Free Rivets and Sprockets by Alexander Key

something unseen rushed below the hillock of trees beyond him. From it came a loud, angry Beep! B-e-e-p! B-E-E-P ! Then, with a quick hum and whir, it was gone.
    The racket brought Jim and the doctor to their feet.
    Jim was instantly wide awake. “W-what in the walloping doodads was that? ”
    â€œGreat gracious!” muttered the doctor, still a bit foggy. “Who—what—why—”
    â€œI was unable to see it,” said Sprockets. “But something came and went, very loud and fast.”
    â€œBless me! Could it have been the Something?”
    â€œIt was a machine,” said Sprockets. “It hummed and whirred.”
    â€œAnd it beeped,” said Jim. “It nearly beeped me out of my skin.”
    â€œThen it couldn’t have been the Something. This will have to be investigated.”
    â€œI—I’m not investigating any Beeper,” Jim said uneasily. “At least not till I’ve had some breakfast. What time is it, Sprockets?”
    â€œAs nearly as I can calculate,” Sprockets told him, “it’s seventeen minutes to six, underground Martian time. It’s no longer the same as Martian surface time—which naturally has changed through the years. The imitation sun will be up in fourteen minutes.”
    â€œImpossible!” exclaimed the doctor. “I’ve hardly slept a minute!” The doctor always said this, even after he had napped for hours. “How do you figure this, anyway?”
    â€œQuite simple, sir. The Martian day is nearly the same length as an Earth day. The imitation sun is made to rise and set exactly like the real sun, and its purpose is to make underground Mars look exactly like surface Mars—I mean, like the surface of Mars used to look.”
    â€œEh? But why?”
    â€œI’m sure, sir, the reason will be evident as soon as we explore ahead. I’ve been talking to Ilium and Leli. They think they are nearing the power source, and they are very anxious to hear what we find.”
    Dr. Bailey was suddenly wide awake, his nose twitching eagerly. Hurriedly he plucked a handful of plapples, stuck one in his mouth, and started across the glade. Jim did likewise.
    The imitation sky was turning pale as they reached a flight of carved stone steps leading down to a path. The glade ended here. But the path and the underground world were just beginning.
    Before them stretched a long valley. The brook went tumbling through it, and was soon lost in groves of trees where robot birds were beginning to sing. On either side of the valley were hundreds of small houses, as bright as frosted cakes. They nestled into the hillsides, one above the other, all the way up to the sky—or what seemed to be the sky.
    Jim stared at it. “Great crickety crimble!” he whispered in an awed voice. “Now I get it. This is where the Martians came to live !”
    The doctor nodded. “Of course! They dug the canals when the seas dried up, and when the air was nearly gone, they came underground.”
    â€œB-but, Daddy, that was a long, long, long time ago. So long ago that time has rubbed out everything above us that used to be, except the canals. Why does everything down here look so bright and fresh and new? D’you s’pose the Martians are still living here?”
    As he spoke, the imitation sun peeped over the hill, and the valley suddenly awoke. Sprinklers came on, watering trees and flowers. Dozens of bottle-shaped machines scooted from openings under the houses and began scurrying about, planting, picking, mending, cleaning, painting, and putting things in order. All had six rubbery wheels, two pincherlike arms, and wiggling snouts that could do anything from root in the ground to spray fresh plastic on the houses. When they wanted to climb, they merely hummed mysteriously and rose into the air like bees.
    Other than the bottle-shaped workers, and the robot birds, there was no sign of life.