A Machine in the Walls:

Ignorance does not discourage him, however. Jacob continues to walk the cave floor, watching the wax textures of the stone and the white shapes beyond. He begins to understand the philosophy of their architecture, a machine for living in the non-spaces of post-life.

These visions, and his endless walking, tire Jacob, so he leans on slanting stone and takes a short nap. Time passes quickly, and when he wakes, the light is still shining. In this faux morning, Jacob feels anew the mission that he gave himself at his hives on that last morning in Alamogordo. He knows that now is the time to find the person he is meant to kill.