O'Hara felt positive now that he had established a second major fact. For almost two hours he had been flying these forbidden skies, yet no patrol--save the atomic radiation of the dead city--had so far challenged him. It was inconceivable that any important civilization could exist upon the northernmost of the two Lost Continents without patrolling its skies, and consequently, if a civilization did exist, it must have shifted to the south, leaving these wastes. But for some minutes past, he had observed a peculiar series of geometric designs, resembling the drawings of ancient Cretan labyrinths, far below him on the frozen surface. He now was approaching another of these, extending for miles upon the floor of a wide valley, and very cautiously, with his eyes constantly flicking back to his scintillometer, he began to descend toward it, diving. As rapidly, his milliroentgen count began to climb, reaching .290 within the first five thousand feet of descent, then to .300 when his altitude was down to 30,000 and jumping very sharply at 24,000 feet to .325. The strange labyrinth was much too hot for inspection. But the regularity of its form seemed proof that it was man-made, and that decided O'Hara. He continued down until his altimeter registered 5,000 feet. The labyrinth's pattern was precise. It was composed of an almost infinite number of thick parallel lines, joined at the ends, so that it actually resembled an endless pipe, though various segments of it were colored in a multiplicity of pastel shades--a vast farm of pipes, reaching for miles across the valley's floor, its purpose not apparent to O'Hara. His milliroentgen count now was approaching .400, and again he made that steep climb back to 40,000 feet. These pipe farms soon were visible in every major valley, and further toward the east, upon the great slope of the plains toward the Mississippi, they were everywhere like the squares of a continental checkerboard, in some places partly obscured by snow drifts and in others lying exposed to the sun's brilliance.