Mt. El Sombroso Loop via Limekiln Trail and Priest Rock Trail
This is a moderate loop trail to Mt. El Sombroso in Sierra Azul Open Space, starting on Alma Road in Lexington Reservoir County Park.
The trail starts at Lexington Reservoir, near Saint Josephs Hill. There are two trail entrances on Alma Road, park at either one and walk between them to complete the loop. Either way, the trail heads uphill immediately and keeps climbing. If you want to hike the steepest sections downhill there isn't a lot of difference which way you go, but the climb up Priest Rock Trail to the Kennedy Trail is more exposed an may be easier going down on a hot summer day.
There isn't much of a view from Mt. El Sombroso—the trail is a utility access road for the power lines—you mostly see Mt. Umunhum and the southern end of Santa Clara Valley. There are nice views of Santa Clara Valley on the approach to El Sombroso, you can't miss them on the way back.
For the really adventurous, the trail continues on past Mt. El Sombroso, with access to Mt. Umunhum or Almaden Quicksilver Park.
Park on Alma Road at either access point. The trails are clearly visible from the road. Geo locations are 37.20018, -121.98016 and 37.196645,-121.983940.
The full loop is challenging in both distance (12 miles) and ascent (over 2,500'). You don't want to have to carry children on the steepest sections.