With the aid of Nellie's keychain flashlight, the trio finds a plaque dated 1785. A-ha. Exactly what they're looking for.
Near the plaque is a 4x4 square of skulls. Most of them, except for three, have Roman numerals chiseled into the foreheads. We hope this happened after death.
Amy remembers the clue about "Coordinates in a box" (16.42), and realizes this must be what they're talking about. It's not just any box, it's a magic square.
If you look at the picture long enough, you can solve the puzzle yourself before Dan does so a few pages later.
To throw him a bone, we should say that he probably would have solved it quicker were it not for the untimely arrival of the Kabra siblings.
Natalie and Ian have poison darts. Amy, Dan, and Nellie have a backpack and a flashlight. This is not a fair fight.
Thankfully, Uncle Alistair shows up and knocks Natalie to the ground. If there's anything Uncle Alistair is good for, it's pushing over eleven-year-old girls.
They escape and end up in the subway (or as the French call it, the Metro) tunnels.
These tunnels are active, with trains speeding by every four minutes. The kids have to time it just right to race across the tracks and onto the subway platform.
When Dan runs across the tracks, he drops his backpack. He's unable to retrieve it before the train comes, sweeping the backpack—and the only photo he has of his parents—away into the darkness of the tunnels.
Dan tells Amy what the missing skull numbers should be, and she suspects that it might have something to do with Paris's arrondissement system. It's back to the library.
As they do more Ben Franklin research, they come across a "rare grocery list" (16.158) of ol' Ben's. One of the items, Iron Solute, stands out to Amy, but she doesn't know what it is. (You might remember this from Chapter 8).
Nellie's taken Chemistry, and she tells them it's "for metalworking and printing and a bunch of other stuff" (16.166). Whoa, Nellie. Way to pay attention in class.
Dan reminds them that they're supposed to be tracing coordinates, so Amy finds a spot on an ancient map of Paris that matches the coordinates: St-Pierre de Montmarte. A church.