Lookout Mountain, a 6,812-foot peak on the shoulder of Mount San Antonio in the San Gabriel Mountains, served both practical and scientific purposes. In 1914 the first fire lookout in the Angeles National Forest was built there and remained in use until it burned in 1927. The mountain later became closely associated with Albert A. Michelson’s experiments measuring the speed of light from 1925 to 1929. Those tests relied on a highly accurate baseline established by the United States Coast and Geodetic Survey in 1923 and triangulation to Mount Wilson Observatory. Surviving concrete piers connect the site to a landmark achievement in modern physics.