Letterboxing originated on Dartmoor in the 19th century and has become increasingly popular in recent decades. Watertight containers, or 'letterboxes', are hidden throughout the moor, each containing a visitor's book and a rubber stamp. Visitors take an impression of the letterbox's rubber stamp as proof of finding the box and record their visit by stamping their own personal stamp in the letterbox's logbook. A recent related development is geocaching. Geocache clues make use of GPS coordinates, whereas letterboxing clues tend to consist of grid references and compass bearings.