboxing