Mt D has always stood as the most significant landmark in the East Bay, towering over all of the other Ridges of Danville around it. However, for whatever reason, it's always been under-appreciated from a caching perspective... very few cachers come up here. Our theory has long been that it has two issues -- it costs $monies for park fees when you drive up, and it's a long drive up a windy mountain road.
This cache series solves both of those problems -- introducing the Mt Diablo 80 "Free Way" to the summit. Hidden roughly along the Summit Trail of Mt D, it takes you from the foothills of Diablo, all the way up the Mt, climbing over 3000ft of vertical from bottom to top. And since it's a hike, it bypasses all of those parking fees levied by the local rangers - it's "Free". There's over 20 caches part of the MTD80, plus several dozen easily accessible along the way.
This cache is hidden on the Mt D Summit Trail -- rather, it's hidden UNDER the Summit Trail, in the rock retaining wall that holds up the trail. It shouldn't be difficult to find, if you look at it from below -- but you can find it from trail level too, if you don't mind blindly sticking your hands into rock holes on the wall (not always wise, in this territory).
Note: most of the caches in this series are fairly simple finds. However, the terrain rating of each is set with the assumption that you have driven up the hill, and parked in a nearby parking spot. However, if you are hiking up the Free Way from the bottom, consider this a 4.