Tom shuddered.  He remembered how narrow had been their escape when Earthquake Island sank into the sea.  And that some terrific upheaval was now imminent might be judged from the awful reports that sounded more plainly as the adventurers raced toward the opening of the cave.  It was like the bombardment of some doomed city.

Mr. Jenks and Tom cast one longing look behind at the complicated and expensive machinery that had been installed in the cave by the diamond makers.  They had abandoned it, and in it lay the secret of making precious gems.  But there was no time to stop now, and investigate.

“This way,” urged Bill Renshaw.  “We’ll soon be out.”

“But won’t it be dangerous to go outside?” asked Mr. Damon.  “Shan’t we be struck by lightning?  There is some protection in here.”

“None at all,” said Mr. Parker, quickly.  “This mountain is a natural lightning rod.  To stay here in this cave will be sure death when the storm gets directly over it.  And that will be very soon.  We must get on insulated ground.  Is there any part of this mountain that does not contain iron ore?” the scientist asked of the former spirit.

“Yes; the way out by which we are going lands on a dirt hill.”

“That’s good; then we may be saved.”

On they ran.  They had no lanterns, but the blue light of the electricity, as it leaped from point to point inside the cave, where there were outcroppings of iron ore, made the place bright enough to see.

“Here we are!” cried Bill Renshaw at length.  “Here’s the way out!”

Making a sudden turn in the winding passage he showed the adventurers a small opening in the side of the crag.  In an instant they had passed through, and found themselves in daylight once more.  The sudden glare almost blinded them, for, though the sky was overcast by clouds, from which jagged tongues of lightning played, the outside was much lighter than the dark cave.

“I should say it was a storm!” cried Tom Swift.  “See, it is striking every minute, and all around us!”

In fact, lightning bolts were falling on every side of the adventurers.  Every time the balls of fire struck, they burst open great stones, or seared a livid scar on the face of some cliff.  As for Tom and the others, they stood on a dry dirt hill, in which, fortunately, there was no iron ore.  To this fact they undoubtedly owed their lives, though had there been rain, to moisten the ground and make the earth a good conductor of electricity, they probably would have been badly shocked.  But the electrical outburst was not accompanied by rain.

Tom looked up.  He saw a compact mass of cloud moving toward the summit of the mountain on the slope of which they stood.  From this cloud there played shafts of reddish-green fire.

“Look!” called the young inventor to Mr. Parker.  The instant the latter saw the cloud, he cried: 

“We must get away from here by all means!  That is the center of the storm.  As soon as it gets over the mountain, where that lightning rod is, all the electrical fluid will be discharged in one bolt at the mountain, and it will be destroyed!  We must run, but keep on the dirt places!  Run for your lives!”