 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. 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. 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.
 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.