In Chapter 31, how does David finally get out of the room where he awakened?

David puts on the uniform of the Immari colonel. The tubes crack and splinter. Then a door to his right hisses as it breaks free from the wall and opens. David walks until he reaches a round room that seems to be an elevator. It takes him to a subterranean tunnel. David touches the walls, and an electric wave sweeps over him and leaves him standing outside in a mountainous place.