Guns mounted among the hills opened fire. Vast columns of flame burst up in the path of the advancing cars. The cars hesitated and retreated, as the plain was churned up by a howling vortex, a thundering chaos of explosions. Here and there a car vanished in a cloud of particles. A group of cars moving away suddenly scattered, caught up by a giant wind that lashed across them and swept them up into the air. Dixon gave orders to have the cannon silenced. The police air arm again swept overhead, a sullen roar of jets that shook the ground below. The police ships divided expertly and hurtled down on the cannon protecting the hills. The cannon forgot the surface cars and lifted their snouts to meet the attack. Again and again the airships came, rocking the mountains with titanic blasts. The guns became silent. Their echoing boom diminished, died away reluctantly, as bombs took critical toll of them. Dixon watched with satisfaction as the bombing came to an end. The airships rose in a thick swarm, black gnats shooting up in triumph from a dead carcass. They hurried back as emergency anti-aircraft robot guns swung into position and saturated the sky with blazing puffs of energy. Dixon checked his wristwatch. The missiles were already on the way from North America. Only a few minutes remained. The surface cars, freed by the successful bombing, began to regroup for a new frontal attack. Again they crawled forward, across the burning plain, bearing down cautiously on the battered wall of mountains, heading toward the twisted wrecks that had been the ring of defense guns. Toward the entrance tunnel. An occasional cannon fired feebly at them. The cars came grimly on. Now, in the hollows of the hills, Sherikov's troops were hurrying to the surface to meet the attack. The first car reached the shadow of the mountains.... A deafening hail of fire burst loose. Small robot guns appeared everywhere, needle barrels emerging from behind hidden screens, trees and shrubs, rocks, stones. The police cars were caught in a withering cross-fire, trapped at the base of the hills. Down the slopes Sherikov's guards raced, toward the stalled cars. Clouds of heat rose up and boiled across the plain as the cars fired up at the running men. A robot gun dropped like a slug onto the plain and screamed toward the cars, firing as it came. Dixon twisted nervously. Only a few minutes. Any time, now. He shaded his eyes and peered up at the sky. No sign of them yet. He wondered about Reinhart. No signal had come up from below. Clearly, Reinhart had run into trouble. No doubt there was desperate fighting going on in the maze of underground tunnels, the intricate web of passages that honeycombed the earth below the mountains. In the air, Sherikov's few defense ships were taking on the police raiders. Outnumbered, the defense ships darted rapidly, wildly, putting up a futile fight.