It was presented by the comedian and author Pete McCarthy until his death in October 2004.
The game consisted of three celebrities identifying four places around the United Kingdom, as well as a supplementary question set in those locations.
The panel have to identify the treasure using up to four lines of a riddle gained by answering the supplementary questions correctly.
It was inspired in part by the book Lights out for the Territory by Iain Sinclair, in which the author walks across London and at one point takes a route that marks an 'X' across the ground.
The questions were written by David J. Bodycombe (and, for one series, treasure hunt author Dan James) and the programme was an independent production by Hewland International for BBC Radio 4.