Thx to Jonathan for pointing out the long history (150 years!) of Letterboxing which was invented Devon UK in 1854. Awesome! I have to admit I didn’t know about it but now I understand how Geocaching came up :). Love the stamps <3 !

Wikipedia:
Letterboxing is an outdoor hobby that combines elements of orienteering, art, and puzzle solving. Letterboxers hide small, weatherproof boxes in publicly-accessible places (like parks)^[1] and distribute clues to finding the box in printed catalogs, on one of several web sites, or by word of mouth. Individual letterboxes usually contain a notebook and a rubber stamp.^[2] Finders make an imprint of the letterbox’s stamp, either on their personal notebook or on a postcard, and leave an impression of their personal stamp on the letterbox’s “visitors’ book” or “logbook” — as proof of having found the box and letting subsequent letterboxers see who have visited. Many letterboxers keep careful track of their “find count”.