California, United States
Mount Disappointment is a mountain in the San Gabriel Mountains in Los Angeles County, California with a summit elevation of 5960 feet. It was named "Disappointment" in 1894 when USGS surveyors sighted it from Santa Susana Mountain, believing it to be the highest point in the immediate area, decided to use it as their next triangulation point. When they reached the summit, however, they discovered that San Gabriel Peak half a mile to the east was 167 feet higher and so moved there instead.
A Nike missile site was located there in 1955 and the summit was flattened to accommodate it. In 1965 the site was converted into a military radio relay site.