Basically, I've been doing variations of Lobster's post above for the past month or so, but didn't hit upon the right combination. Congrats to Gisela!
Here are a few indicators of the solution I found within the book itself, along with confirming indicators from the Facebook clues:
1) There are multiple indicators to look on the edges: "...perched as she was on the edge of the settee. She was sending me invisible panic signals" (Girls version, p32). "...we saw symbols etched vertically along the edge of the fissure: form top to bottom..." (Guys version, p41). Also searching along the shore (which is another edge) in Shorebird park.
2) The specific start location may be indicated by the phrase "ten subs" read vertically downward on the left edge of lines 6-13 on page 32 in the girls' version, and a similar phrase "thnwstbs" on the left edge of lines 6-13 on page 34 in the boys' version ("ten subs" is disguised as "thnwstbs" by replacing the e with h, space with w, and u with t). This indicates that there are ten letters to find in each version. "subs" = submarines that can dive downwards, so it can also imply vertical descent.
3) I interpreted the 8/31 clue "After the end of "The Pig War" on November 25th 1872 the plot truly starts to thicken" as a pointer to look just after the "ten subs" location, since according to Wikipedia, "On November 25, 1872, the Royal Marines withdrew from English Camp." So after the sub[marine]s that's where to start looking.
Anyhow, that's a sample of things that I'd found.
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