It was just before six in the morning when we stopped at a rest area off Interstate 8, near where California, Arizona, and Mexico meet in the desert. A line of U.S. Border Patrol dune buggies with flat tires slumped at one end of the parking lot. Across the road, a mobile watchtower with dark windows loomed over an SUV. Jad Bean flipped open a three-ring binder on the hood of our rented Trailblazer to reveal a satellite map, most of which was taken up by a waxen emptiness that crinkled like the surface of a brain. The Imperial…