The hike starts by the south entrance to Mt. Diablo State Park and follows the Summit trail past the Live Oak camping area. From there it's just a short walk to the Rock City area in the park which is marked by a wide variety of rock structures including the so-called 'wind caves'. These were actually formed through erosion by flowing water creating large hollowed-out areas in the sandstone.
A prominent feature is Sentinel Rock that stands well above the surroundings and provides a panoramic view from its summit. There are steps carved into the rock and iron railings at key points for safety when climbing to the top.
Although much of this trail is shaded, it can still get quite warm during the summer months. Carry enough water. Drinking water is available at Live Oak camp and at Rock City but those are over two miles into the hike.
The trailhead is reached by taking Diablo Road east from Danville and turning left on Mt. Diablo Scenic Drive. This takes you past Athenian School and a residential area (very rough road in spots). The trail is on the left side when you pass the last private residence. There are several small turnouts available along the side of the road for parking (no fee).
I just recently did this as a loop (up to summit, back to rock city) very nice hike. The rock city area is awesome ! even if you are not looking for a hike, rock city is a great place to just climb rocks for a good hour or two to clear the mind. Took summit up right after the recent fire and you can clearly see the impact that it had. NOTE: there is another trail that circles the top of the summit called the "marry Bowerman trail" its a short 0.7 loop around the top. its a must-do trail after you visit the visitor center. make sure you grab a brochure from the gift shop for this trail, as it will be your personal tour guide. Anyway, this is one of many hikes hikers should try on the Mt. Diablo experience, NOTE 2: the Live oak campgroundis right next to rock city, would be a great place to camp with family and a great day of adventure to take this moderate hike up and down. Visited on Nov 11, 2013