For 142 years walkers have searched out the tiny containers - or letterboxes - that are now hidden all over the 365 square miles of wilderness that makes up the Dartmoor National Park.
The letterboxes are hidden under rocks, in holes in the ground, near landmarks - and occasionally in pubs. They contain a rubber stamp, which enthusiasts collect in a personal book to show they have found them.
Letterboxing was begun by Dartmoor guide James Perrott in 1854 who placed a bottle at the remote Cranmere Pool and circulated clues as to its whereabouts. Dartmoor's peculiar type of treasure hunt had a slow start. Perrott's "letterbox" remained the only one until around 1932. By 1969 there were 13, and eight years later just 32. However, since then the number has mushroomed and there are now 3,500 to 4,000 - nobody is exactly sure - with thousands of enthusiasts hunting for them.
Pat Clatworthy, who has collected 22,000 of these rubber stamps, loves it. "There is a wide cross section of people who take part. You can get a nine year old talking happily about boxes he has found with a doctor or solicitor. It also gives the chance to visit parts of the moor that are off the beaten track."
Sam Mulligan, a 45-year-old HGV driver from Newton Abbot, says he first became interested in letterboxing he and his wife visited Dartmoor simply to walk their dog. "The attraction is that you get out on your own into some of the most remote areas."
Such popularity, though, is taking its toll. Rather than follow well- laid paths clearly marked by the park authorities, letterboxers prefer to go it alone, using compasses and ordnance survey maps.
The park authorities are objecting. The invasion of remote and often environmentally sensitive sites is of increasing concern, especially since birds such as the golden plover and the ringed ouzel have been badly affected, their nesting areas disturbed by enthusiastic letterboxers. As a result, this summer, devotees of what is becoming a craze face a revised code of conduct.
At the moment, anybody can put a letterbox out and give clues to a few other people as to its whereabouts. A more formal arrangement also exists with a letterboxers' Bible - a catalogue of Dartmoor letterboxes and a list of clues - that is published each year. The national park wants less ad hoc placing of boxes and a more regulated system. The authorities also want to draw up new rules to place a time limit on boxes and to provide guidance on where new boxes should be sited.
The letterboxers, though, are as unhappy as the authorities. At the moment the ultimate sanction of limiting the number of boxes has been held in abeyance, but even so the new rules are only reluctantly being accepted.
Pat Clatworthy, the conduit between the park authorities and letterboxers explains: "We've always had a good relationship with the park authority, but the truth is they don't like the unofficial nature of letterboxing. They like everything to be regulated, but the fact our pastime isn't is a great part of the attraction."
She isn't convinced by the park authority's argument over damage caused by numbers. She maintains that although thousands of people take part, few are on the moor at any given time. Tourists like to have a go, but the number of die-hard enthusiasts is much smaller. Even so, the 100 Club, which issues a badge to those who have amassed a century of boxes, has 11,000 members.
The word "club", though, is a misnomer - there is no formal organisation, and no regular meeting area. On the Sunday the clocks change, however, letterboxers traditionally meet at the Prison Officers' Club in Princetown to swop clues and discuss exploits.
The lack of formality lies at the heart of the park authorities' concern. Jeff Haynes, assistant park officer, explains: "We see letterboxing as an appropriate activity within the national park, but we wanted to look at how we could effect rather more control than has been the case in the past."