"Commander Huges," he addressed the man in charge of the _Los Angeles_, "you will proceed toward Mount Locke, and continue in that line until you reach the spot marked on this chart, which is directly opposite our present position. Rogers, you take the _Ganymede_, and go at an angle of 120 degrees to Huges' course, toward Mount Zoga. I will continue over the Crater of Aristotle. We will keep in constant communication with each other by means of the space phone. Time the charges so that they will commence to react on the afternoon of the twenty-eighth, thus giving a sufficient margin of time in case of delays due to parties getting lost. That's all." The _Ganymede_ and the _Los Angeles_ left almost immediately, while men from the _San Francisco_ set out to plant the first charges. There were four men to each cartridge, since it was necessary that they travel fast. Holden smiled as the lean figure of Professor Erickson, almost lost in his space-suit, bounded away in great leaps at the head of his party. In five hours they returned, having had no trouble at all. Edwards manipulated the controls, and the ship rose quickly to an altitude of about five thousand feet and headed for the rim of the Crater of Aristotle, barely visible in the distance. As they neared the rim, they rose higher and higher. The mammoth cliffs of black rock towered above them, and the meters registered a height of five miles as they passed through a crack in the cliffs and looked down on the level floor beneath them. Suddenly Holden, who had been inspecting the country from one of the bow ports, uttered an exclamation of astonishment. "A tiny ship is rising toward us from the floor of the crater, near the cliffs!" There it was, a speck rapidly growing larger, headed straight for them, and gaining velocity with every foot it covered. Edwards worked frantically with the controls, diving in a zig-zag path toward the strange craft. Captain Linet rushed in, carrying one of the light _hexoxen_ guns. Holden hurried to help him place it in a specially designed aperture in the bow, while Erickson and the regular radio man endeavored to establish communications with the intruder. A voice suddenly spoke from their instrument. "You will consider yourselves our captives. Land at once as close as possible to the white spot you see at the base of the cliff. If you do not obey instructions, we will ram you immediately." "Don't reply for a moment," Holden commanded, focusing his glasses in the direction indicated. As the powerful lenses brought out every detail of the scene below, he paled visibly. "What's the matter?" demanded Erickson. "Matter enough," was the amazing reply. "We've run into a den of some bandits. They must be the fiends who have been preying on the Earth-Mars shipping!" * * * * *