Mark Denning pursed his lips. "We'll have to start tomorrow, and we'll have to go fast. A loss of light and heat, however slight, could have very serious effects on life if continued too long. We can make it by tomorrow night, if we start early and leave the Indians and pack animals behind." The other two nodded. Mark looked at them in the half-light of the Moon. "You'll have to stay with the equipment, Pedro, otherwise the Indians might abandon it. Burl and I will start out at dawn." Gonzales agreed and the three made their way back to the camp. At the first sign of light breaking in the morning horizon, Burl and his father started off. They carried only enough equipment for survival, plus the additional items that might be needed for the emergency ahead. The trek over the mountains was a hard one, the path narrow, steep, sometimes nonexistent. There were few signs of Indians or animals, and it was plain that few ever traveled over this range. The air was cold and thin, vegetation sparse and hardy. All around them was the cold blue of the sky--a shade darker than usual--and the gaunt peaks of ancient mountains. The Inca kings may have claimed the land here, but even their hardy legions had never conquered these lonely and hostile sky domains. Panting and weary with hours of climbing, Burl and his father made a quick lunch in a sheltered jumble of rock near the top. Then, shouldering their packs again, they trudged on. At last they reached a point where the view of the other side spread out before them--a breathtakingly clear vision of the little valley below. As they looked down, the air seemed to shimmer and vibrate. Burl rubbed his eyes. "It hurts," he said. His father squinted. "There's a powerful vibrational effect. It may be a very dangerous concentration of the invisible rays of the Sun as well as of light." Once Burl had gotten used to the odd visual effect, which was like gazing into the twisting heat rays rising from an overheated oven, he saw that there was a small flat region between the mountains. And in the center of this valley was a large black structure of some sort. The twisting effect of the light around it made it impossible to tell more. "That's it," said Burl. His father nodded, shifted the pack to ease his shoulders, unstrapped the hunting rifle slung over his back, and carefully checked its loads. Burl saw what his father was doing and suddenly understood the danger. What could be doing a thing like this? What but something not of this Earth? Something of distant space, of a science beyond that of man--and _unfriendly_ besides. Now, for the first time, Burl realized what he had not had time to before--this was an enemy he and his father were facing--an enemy of all mankind--and utterly unknown. He gulped, gripped his rifle, and followed his father down the sliding rocky trail.