"Oh, let them shoot us! I can't go through that again!" "You don't have to. Wait here." He climbed back into one of the ships and cut the richness of the fuel mixture down till the exhaust was a lambent, shuddering stutter, verging on extinction. He dashed to the other ship and repeated the maneuver, fussing with the throttle till he had the fuel mixture adjusted to critical fineness. The beat of the stuttering exhaust seemed to catch up to the other and built to an aching pulsation. In a moment the whole mass of air in the cavern hit the frequency with a subtle, intangible thunder of vibration. Crystal screamed. "Brian! There's more police cutting in around the entrance." Brian clambered out of the ship and glanced at the glowing points in the rock where the police were cutting their way through outside the line of the exhaust flames. The pulsating thunder in the cavern crescendoed to an intolerable pitch. A huge mass of stalactites crashed to the floor. "It's time to check out," Brian shouted. Crystal led the way as they fled down the escape tunnel. The roaring crash of falling rock was a continuous, increasing avalanche of sound in the cavern behind them. They emerged from the tunnel on the face of the mountain, several hundred yards to the east of the cavern entrance. The ground shook and heaved beneath them. "The whole side of the mountain's sliding," Crystal screamed. "Run!" Brian shoved her and they plunged madly through the thick tangle of jungle away from the slide. Huge boulders leaped and smashed through the matted bush around them. Crystal went down as the ground slipped from under her. Brian grabbed her and a tree at the same time. The tree leaned and crashed down the slope, the whole jungle muttered and groaned and came to life as it joined the roaring rush of the slide. They were tumbled irresistibly downward, riding the edge of the slide for terrifying minutes till it stilled and left them bruised and shaken in a tangle of torn vegetation. The remains of two police ships, caught without warning in the rush as they attempted to land, stuck up grotesquely out of the foot of the slide. The dust was settling away. A flock of brilliant blue, gliding lizards barking in raucous terror, fled down the valley. Then they were gone and the primeval silence settled back into place. Brian and Crystal struggled painfully to solid ground. Crystal gazed with a feeling of awe at the devastated mountainside. "How did you do it?" "It's a matter of harmonics," Brian explained. "If you hit the right vibratory combination, you can shake anything down. But now that we've made a mess of the old homestead, what do we do?" "Walk," Crystal said laconically. She led the way as they started scrambling through the jungle up the mountainside. "Where are we heading for?" Brian grunted as he struggled along.