I lived in Walnut Creek for about 3 years and use to mtb Diablo all the time. I remember one wet fall day riding down a road they had recently graded and was watching the mud pack up on my tire when next thing I knew the front wheel stop and I was ejected over the front of the bike. We literally had to pull the bikes down off the side as the mud would pack up so bad. And a bit of trivia - Mt Diablo is the second peak where you can see the most square miles - Mt Kilimanjaro is first.