"They may have found the sunken seas of Mars," thought Rolf as he moved alertly forward, "water that would give the mining domes new life." His fists clenched dryly. "Water that should be free!" Then the light brightened before him as he rounded a shouldering wall of smoothly trimmed stone, and the floor fell away beneath his feet! He found himself shooting downward into a vast void that glowed softly with a mysterious all-pervading radiance. His eyes went searching out, out into undreamed distance. For miles below him there was nothing but emptiness, and for miles before him there was that same glowing vacancy. Above the cavern's roof soared majestically upward; he could see the narrow dark slit through which his feet had betrayed him, and he realized that he had fallen through the vaulted rocky dome of this fantastic abyss. It was then, even as he snapped the release of his spinner and the nested blades spun free overhead, that he saw the slowly turning bulk of the cloud-swathed world, a tiny five mile green ball of a planet! The weird globe was divided equally into hemispheres, and as the tiny world turned between its confining columns a green, lake-dotted half alternated with a blasted, splintered black waste of rocky desert. As the spinner dropped him slowly down into the vast emptiness of the great shining gulf, Rolf could see that a broad band of stone divided the green fertile plains and forests from the desolate desert wastes of the other half. Toward this barrier the spinner bore him, and Rolf was content to let it move in that direction--from the heights of the wall he could scout out the country beyond. The wall expanded as he came nearer to the pygmy planet. The spinner had slowed its speed; it seemed to Rolf that he must be falling free in space for a time, but the feeble gravity of the tiny world tugged at him more strongly as he neared the wall. And the barrier became a jumbled mass of roughly-dressed stone slabs, from whose earth-filled crevices sprouted green life. So slowly was the spinner dropping that the blackened desolation of the other hemisphere came sliding up beneath his boots. He looked down into great gashes in the blackness of the desert and saw there the green of sunken oases and watered canyons. He drifted slowly toward the opposite loom of the mysterious wall with a swift wind off the desert behind him. A hundred yards from the base of the rocky wall his feet scraped through black dust, and he came to a stop. Deftly Rolf nested the spinners again in their pack before he set out toward the heaped-up mass of stone blocks that was the wall. Ten steps he took before an excited voice called out shrilly from the rocks ahead. Rolf's slitted gray eyes narrowed yet more and his hand dropped to the compact expoder machine-gun holstered at his hip. There was the movement of a dark shape behind the screen of vines and ragged bushes. "Down, Altha," a deeper voice rumbled from above, "it's one of the Enemy."