 What caused the avalanche of rocks in the cavern?
He strode out of the huge laboratory, but his mind was still on the vitamin research they had been conducting, he barely heard the remarks that followed him. True, there had been others, pioneers, before the company came, who objected to the expansion of the monopolistic control. But, if they could not realize that the company's regime served the best interests of the planet, they would just have to suffer the consequences of their own ignorance. This walk was part of a regular routine of physical exercise that kept his body hard and resilient in spite of long hours spent in the laboratory. He was making that walk in precisely seven minutes, four and four-fifths seconds. The battering thunder continued persistently. He swore some more and grabbed a towel, wrapping it inadequately around him; it didn't quite meet astern. Through the open door of his bedroom he could see his bed neatly turned down as it should be, but the outline under the counterpane and the luxuriant mass of platinum-blond hair on the pillow was certainly no part of his regular routine. Pete Brent's joking words flashed back to him. He struggled out of the bath, wrapped his damp bathrobe clammily around him and marched to the door with a seething fury of righteous anger burning within him. Four police constables and a sergeant swarmed into the room, shoving him away from the door. "Wherethehell's who?" "Quit stallin', bud. I don't care if you are—" Brian's eyes almost popped out as he saw a gloved hand reach around the guard's neck and jam a rag over his nose and mouth. Swift shadows moved expertly before his astonished gaze. Another guard was caught and silenced as he came around the end of the corridor. "Somebody by the name of Pete Brent tipped us off that you were in trouble because of me. He had designed these escape-proof locks himself. In a few seconds the door swung open and they were fleeing silently down the jail corridor. "Right with you," the guard snapped and ran a few steps with them before a blackjack caught up with him and he folded into a corner. The electric eye tripped a screaming alarm, but the broken key in Brian's hands opened the complicated lock in a matter of seconds. They were outside the jail on a side street, the door closed and the lock jammed immovably behind them. Brian faltered to a stop and Crystal James pushed past him. "We've got reinforcements down here," she said, then skidded to a halt.
 Vibrations from the ships’ exhaust.
 An atomite bomb.  A chemical reaction with the ships’ exhaust.  The vibrations of the advancing police.  Vibrations from the ships’ exhaust.