Here, on this tiny continent, the prison ship came once a month, to hover over the tiny islet in the middle of the swamp, the only spot of firm ground for untold miles. Here it dropped supplies and food, and occasionally picked up the little heaps of fabulous Venusian pearls. There were no guards and none were needed, for at night when the awful humidity increased, the men worked or died. With night came the dreaded fog, lurid in the ghostly illumination of the _igniis fatui_, the phosphorescent radiance of this vast graveyard. And the idle died. Decomposition of the blood set in; essential salts within their bodies were dissolved, cellular activity ceased, and their bodies bloated. Not many, however, were idle. Escape? For years it had been thought a virtual impossibility. The very thought would have brought smiles to the grim faces of that august body, the Tri-Planetary Bureau of Prisons. And yet--a notorious killer who had been sent to this swamp only a year ago, had recently been found dead--out in space! * * * * * A patrol ship had found the body floating a few thousand miles off Callisto, an atom-blast hole drilled neatly through the forehead. There was not the slightest doubt that this was the same man. How had this criminal been able to escape the swamp and travel to Callisto, millions of miles away? It was a mystery and above all, a challenge. Apparently the Venus Prison had ceased to be impregnable. And that was why Mark Denning, the Prison Bureau's leading investigator, was here. "Guard your pearl, middle species," Aladdo's voice was ironic once more. "Escape, and with it you may buy a pardon!" Without a backward glance, the Venusian moved on with nightmare slowness through the swirling mists, pushing his basket into which the Josmian globes were loaded. Escape, Mark thought, following the Venusian. He did not need to escape, he could signal the prison ship to pick him up the next time it arrived. He wondered if he should. He had been here two months, and they were an eternity that dwarfed any concept of hell. But he hadn't any clue to the mystery of the escaping convicts, and he could hardly return with a confession of failure. He looked ahead through the mists, at the slender body of Aladdo in its tight-fitting sheath of metallic blue. "I _would_ miss Aladdo," Mark whispered to himself; "and if he can stand it here, I should be able to!" "What are you mumbling about to yourself?" Aladdo's mocking voice came back to him. "That lowers you from the middle species to the sub species again." He held up a Josmian globe against the greenish swamp glow. "White," he said contemptuously and threw it into the basket. Pushing through the muck with his tremendous strength, Mark cut the distance that separated them. "You may have my purple one, Aladdo. I will not need it, and perhaps you ... with it you might...."