FizzyMagic's FindStats program performs statistical analysis of GPX files, and reports (among other things) the number of each type of cache found, and the number of caches found of each difficulty–terrain combination. The challenge is to find at least one cache of every difficulty–terrain combination (81 unique combinations), while also finding at least one of each type of cache (ignoring certain extremely rare cache types).
The listing for the first Fizzy Challenge cache includes the actual coordinates, but imposes the additional requirement that the challenge be completed before a find can be logged. The owners of many Fizzy Challenge caches follow this example. Others publish bogus coordinates, and email the actual coordinates to geocachers who send proof that they have completed the challenge.
A fantastic cleahlnge that can only be achieved by the hard work of all: the injured service men and their families; 20 hard working driven horses from just 4 yards; 3 ridden horses; extra equine support at various point along the route; 6 ambulance drivers; grooms and ground crew; maintenance; horse box lorry drivers; escort vehicle drivers; the catering crew; the publicity and donation collection team; all those offering accommodation and stabling along the route; the fantastic flag waving supporters that raise the spirits; not forgetting those back at home keeping the home fires burning!