Sierra Azul Open Space Preserve is a large protected area in California managed by the Midpeninsula Regional Open Space District, covering about 19,300 acres in the southern half of the Santa Cruz Mountains. Named for the Sierra Azul, or Blue Mountains, it includes prominent summits such as Mount Umunhum, Mount Thayer, and El Sombroso. Trails and old roads connect its western and eastern sides near Lexington Reservoir and Hicks Road. The preserve also protects the upper watersheds of Los Gatos Creek and Guadalupe Creek, and its varied terrain ranges from wooded canyons to chaparral hills and open meadows, with many accessible sections offering miles of routes and broad views.