Mount Diablo as seen from Pleasanton Ridge overlooking Tri-Valley
Interesting facts about Mount Diablo from TrailStompers (bold emphasis mine):
The “Devil Mountain”
According to the most widely accepted story, the reference to “diablo” or “devil” can be traced back to 1804 or 1805 when a Spanish military expedition visited the area in search of runaway mission Indians. At a willow thicket near present-day Buchanan Field, the soldiers encountered a Village of Chupcan people and surrounded it. But night came, and evidently all the Indians escaped unseen. Angry and confused, the Spanish called the site “Monte del Diablo,” or “Thicket of the Devil”. Later, English-speaking newcomers mistakenly assumed the word “monte” to mean “mountain” and applied the title to this prominent East Bay peak. A linguistic accident thus gave California its “Devil Mountain.”
The “Mount Diablo Meridian”
Just after the Gold Rush, federal land surveyors began the momentous task of surveying out the lands of the Wild West. To do this, reference points were chosen and lands were surveyed with respect to those “initial points” as they are called. Lines called “meridians” (north-south lines) and “base lines” (east-west lines) were extended out from the initial points. Because of the tremendous distances from which Mount Diablo was visible, in 1851 it was selected to be the initial point for the federal land surveys for northern California and Nevada.
Ignoring the excitement of the Gold Rush, Leander Ransom and his men erected a flagpole at the summit of Mount Diablo and began to extend the base and meridian lines that are used to this day in our official land surveys. As a matter of fact, Mount Diablo base and meridian lines are referred to in legal descriptions of real estate throughout two-thirds of California and parts of Nevada and Oregon!