Mountain's mystery has likely sources
There are a lot of stories on the Internet, told around campfires and happy hours about the history of Spook Mountain. You can hear about the American Indians considering it a sacred place, about how the Spanish Explorers and early stagecoach passengers awed at the phenomenon, an eerie white haze emanating from the mountain (some say the valley) on a moonless night. I and librarians and historians that I have checked with cannot find any reference to the phenomenon. Most think that the first published reference to it is in my book “Following Footprints around Yuma, Arizona.” I think that there was an earlier letter to the editor about it. If you want to awe at the mystery, don't read further.
I have identified the five most common sources of the phenomenon. One is eastbound trains. If you look north from the viewing area, on a moonless night with a little haze you can see an eerie moving glow emanating from an eastbound train's headlight. As it rounds the corner, heading south, the train can be seen and light is projected onto Spook Mountain. As the train approaches, the light is hidden by the terrain. Another is traffic southbound on Avenue 16E, one half-mile off it is Dome Valley Road. Light from these vehicle headlights projects onto Spook Mountain. Because of the movement of the vehicles and the variation of the headlights, the projected light gives the effect of movement. Another is traffic westbound on County 5th Street. The volume of traffic is less, but the effect is similar. A more rare source is aircraft landing lights as they approach the Yuma Proving Ground Airport. An even less frequent source is crop duster aircraft. There are other sources.
If you wish to visit the site, travel on Highway 95 to 4 miles northeast of the signal at Fortuna Road, turn right and go 6.2 miles easterly along the old Butterfield Stage line to N32 45.02' W114 21.4'. There is a culvert under the railroad tracks, go through it and continue about 1,000 feet west to the viewing area at: N 32 45.0' W 114 21.5'