When we catch up with the robot, it is poking along in a herky-jerky and rather flummoxed fashion through the Atacama Desert, which covers much of far northern Chile. The Atacama is reputedly the driest place on Earth, with rainfall measured in millimeters per decade. It is a rough place for man or robot, a tawny maze of high plateaus and shaley foothills under constant sun and an enormous cobalt-blue sky.

According to the U.S. Department of Defense, hundreds of
underground bunkers in enemy territories serve as weapons silos, command
centers and safe havens for rogue leaders. Drilled several hundred feet or deeper into the ground, many of the hideouts are far beyond the reach of conventional weapons. The Pentagon’s solution: Build a super-slim bomb called a Robust Nuclear Earth Penetrator (RNEP) capable of piercing 20 feet of solid rock and unleashing shock waves on par with a magnitude-7 earthquake.

This fall Congress will decide whether to approve $8.5 million to complete a