It wasn't until refrigerator-size boulders began hurtling down from above that the scientists sitting in an Anchorage, Alaska, control room started to get seriously worried. Until then the robot known as Dante II had successfully negotiated a steep, muddy descent and ambled unconcernedly through hot steam and poisonous gases. But even a 10-ft.-tall, 1,700-lb. automaton has its limits, and multiton chunks of rock moving at high speed were beyond Dante's. "That big one," said Carnegie Mellon University robotics expert John Bares, pointing nervously at a video screen after a rockslide, "would've wiped us out."
In the end, it was a misstep,...