The original airline navigation beacon on Mt. Diablo was turned off on Dec.
7, 1941. Could be seen miles out to sea. Originally was one of the
beacons airplanes used to navigate across the country. It is lit each year
in honor of Pearl Harbor. Used to have a great view of the mountain, but
now have a tree obscured view.

https://youtu.be/l1zdSEFhSek