This bunker is located above the pacific ocean at a part of the coast called "devil's slide" few miles south of San Francisco. I want to thanks Kevin L. for sending me this tip, this is truly an amazing location. I had the chance to go there and take some pictures, I can tell that the name "devil's slide" is perfectly appropriate!
HIGH ATOP CALIFORNIA’S COASTAL PEAK known colorfully as Devil’s Slide are the crumbling remains of a squat cement building that looks as though it is about to tip off the edge of the summit and slide into the sea. The bunker on Devil’s Peak was originally built during World War II as a triangulation and observing station and was once simply a piece of a much bigger set of buildings and facilities. When in service, a watcher equipped with a set of binoculars would keep watch out at sea and if they spotted any enemy ships they simply radioed a massive six-inch gun not far away which would sink them before they got close. Unfortunately, with the advent of more modern missile defenses the station became obsolete and the entire site was abandoned in 1949, leaving an empty bunker atop Devil’s Slide.