What did Thomas have them do the maps in chapter 42?

In a hidden storage closet are eight cardboard boxes, one for each map section. Thomas grabs a box and begins looking at them. He explains his theory that the maps will spell something out. Minho insists they have tried everything. Thomas asks for wax paper, pencils, and markers. Frypan isn't happy about handing over his wax paper with supplies running so low but when he is told what it is for, he relinquishes it immediately.

Thomas instructs them to begin tracing the maps from the past ten days or so. When they are laid over each other, he believes, they may see something. As they do Thomas worries that this may all be a waste of time. He sees Teresa working diligently. She seems confident and this gives him hope.

Thomas orders them to place the sheets on the table in order. As they do so the lines begin to match up. Sitting in the exact center of the page is the letter F.