The Reverse Geocache Puzzle
The Reverse Geocache Puzzle is a fiendish bit of fun: a locked box that only opens at a given location, and only gives the distance to that location — forcing the user to triangulate it over repeated attempts. Oh, and there are only 50 attempts allowed. Not only is it (apparently) real, but Mikal also goes on to explain how he built it. Thanks to Richard for the tip.