There is a challenge related to the survivalist hidden caches which grants XP upon finding six caches. The caches were apparently left behind by Randall Dean Clark, "the Survivalist", whose diary entries can usually be found in the nearby vicinity. They look like duffle bags and contain various items including ammunition, weapons, and apparel.
Six Survivalist Hidden Caches can be found in Zion National Park, primarily in booby-trapped caves.