I have been thinking about the "Mt. Diablo Code" for the past few days, prompted by some interesting ideas recently presented within the community of armchair "Zodiologists." I believe I have a novel and plausible solution to this fifty year-old riddle, and here offer a brief summary.
Between November 1969, following his last known murder, and July 1970, the Zodiac killer wrote a series of letters to the San Francisco Chronicle referencing a "death machine” he had purportedly devised to blow up a bus full of schoolchildren. This was supposed to be a Rube Goldberg-style contraption comprising a photoelectrically-controlled string of bombs, buried at an unrevealed roadside location. The series of threats tailed off uneventfully as Zodiac switched to a new tease before going silent for several years. A glance at diagrams of the “death machine” make it obvious in retrospect that it was all a put-on designed to extend Zodiac’s reign of terror over San Francisco and mock the gullibility of his audience. I think there was another purpose, too — to keep up the Zodiac’s familiar game of hinting at his identity.
In his recent book, The Early Years - Part I, Steve Hodel revisits a sensational but largely forgotten San Francisco crime, the 1921 kidnap-murder of Father Patrick Heslin, known as the "Flapjack Murder." The odd moniker comes from the fact that the burial site of the abducted priest was allegedly revealed through a bizarre "clew" involving a roadside billboard portraying a "desert rat" frying pancakes, located by Pedro Road, a coastal mountain pass traversing from Half Moon Bay to Pacifica.
Among other lines of evidence, Hodel points out interesting parallels between the Zodiac's "bus bomb" nonsense and a fantastical aspect of the Flapjack case involving a purported "death machine" or "infernal machine." This was, supposedly, a string-activated roadside contraption that would fire a salvo of shotgun shells to shoo away law-enforcement during a ransom pickup. Steve Hodel's father, George Hill Hodel, was 13 going on 14 at the time of this crime and the associated press furor. If he was the Zodiac (and I think he was), it's conceivable that he could have reached into his childhood memories of the story in formulating the bus bomb hoax. Steve Hodel goes beyond that and boldly proposes that the child prodigy was in fact the assumed "accomplice" of the man convicted of Heslin's murder, William Hightower. This never-apprehended Hightower “pal” was the heavily-disguised motorist who abducted the priest — described as a small, dark, foreigner with a "refined, ladylike" voice. Having reviewed hundreds of contemporary news accounts about the case and the trial and given the matter much thought, I believe this theory may well be correct.
Back to the bus bomb. In a communication dated June 26, 1970, the Zodiac set a famous, unsolved brainteaser about the secret location of his imaginary roadside bomb, which he dubbed the "Mt. Diablo Code." His puzzle is clearly inspired by Edgar Allan Poe's short story, "The Gold Bug," featuring a cipher along with veiled cartographic hints involving an origin point and compass bearing. Zodiac provided a marked up gas station map of the San Francisco Bay Area, identifying Mount Diablo as the origin point — analogous to the "Devil's Seat" in "The Gold Bug." Historically Mount Diablo has served as the "initial point" for large scale land surveys, so this was a clever riff on Poe's story. Zodiac drew a compass rose over this origin, with an annotation, "0 is to be set to Mag. N.," implying that any compass bearing used in solving the code should be adjusted for magnetic declination. That would be a correction of about +17 degrees given the location and year. A letter accompanying the map included a 32-symbol cipher (the "Z32") and stated: "The map coupled with this code will tell you where the bomb is set. You have untill next Fall to dig it up."
Zodiac's "Little List" letter, sent one month after the map and code, added additional hints. The most definitive was the statement, "The Mt. Diablo Code concerns Radians & # inches along the radians," implying that a location was somehow being specified in terms of polar coordinates, in units of map inches (relative to Mount Diablo) and radians (relative to Magnetic North.) The letter also features a hand-drawn compass rose annotated with "SFPD=0" so that the "0" looks like a hint to the compass bearing of the bomb. As it happens, that bearing is a very good approximation to 17 degrees plus 4 radians. The diagram is also annotated with "=13" in such away as to suggest a victim count — but it could be read as "Zodiac=13," which I find quite interesting.
Zodiologists Richard Grinell, "druzer" and "cragle" have proposed that we make an educated guess that a whole number of map inches and radians is entailed in the solution, and see where that leads and if it helps us solve the cryptogram and we get an "a-ha" moment where it all comes together. I agree that is a reasonable line of attack. Interestingly, the Z32 uses the same cipher alphabet as the Zodiac's earlier Z340 cipher, whose key was finally broken in 2020, with the exception of two unknown symbols. All three of the aforementioned commentators favor a solution of 5 map inches and 4 radians, based mainly on the fact that few other whole-number solutions fall on land within the area covered by the supplied map and, amongst those, the pinpointed location near the city of San Francisco seems most pertinent. I find this logic reasonable. I was intrigued to find that this (5,4) solution points to somewhere immediately southwest of San Francisco. Compatible locations seem to have no known connection to Zodiac activities, but they are basically at Ground Zero for the 1921 Flapjack Murder (Pacifica, which encompasses what used to be known as Salada Beach, the Colma necropolis and Thornton State Beach in Daly City.)
I've seen a couple of attempts to use the (5,4) coordinate guess to coax out a Z32 cipher solution which I do not find compelling. Below, I present my proposed solution, referencing the Flapjack "clew" from 1921. If I’m on the right track, the joke is on us.