Packard steps over to a small wooden hatch set low into one of the hut’s exterior walls. Next to it sits an office tray with a cup hook screwed into each end, and a string tied to each cup hook. One of the strings is piled up loose on the floor. The wall hatch has been slid shut on the other string. Packard puts the message slip on top of a pile of similar ones that has accumulated in the tray, then slides the hatch open, revealing a narrow tunnel leading away from the hut.