Free 05-A Gift From Earth by Larry Niven

years men had taken their cars below the Plateau to prove their courage and to gauge what level they could reach before the air turned poisonous. That level was far above the mist. Someone named Greeley had even tried the daredevil stunt of dropping his car with the fans set to idle, falling as far as he could before the poison mist could leak into his cabin. He had dropped four miles, with the hot, noxious gasses whistling around the door, before he had had to stop. He had been lucky enough to get back up before he passed out. The Hospital had had to replace his lungs. On Alpha Plateau he was still a kind of hero.
    Even Greeley would never have flipped his car over and bored for the bottom. Nobody would, not if he knew anything about cars. It might come apart in the air!
    But that wouldn't occur to Matt. He knew little about machinery. Earth's strange pets were necessities, but machinery was a luxury. Colonists needed cheap houses and hardy fruit trees and rugs that did not have to be made by hand. They did not need powered dishwashers, refrigerators, razors, or cars. Complex machinery had to be made by other machines, and the crew were wary of passing machines to colonists. Such machinery as they had was publicly owned. The most complex vehicle Matt knew was a bicycle. A car wasn't meant to fly without gyroscopes, but Matt had done it.
    He had to get down to the mist to hide himself from the police. The faster he fell, the farther he'd leave them behind.
    At first the seat pressed against him with the full force of the fans; about one-and-a-half Mount Lookitthat gravities. The wind rose to a scream, even through the soundproofing. Air held him back, harder and harder, until it compensated for the work of the fans; and then he was in free fall. And still he fell faster! Now the air began to cancel gravity, and Matt tried to fall to the roof. He had suspected that he was making the car do something unusual, but he didn't know how unusual. When the wind resistance started to pull him out of his seat, he snatched at the arms and looked frantically for something to hold him down. He found the seat belts. Not only did they hold him down, once he managed to get them fastened; they reassured him. Obviously they were meant for just this purpose.
    It was getting dark. Even the sky beneath his feet was darkening, and the police cars were not to be seen. Very well. Matt pushed the fan throttles down to the Idle notches.
    The blood rushing to his head threatened to choke him. He turned the car right side up. Pressure jammed him deep in his seat with a force no man had felt since the bruteforce chemical rockets, but he could stand it now. What he couldn't endure was the heat. And the pain in his ears. And the taste of the air.
    He pulled the throttles out again. He wanted to stop fast.
    Come to that, would he know when he stopped? This around him was not a wispy kind of mist, but a dark blur giving no indication of his velocity. From above, the mist was white; from below, black. Being lost down here would be horrible. At least he knew which way was up. It was fractionally lighter in that direction.
    The air tasted like flaming molasses.
    He had the throttles all the way out. Still the gas crept in. Matt pulled his shirt over his mouth and tried to breathe through that. No good. Something like a black wall emerged from the mist-blur, and he tilted the car in time to avoid crashing against the side of Mount Lookitthat. He stayed near the black wall, watching it rush past him. He'd be harder to see in the shadow of the void edge.
    The mist disappeared. He shot upward through sparkling sunlight. When he thought he was good and clear of the foul mist, and when he couldn't stand to breathe hot poison for another second, he put the window down. The car whipped to the side and tried to turn over. A hurricane roared through the cabin. It was hot and thick and soupy, that hurricane, but it could be breathed.
    He saw the edge of the Plateau above