The Bay Area Ridge Trail (Ridge Trail) is a planned 550+-mile multi-use trail (currently over 330 miles are complete) along the hill and mountain ridgelines ringing the San Francisco Bay Area, in Northern California. When complete, the trail will connect over 75 parks and open spaces. The trail is being designed to provide access for hikers, runners, mountain bicyclists, and equestrians. It will be accessible through trailheads near major population centers, while the trail will extend into more remote areas. The first trail section was dedicated on May 13, 1989.
More info: http://www.ridgetrail.org and http://en.wikipedia.org/wiki/Bay_Area_Ridge_Trail