MacVickers stopped at the brink of the dark round shaft. It was cold, and he was stark naked except for the silver collar welded around his neck. But it was more than cold that made him shiver and clamp his long bony jaw. He didn't know what the shaft was for, or where it led. But he had a sudden feeling that once he went down he was down for good. The small, round metal platform rocked uneasily under his feet. Beyond the railing, as far as MacVickers could see to the short curve of Io's horizon