“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.”