The cache is not at the posted coordinates.
This is the bonus cache for the Palouse to Cascades Trail Adventure Lab. I hope you enjoy learning a bit about the history of the trail.
Keep in mind: to find this bonus cache, you must complete all stages of the Adventure Lab first. To get the coordinates for this bonus cache, take the info from all the stages and plug it into this formula:
N 47 AB.CDE W 121 FG.HIJ