"What's that noise?" asked Tom, as their guide flashed the lantern to show them the way.

"That's the men getting ready to make diamonds, I guess," was the answer. "You see it takes quite a while to get the stuff ready. I don't know what they use--they never tell me any of their secrets."

"Oh, I know the ingredients well enough," said Mr. Jenks, "but I don't know the secret of how they apply the terrific heat and pressure necessary to fuse the materials into diamonds."

"Well, you'll soon know," declared Bill Renshaw. "Of course it isn't always successful. I've known 'em to try half a dozen times before they got any diamonds big enough to satisfy 'em. They gave me some of the small ones when I asked for my wages.

"How did you come to get in with these men?" asked Tom, curious to understand how a person seemingly as honest as Renshaw appeared to be had cast his lot in with the men who had broken faith with Mr. Jenks.

"Oh, I've lived around these parts all my life," was the answer. "I knew of this cave before these diamond fellers came to it. In fact, I showed it to 'em. It was several years ago that a party of men who were prospecting around here came to me and asked if I knew of a small cave near the top of a high mountain, where lightning storms were frequent. I told them about Phantom Mountain, as it was called then, and also of this cave. If there's any place where they have worse lightning storms than here, I'd like to know it. They scare me, sometimes, like the night when that landslide happened, and I'm sort of used to 'em.

"Well, I took these men to the cave, and they hired me as a sort of lookout. Then they began their work, and at first I didn't know what they were up to, but finally I caught on. Then Mr. Jenks came, and disappeared mysteriously, though then I didn't know that they had played a trick on him. I was outside most of the time, pretending I was the ghost. So that's how I came to get in with 'em, and I wish I was out."

"You soon will be, I think," declared Mr. Jenks. "But won't our talking be heard by the men?"

"No danger. There is a thick wall between this part of the cave, and the part where they live and work. I'll soon have you well hid, and then you wait until I come back."

"What about Munson?" asked Tom. "He is evidently on his way here to tell his confederates about us."

"He won't know what has happened to us," said Mr. Jenks, "and he won't see anything of us. I guess we're safe enough."

Through the dark passage they followed Bill Renshaw until he came to a halt in a place that suddenly widened and broadened into a good-sized cave.

"Here's your stopping place," said the former ghost. "Now if you follow that passage, off to the left," and he pointed to it, "you'll come to the larger part of the cave where the diamond makers are. But go cautiously, and don't make any noise. I won't be responsible for what happens."

"We'll take all the risk," interrupted Tom.

"All right. Now there's a couple of lanterns around here. I'll light them, and leave you for a while until I can get some grub. I'll be back as soon as I can."

He glided away, after lighting two lanterns, by the gleams of which the adventurers could see that they were in a vaulted cavern that had evidently been fitted up as a living apartment. The sides, roof and floor were of stone. It was clean, and the air was fresh. There were some chairs, a table, and several cots, with pieces of bagging for bedding, though it was warm in the place.

"I guess we can stay here until we discover the secret," spoke Tom.

"Bless my watch! We can if we have something to eat," came from Mr. Damon, with something like a sigh. "I'm hungry!"

"And I want to make some observations," said Mr. Parker. "From what I have seen of this mountain, I would not be surprised if this cave was to be suddenly destroyed by a landslide or a lightning bolt. I will make some further investigations."

"Well, if it's going to cause you to make such gloomy prophecies as that, I'd just as soon you wouldn't look any further," spoke Tom. in a low voice. But Mr. Parker, taking one of the lanterns, set about examining the rock of which the cave consisted.

In a short time Bill Renshaw returned with enough food to last for two days. He said he was going out on the mountain once more to act the part of a lookout, and would visit the adventurers again the next day.

"In the meanwhile you can do just as you please," he said. "Nobody is likely to disturb you here, and you can sneak up and take a look at the men in the other cave whenever you're ready. Only be careful--that's all I've got to say. They're desperate men."

It was not very pleasant, eating in the gloomy cavern, but they made the best of it. They cooked on a small oil-stove they found in the place, and after some hot coffee they felt much better.

"Well," remarked Tom, after a while, "shall we take a chance, and go look at the men at work?"

"I think so," answered Mr. Jenks. "The sooner we discover this mystery, the better. Then we can go back home."

"And recover my airship," added Tom, who was a bit uneasy regarding the safety of the Red Cloud.

"Then, bless my finger-rings! let's go and see if we can find the big cave your friend the ghost told us of," suggested Mr. Damon.

Cautiously they made their way along the passage Bill had pointed out. As they went forward the subdued noise became louder, and finally they could feel the vibration of machinery.

"This is the place," whispered Mr. Jenks. "That sound we hear is one of the mixing machines, for grinding the materials--carbon and the other substances--which go to make up the diamonds. I remember hearing that when I was in the cave before."

"Then we must be near the place," observed Tom.

"Yes, but I didn't have much chance to look around when I was here before. They wouldn't let me. I never even knew of the small cave Bill took us to."

"Well, if we're close to it, we'd better go cautiously, and not talk any more than we're obliged to," suggested Mr. Parker, and they agreed that this was good advice.

They walked on softly. Suddenly Tom, who was in the lead, saw a gleam of light.

"We're here," he whispered. "I'll put out our lantern, now," which he did. Then, stealing forward he and the others beheld a curious sight. The tunnel they were in ended at a small hole which opened into a large cavern, and, fortunately, this opening was concealed from the view of those in the main place.

"The diamond makers!" whispered Tom, hoarsely, pointing to several men grouped about a number of strange machines.

"Yes--the very place where I was," answered Mr. Jenks, "and there is the apparatus--the steel box--from which the diamonds are taken --now to see how they make them."

Fascinated, the adventurers looked into the cave. The men there were unaware of the presence of our friends, and were busily engaged. Some attended to the grinding machine, the roar and clatter of which made it possible for Tom and the others to talk and move about without being overheard. Into this machine certain ingredients were put, and they were then pulverized, and taken out in powdery form.

The power to run the mixing machine was a gasoline motor, which chug-chugged away in one corner of the cave.

As the powder was taken out, other men fashioned it into small balls, which were put on pan, and into a sort of oven, that was heated by a gasoline stove.

"Is that how they make the diamonds?" asked Mr. Damon.

"That is evidently the first step," said Mr. Jenks. "Those balls of powdered chemicals are partly baked, and then they are put into the steel box. In some way terrific heat and pressure are applied, and the diamonds are made. But how the heat and pressure are obtained is what we have yet to learn."

He paused to watch the men at work. They were all busy, some attending to the machines, and others coming and going in and out of the cave. In one part a man was apparently getting ready a meal.

Suddenly there rushed into the cave a man who seemed much excited.

"Are you nearly ready with that stuff?" he cried. "There's a good storm gathering on the mountain!"

"Yes, we'll be ready in half an hour," answered one of the men at the mixing machine.

"Good. It will be flashing lightning bolts then, and we can see what luck we have. The last batch was a failure." The man hurried out again. Mr. Parker touched Tom and Mr. Jenks on their shoulders.

"What is it?" asked Tom.

"I know the secret of making the diamonds,~ said the scientist.

"What?" cried Mr. Jenks.

"It is by the awful power of the lightning bolts!" whispered Mr. Parker. "Everything is explained now--the reason why they make diamonds in this lonely place, near the top of the mountain. They need a place where the lightning is powerful. I can understand it now--I suspected it before. They make diamonds by lightning!"

"Are you sure?" cried Mr. Jenks.

"Positive."

"I agree with you," said Tom Swift. "I was just getting on that track myself, when I saw the electric wires running to the steel box. That explains the upright rod on the top of the mountain. The man says a storm is coming --very well; we'll stay here and watch them make diamonds!"

As he spoke there came the mutter of thunder, and the mountain vibrated slightly. The men in the cave redoubled their activity. Tom and his friends felt that the secret process they had so long sought was about to be demonstrated before their eyes.