"I think he understands," she said after a little. "Keep your hand on him and follow." Nick was hesitant, but the only alternative was to remain in the pitch black, musty tunnel. For hours they shuffled blindly along, their hands meeting in the loose fur of the beast's neck. The tunnel sloped downward, turning right and left so that within minutes Nick was hopelessly lost. Time and again his outstretched fingers, trailing along the wall, encountered the emptiness of side tunnels and branchings, but the _cajora_ moved purposefully ahead. Several times Nick tried to talk, to ask the questions which were perplexing him, but each time the girl silenced him. "You'll distract the _cajora_," she warned. The animal stopped short as they rounded a turn and saw a glimmer of light ahead. "They don't like light," she explained. "We'll have to go on alone." The light came from a cross tunnel, from patches of some glowing substance in the hard, smooth walls. The tunnel was roughly circular in section, large enough for Nick to walk upright despite his height. He whistled in amazement. "Who built these?" he asked, for they had come several miles in darkness and now the lighted tunnel stretched away into the distance, a major engineering project. "The Martians." "How?" "With their _voras_." He wanted to ask her to explain, but she was examining some markings on the walls, combinations of triangles and curved lines that were obviously writing. She seemed to understand them, and Nick began to understand now how she and her father had evaded the Mec patrols so long. The leaders of the Martian Exploitation Company did not even suspect the existence of this extensive underground labyrinth. "We're a long way from Klev's home," the girl declared. "The faster we get there the better." "Why? What's the danger? The Mecs won't follow us down here?" "Martians." "Huh? I thought you were friends with them." She shook her head sadly. "Only a few now. The rest have grown to hate us. Come on." They had covered several more miles when they were stopped. Susan's faint gasp sent Nick's hand automatically to his holster and he looked up to see three Martians emerging from a side tunnel just ahead. He stared. They were the first living Martians he had seen at really close range, and the bodies of those hunted down by the patrols had always been as crumpled and collapsed as spiders caught in the flame of a blowtorch. * * * * * They were slightly taller than humans, with great glowing eyes in their bulging heads and thin, many-fingered arms that reached almost to the knee joints of their stubby legs. Their noses were almost flat and their mouths too small, and their heads were topped by erect crests of skinlike material. Two of them were a dull greenish color, but the third, evidently the leader, had a marked bluish tinge to his face. All three wore shapeless brown clothing.