For Doc Willard might still be alive. Even if he wasn't, there was that notebook the Doc had always carried around with him--a book that contained the medico's theories about the Silver Plague. Even if that ghastly dream-like memory were not merely delirium--even if Garth, witless and unknowing, had killed Willard--there was always that dim, desperate chance that the cure for the Plague might be found in the Black Forest. So--damn Paula Trent! She didn't matter, when the lives of millions might depend on Garth's penetrating the jungle that had baffled him for five years. Without a word he turned and started back, Brown keeping close beside him. The huge chamber loomed before them, filled with its cryptic shadows. There was time now to see what they had missed in their quick flight a few moments ago--though not much time, for pursuit might start at any minute. Dead silence, and darkness, broken by the crossing beams of the brilliant lamps. Garth listened. "Hear anything?" Brown shook his head. "Nothing." "Okay. We'll try this way." Then went into a passage that sloped down, ending in a vaulted room larger than the first. Brown swung up his gun abruptly as a figure seemed to leap from blackness in the ray of the lamp. Garth caught his arm. "Robot. Unpowered. They're all over the city." The robots--slaves of the Ancients, Garth thought, who had died with them, lacking the fuel that could quicken them to life. No Earthly scientists had ever been able to analyze the construction of the machines, for they were built of an alloy that was apparently indestructible. Acid and flame made no impression on the smooth, glittering black surface. This one, like all the others, was roughly man-shaped, nearly eight feet tall, and with four arms, the hands extended into limber jointed fingers almost like tendencies. From the mask-like face complex glassy eyes stared blankly. It stood motionless, guarding a world that no longer needed guardians. With a little shrug Garth went on, his ears alert for sounds. From the walls bizarre figures in muraled panels watched. Those murals showed a world of incredibly advanced science, Garth knew. He had seen them before. He spared them not a glance now. The machines-- What were they? They loomed like dinosaurs in the endless chain of high-domed vaults. They had once given Chahnn power and life and strength. The murals showed that. The Ancient Race had used antigravity--a secret unknown to Earthmen--and they had created food by the rearrangement of atomic patterns, not even requiring hydroponic tank cultures. They had ruled this world like gods. And they had passed with no trace, leaving only these silent monuments to their greatness. With the power of the Ancients, Earth's lack of fuel-reserves would not matter. If the secret of atomic power could be found again, these machines would roar into thundering life--and machines like them would rise on Earth.