Powell stared and pulled his moustache. "An old robot. Of course! He can't move without human control. Someone has to ride him like a horse. What do we do now? "We can't go thirty kilometres outside, with a robot or without," Donovan said. "Wait a minute. Give me that map! This is a mining station. What's wrong with using the tunnels?' Donovan studied the map. The mining station was a black circle, and black lines showing the tunnels stretched out from it. "Look, we can come out here, at exit 13A, five kilometres from the selenium pool. "Right. Get your insosuit," Powell said. It was the first time they had worn the insosuits. They were light, but big and ugly. A human being wearing an insosuit could survive the sun on Mercury for twenty minutes. "Are you ready to take us to exit 13A?" Powell asked the robot. "Yes, Master." Powell climbed up on to one robot and Donovan got up on another. There were seats on the backs of the robots, and two long ears to hold on to. The two huge robots moved through the door and along a narrow passage into the tunnel. They moved slowly, at a speed that never varied: they were unable to hurry After a time, they came to the end of the tunnel and they went up some stairs to a tiny substation, empty, airless. A tall cliff of black rock cut off the sunlight, and the deep night shadow of an airless world enclosed them. Before them, the shadow of the cliff reached out and ended suddenly in a hard