“Foothill BLVD Hillcrest 16 miles 570°”. This the solution of the Mt Diablo cipher, which Zodiac sent on 26 June 1970 to the San Francisco Chronicle. In the letter, Zodiac wrote a 32 symbols cipher and a compass in a map of the San Francisco Bay Area attached to his letter: “The map coupled with this code will tell you where the bomb is set. You have until next Fall to dig it up”.
There are no doubts on the position of the bomb, and not only because in the solution the arrow of the compass of Zodiac points a place between two buildings of the Sheriff’s office in Foothill BLVD. The mass murder used a pair of tricks and a couple of clues to enhance the solution of the Mt – Diablo cipher.
1. Four substitutions in the Mt – Diablo cipher had been already used by Zodiac in the first cipher he sent, cracked by the Harden couple.
2. The H for I is the same substitution I’ve found in the last 18 symbols of the Z408 (here’s the solution).
3. Zodiac used Delta for O and the symbol of degree; and I for O (two times).
The substitutions I-O and J-O are completely in accordance with the solution of the My name is “cipher” (point 42). “Io” is the italian translation of “I” and in the letter of 20th April 1970 is just linked to the word “JO”, diminutive of Joseph.
Zodiac used different symbols for the L probably because the previous cipher of 31th July 1969 had been deciphered also thanks to the “ILL” recurrence, here present twice in a row (to mock the readers, it seems).
The substitutions symbols to numbers follow: the descending alphabetical order Z (-3) → P (7), from which they results W = 0; V = 1; P = 7; the ascending alphabetical order A (-3) → J (6), from which results J flipped=6. The Celtic cross is not a symbol of the Latin Alphabet.
There is a big mistake miles/inches in the Phillips 66 map and Zodiac knew it. It’s the reason why he chose this map. You can read at the bottom: 1 inch / 6.4 miles. It’s completely wrong (almost of x10). But the solution is correct. So Zodiac made fool of the readers on purpose. If you use that indication you can’t lead the solution.
The 0 of the compass of Zodiac was “to be set to the magnetic north”. That implies the declination (+17 degrees) is important to lead the solution.
After having set the 0 to + 17 degrees, the solution is 570 degrees (210 and a turn) on the right, as the solution of the cipher says. In the tip of the 26th July 1970, to help the readers, Zodiac put the M of “Mt” just over the position indicated by the solution.
Finally, adding the four lines of the compass it’s possible see that they represent exactly the distance between Mt – Diablo and Foothill BLVD + Hillcrest.
In the map, the arrow’s tip cross exactly the words Lake Chabot (a lake, again).