 What caused the avalanche of rocks in the cavern?
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 had designed these escape-proof locks himself. "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. "We've got reinforcements down here," she said, then skidded to a halt. The sharp, ripping blast of an atomite bomb thundered along the street and slammed them to the ground. The guards had vanished, a shallow crater yawned in the road where they had been.
 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.