Around the "sugar cube"

The "sugar cube" on Mount Umunhum southwest of San Jose, California

The historic “sugar cube”, a cold-war radar tower, is located on top of Mount Umunhum (3,486 feet or 1,063 m) in the Santa Cruz Mountains in California. This landmark cube can be seen from all over Silicon Valley and neighboring locations around San Francisco's South Bay. By making it to the top of Mount Um—as the mountain peak is called for short—you will have panoramic views of two bays: San Francisco Bay and Monterey Bay [1,2].

Stairway leading from the summit parking lot to the cube

From the summit parking, you may want to experience the scenery by taking the stairs up to sugar cube structure, where platform. From the cube's south side a downhill trail leads back to the parking area. At various site you will find illustrative information panels to immerse yourself in the local history to get engaged with the areas geography, ecology and recreational opportunities. 

Mount Umunhum Summit Shelter with viewing platform

Inspired by this exceptional place and the rich contextual displays, I have selected some topics I am sharing in the following. 

Once a military community

When I was living on the San Francisco Peninsula in the early 1990s, the sugar cube and its surroundings was still “off limits” —being a fenced-off Air Force Station from 1957 to 1980, which continued to be closed to the public until 2017 while proceeding with clean-up: removal of  3,000 cubic yards of hazardous materials, including lead paint, asbestos, fuel storage containers, polychlorinated biphenyl (PCB) transformers and other waste [2]. Finally, in 2025 I made it to the top and visited the site. 

During the time of the operation of an early warning radar station (1957-1980) hundreds of people lived in military housing. A panel describes the live of the military community at the Almaden Air Force Station: 

People recreated and entertained on the summit of Mount Umunhum in in the swimming pool, bowling alley, dining hall, non-commissioned officers' club, and outdoors. Military families who lived here appreciated the remote feeling and the magnificient views. Some people often drove to the valley below while others found all the community and recreation they needed right here at “home.” Children who took the Air Force bus to schools in the vallay often experienced a dramatic change of climate, sometimes leaving a snow covered mountain and arriving at a warm schoolyard below. This rich, thriving community forever shaped people's lives, as heard in the unique stories told by those who lived here.

Almaden Air Force Station swimming pool on the summit of Mt. Umunhum (displayed at kiosk near summit parking lot)  

Remnants of the swimming pool are still visible on Mt. Umunhum. 

Now an open space preserve 

The preserve around Mt. Um is named for the Sierra Azul: Sierra Azul Open Space Preserve. Sierra Azul means “Blue Mountains”. This is the name the Spanish-speaking colonist gave to the southern range of the Santa Cruz Mountains.

Sierra Azul with Mt. Um and “sugar cube”

The preserve protects more than 19,000 acres of wilderness and features 26 miles of multiuse trails for hikng, dog walking, biking and horseback riding. It is prime mountain lion habitat and also proveds homes for bobcats, coyotes, deer and many other animal species [3].

Mount Umunhum wildlife

The “Welcome to Mount Umunhum” panel summarizes Mt. Um's plant and animal life:

Visitors will discover that a bounty of wildlife lives among the many plant communities and habitat types here. The chaparral zones on the mountain are characterized by dense, impenetrable scrub and dominated by chamise, big berry manzanita, birchleaf mountain mahagony, and poison oak. Chaparral is usually found on dry, southern-facing slopes that endure long periods of daily heat from the sun. Gray pines and knobcone pines offer limited shade beneath their sperse canopies to may plants including golden fleece, venus thistle, and foothill penstemon.

While chaparral occurs mainly on south-facing slopes, cooler and moister north-facing slopes are typically covered by mixed evergreen forest composed of bay laurel, coast and canyon live oaks, madrone, and big leaf maples. Their shade attracts California nutmeg (a coniferous evergreen tree), western heart's ease (a violet), elk clover (not actually a clover, but a herbaceous plant), ground rose (a coastal shrub with prickly stems), and many smaller plants.  

The majority of animals living on Mt. Um belong to invertebrates, birds and small-to mid-sized mammals. The bird life is abundant:

Anna's hummingbird is present year-round, but seasonal hummingbird visitors include Allen's, rufous, calliope, and black-chinned, which can be seen in the spring. California quail, California trasher, spotted towhee, fox sparrow, wrentit, and blue-gray gnatcatcher forage around the chaparall for their meals. California scrub jay, Steller's jay, northern flicker, dark-eyed junco, varied and hermit thrush, band-tailed pigeon, western screech owl, and Cooper's hawk fly through the mixed evergreen forest. Look up to spot golden eagle, hawks, and common raven soaring high above the summit.

A sacred site: reaching out for balance and harmony

Mount Umunhum holds great spiritual value for local tribal bands such as the Amah Mutsun and Muwekma. This mountain is where visions are sought and communion with the spiritual world is made. It is also where relationships with ancestors and future descendants are their strongest, where Hummingbird serves to remind those who see the mountain of the old teaching. It also serves as a place of ceremony and prayer even today. The Amah Mutsun hold ceremonies here because it is close to their Creator, because of its resonance to the spiritual self, and because it is where the children of their ancestors continue to reach out for balance and harmony. 

Getting there

Here is a link to a map showing the Mount Umunhum Area with roads, trails, utilities and sites of interest;

https://www.openspace.org/sites/default/files/Um_Trail_map_0.pdf .

Take Mt. Umunhum Road to drive up to the summit or park you car at the Bold Mountain Parking Area and use Mt. Umunhum Trail to approach the summit. 

The following map puts Mt. Um and the Sierra Azul Open Space Preserve into context with other locations in the Santa Cruz Mountains: 

Map with Santa Cruz Mountans including Sierra Azul area and “You are here” pointer locating Mount Umunhum

References and further reading

[1] Mount Umunhum: Exploring San Jose's Historic Cube. California Through My Lense by Josh. URL: https://californiathroughmylens.com/mount-umunhum/ (accessed January 18, 2026).

[2] Opening Mount Umunhum. Sierra Azul Open Space Preserve | Opening 2017. Midpeninsula Regional Open Space District. URL: https://www.openspace.org/sites/default/files/OpeningMountUmunhum.pdf (accessed January 19, 2026).

[3] Sierra Azul Preserve. Midpeninsula Regional Open Space. URL: https://www.openspace.org/preserves/sierra-azul (accessed January 19, 2026).

View of the San Jose area