I finally finished the 81 Matrix by finding a cache at an almost 800 year old castle and had to climb into the hole they used as a toilet back in the 13th century. Here is my log: http://coord.info/GLN4J0FT.
To complete this matrix you must find at least one cache for each Geocache Difficulty and Terrain rating which begins a 1 Difficulty / 1 Terrain and goes up by .5 to 5 Difficulty / 5 Terrain. There is a total of 81 combinations.
Glad I have finally completed this as it was difficult to find caches that fit the last ten or so spots. It only took me 7 years and 3145 caches!