TOWARD THE END of the long day’s drive, there had been an interlude that had posed Zula with a minor brainteaser. The Suburban had been cruising down a highway for some time. She could tell it was a two-laner because of the sound made by oncoming vehicles as they zoomed by a few feet away, and by the fact that it wound from side to side more than a freeway. But at one point they had slowed down, without turning off the road, and descended a long straight slope, losing speed the whole way, and finally come to a halt, still sloping downhill. Nothing had happened for a quarter of an hour or so. Then she had heard the engines of other cars and trucks starting up all around. A series of vehicles had passed them coming up the other way. The Suburban had descended some distance farther, then leveled out, clanking over steel plates, and then parked again. Presently a deep rumbling had started and continued for twenty minutes or so.