Mt. Umunhum is within the Mid-peninsula Regional Open Space District's Sierra Azul Open Space Preserve. The 3,486 ft high summit was once the Almaden Air Force Station (USAF 682nd Radar Squadron) with a radar surveillance post which operated from 1958 to 1980. The "Cube" is a five story concrete tower which had an 85 ton AN/FPS-24 radar dish on top - designed to detect hostile aircraft or missiles during the Cold War. However, it was made obsolete by satellite reconnaissance.
Long before that Mt. Umunhum was a sacred site for the Ohlone tribe (whose modern descendants are the Amah Mutsun and Muwekma tribal bands). They have many stories concerning the pivotal role of Mt. Umunhum in the creation.
In 1986 MROSD bought the 44 acre site for $260,000 and no development was done because the federal government promised to remediate any hazardous contaminants, but failed to do so until 2009 when federal money was earmarked to clean up the site, which demolished the recreation center, the pool and bowling alley, the commissary, the motor pool, the electrical generator plant, and all the residences - everything except the Cube, which was deemed a site of historic significance by the Santa Clara county supervisors. Measure AA approved by the voters in 2014 provided $15.8M dollars to provide public access.
MROSD also put in a ceremonial prayer circle of large stones placed without mortar and the four cardinal points of the compass are marked. The four directions have significance to the Ohlone.
You can hike the 4.5 mile trail from the Bald Mountain Trail parking or drive up Mt. Umunhum Road and park just below the summit and hike the 159 stair steps. The summit, Cube, and prayer circle are ADA accessible.