"Can't we get some of the diamonds?" cried Mr. Damon, as he raced along behind Tom. "Now's our chance. Those fellows have all gone!" The odd man made a grab for something as he ran.

"It's as much as our lives are worth," declared the young inventor. "We dare not stop! Come on!"

"I'd like to investigate some of the machinery," spoke Mr. Jenks, "but I wouldn't stop, even for that."

"The storm is too dangerous," called Bill Renshaw. "I can show you a shorter way out than the one those fellows have taken. Follow me."

"No way can be too short," said Mr. Parker, solemnly. "This mountain will go to pieces shortly, I think!"

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!"

They needed no second warning. Turning, they fled down the steep side of the mountain, slipping and stumbling, but taking care not to step on any iron ore. Behind them flashed the lightning bolts.

Suddenly there was a most awful crash. It seemed as if the end of the world had come, and the ear drums of Tom and his companion almost burst with the fearful report. The concussion knocked them down, and they lay stunned for a moment.

Following the terrible report there was a low, rumbling sound. Hardly knowing whether he was dead or alive, Tom opened his eyes and looked about him. What he saw caused him to cry out in terror.

The whole mountain seemed bathed in fire. Great blue, red and green flashes played around it. Then the towering cliff seemed to melt and crumble up, and the great peak, the top of it containing the diamond makers' cave, from which they had fled but a few minutes before, the entire summit was toppled over into the valley on the other side, and in the direction opposite to that where the adventurers stood.

Then came a profound silence, and the lightning ceased. The storm was over, and only the rattle of stones and boulders, as they came to rest in the valley below, reached the ears of our friends.

"Phantom Mountain has been destroyed, just as I said it would be," spoke Mr. Parker, solemnly. Once more he had prophesied correctly.

For a few minutes the adventurers hardly knew what to say. They arose awkwardly from the ground where the shock had tossed them. Then Tom remarked, as calmly as possible:

"Well, it's all over. I guess we may as well get back to our airship."

"What became of Munson and the others?" asked Mr. Damon.

Mr. Jenks pointed to the trail, far below. The figures of some men, running madly, could be seen.

"There they go," he said; "I fancy we have seen the last of them." And they had, for some time at least.

There was little use lingering any longer on Phantom Mountain-- indeed little of it was left on which to remain. Looking back toward the place where the cave had been, Tom and the others started forward again. The diamond-making machinery had all been destroyed. So, also, had the finished diamonds stored in the cavern and the large supply which had probably been made by the last terrific crash. No one would ever have them now. Tom and Mr. Jenks felt a sense of disappointment, but they were glad to have escaped with their lives. They sought their former camp, but the tent and all their food was buried under tons of earth and rocks.

Three days later, after rather severe hardships, they were near the place where they had left the Red Cloud. They had suffered cold and hunger, for they had no food supplies, and, had it not been that Bill Renshaw knew the haunts of some game, of which they managed to snare some, they would have fared badly, for they had left their guns in the cave.

"Well, there are the trees behind which I hope my airship is hidden," announced Tom, as they came to the spot. "Good old Red Cloud! Maybe we won't do some eating when we get aboard, eh?"

"Bless my appetite! but we certainly will!" cried Mr. Damon.

"There's somebody walking around the place," spoke Mr. Jenks.

"I hope it's no one who has damaged the ship," came from Tom, apprehensively. He broke into a run, and soon confronted an aged miner, who seemed to have established a rude sort of camp near the airship.

"Is anything the matter?" asked Tom, breathlessly. "Is my airship all right?"

"I guess she's all right, stranger," was the reply. "I don't know much about these contraptions, but I haven't touched her. I knowed she was an airship, for I've seen pictures of 'em, and I've been waiting until the owner came along."

"Why?" asked Tom, wonderingly.

"Because I've got a proposition to make to you," went on the-- miner, who said his name was Abe Abercrombie. "I've been a miner for a good many years, and I'm just back from Alaska, prospecting around here. I haven't had any luck, but I know of a gold mine in Alaska that will make us all rich. Only it needs an airship to get to it, and I've been figuring how to hire one. Then I comes along, and I sees this big one, and I makes up my mind to stay here until the owners come back. That's what I've done. Now, if I prove that I'm telling the truth, will you go to Alaska--to the valley of gold with me?"

"I don't know," answered Tom, to whom the proposition was rather sudden. "We've just had some pretty startling adventures, and we're almost starved. Wait until we get something to eat, and we'll talk. Come aboard the Red Cloud," and the lad led the way to his craft which was in as good condition as when he left it to go to the diamond cave. Later he listened to the miner's story.

Tom Swift did go to the valley of gold in Alaska, and what happened to him and his companions there will be told of in the next volume of this series, to be called "Tom Swift in the Caves of Ice; or, the Wreck of the Airship."'

It did not take our friends long, after they had eaten a hearty meal, to generate some fresh gas, and start the Red Cloud oh her homeward way. Tom wanted to take Bill Renshaw with him, hut the old man said he would rather remain among the mountains where he had been born. So, after paying him well for his services, they said good-by to him. Abercrombie, the miner, also remained behind, but promised to call and see Tom in a few months.

"Well, we didn't make any money out of this trip," observed Mr. Jenks, rather dubiously, as they were nearing Shopton, after an uneventful trip. "I guess I owe you considerable, Tom Swift. I promised to get you a lot of diamonds, but all I have are those I had from my first visit to the cave."

"Oh, that's all right," spoke Tom, easily. "The experience was worth all the trip cost."

"Speaking of diamonds, look here!" exclaimed Mr. Damon, suddenly, and he pulled out a double handful.

"Where did you get them?" cried the others in astonishment.

"I grabbed them up, as we ran from the cave," said the eccentric man; "but, bless my gaiters! I forgot all about them until you spoke. We'll share them."

These diamonds, some of which were large, proved very valuable, though the total sum was far below what Mr. Jenks hoped to make when he started on the remarkable trip. Tom gave Mary Nestor a very fine stone, and it was set in a ring, instead of a pin, this time.

On their arrival in Shopton, where Mr. Swift, the housekeeper, Mr. Jackson and Eradicate Sampson were much alarmed for Tom's safety, an attempt was made to manufacture diamonds, using a powerful electric current instead of lightning. But it was not a success, and so Mr. Jenks concluded to give up his search for the secret which was lost on Phantom Mountain.

And now we will take leave of Tom Swift, to meet him again soon in other adventures he is destined to have in the caves of ice and the valley of gold.