Doug Norris hesitated for an instant. He knew that another movement might well mean disaster. Here deep in the cavernous interior of airless Mercury, catastrophe could strike suddenly. The rocks of the fissure he was following had a temperature of hundreds of degrees. And he could hear the deep rumble of shifting rock, close by. But it was not these dangers of the infernal underworld that made him hesitate. It was that sixth sense of imminent peril that he had felt twice before while exploring t