"Hill 640" is the miitary designation for a cluster of five World War II-era observation stations located at the former White Gate Ranch overlooking Stinson Beach, California. Commonly mistaken for machine gun nests, these stations actually served as the eyes and ears for Coast Artillery batteries located at the entrance to San Francisco Bay. Because of their role in directing artillery fire, the structures were technically called Fire Control Stations.

During World War II, soldiers inside the stations continually scanned the horizon looking for the approach of enemy ships. Had any appeared, the soldiers would have telephoned range and compass bearing information back to their gun batteries. Three or four men were assigned to each station, taking turns watching through the telescope or standing guard duty nearby.

No enemy ever attacked San Francisco and the crews' only real experiences in rangefinding occured during target practice sessions. Following the end of World War II, the coastal guns were scrapped and the fire control stations became obsolete. Today, these five stations and the remains of a nearby radar site are preserved as part of the National Park Service's sprawling Golden Gate National Recreation Area.