On a red-sand world that will one day be called Mars, there are the Three Lands. The Land Above is a vast desert full of thirst and violence and deadly creatures, where nomadic tribes eke out a bare existence. The Great City of Geb sits in a vast sinkhole, where King Tuon rules; other cities likewise rest in deep valleys carved by the subterranean waters which rush beneath them. Then there is the Land Below, a labyrinth of caverns and tunnels where...