For a time the roiling, slipping, sliding and tumbling of the mass of earth and stones, down the side of the mountain, effectually drowned all other sounds. Even the thunder was stilled, and though Tom and his companions called to one another in terror, their voices could not rise above that terrific tumult.

Finally, when they found that the direction of the slide was away from their tent, and that they were not likely to be engulfed, they grew more calm.

Gradually the noise subsided. The great boulders had rolled to the bottom of the valley, and now only a mass of earth and stones was sliding down. Even this stopped in about five minutes, and, as though satisfied with what it had done, the electrical storm passed. Not a drop of rain had fallen.

"Bless my shirt studs!" exclaimed Mr. Damon, who was the first to speak after the din had quieted. "Bless my soul! But that was awful!"

"It was just what I expected," said Mr. Parker, calmly. "I knew, from my observations, that we were in a region where landslides and terrific electrical storms may be expected at any time. I fully looked for this."

"Well," remarked Mr. Jenks, rather sarcastically, "I hope it came up to your expectations, Mr. Parker."

"Oh, fully," was the answer, "though I wish it could have happened in daylight, so that I could better have observed certain phenomena regarding the landslide. They are very interesting."

"At a distance," admitted Tom, with a laugh of relief. "Well, I'm glad it's over, though we'll have to wait until morning to see what damage has been done. Lucky we weren't struck by lightning. I never saw such bolts!"

"Me, either!" declared Mr. Damon. "This mountain seems to attract them."

"It is like a magnet," said Mr. Parker. "I think I shall be able to make some fine observations here."

"If we live through it," murmured Mr. Jenks.

They watched the play of lightning about a distant bank of clouds, but the storm was now far away, only a faint rumbling of thunder being heard.

"I'm wondering what happened to the phantom," said Tom, after a pause. "Seems to me he was right in that track of the storm."

"Do you think it was a 'he'?" asked Mr. Jenks.

"I think we'll find that it's some sort of a man," answered the young inventor. "We may find out very soon, now. I've changed my theory about the ghost being reflections of light."

"How's that?" Mr. Damon wanted to know.

"Well, I think we are on the side of Phantom Mountain where the diamond cave is," went on the lad. "The fact that the phantom appeared here, soon after we arrived, shows that the men kept close track of our movements. It also shows, I think, that the phantom did not have to travel far to be on the spot, whereas we had to make quite a trip to get around the base of the mountain. I think the cave is up there," and Tom pointed toward the spot where the weird figure had been last seen, before the storm drove it back.

"There may be two phantoms," suggested Mr. Jenks. "They may keep one on this side of the mountain, and one on the other, to warn intruders away.

"It's possible," admitted Tom. "Well, we'll see how things look in the morning, when we'll take up our march again, and go up the mountain. We'll reach the top, if possible, which we couldn't do from the other side, as it was too steep."

"I hope we shall be able to go forward in the morning," came from Mr. Jenks.

"What do you mean?" asked the lad, struck by a peculiar significance in the diamond man's tones.

"Why, that landslide may have opened a great gully in the side of Phantom Mountain, which will prevent us from passing. It was a terrific lot of earth and stones that slid away," answered Mr. Jenks.

"It certainly was," agreed Mr. Parker. "I would not be surprised if the mountain was half destroyed, and it may be that the diamond cave no longer exists."

"Not very cheerful, to say the least," murmured Mr. Jenks to Tom, and, as it was getting quite chilly, following the storm, they went inside the tent.

Tom could hardly wait for daylight, to get up and see what havoc the landslide had wrought. As soon as the first faint flush of dawn showed over the eastern peaks, he hurried from the tent. Mr. Damon heard him arise, and followed.

A curious scene met their eyes. All about were great rocks rent and torn by the awful power of the lightning. The fronts of the stone cliffs were scarred and burned by the electrical fire, and fantastic markings, grotesque faces, and leering animals seemed to have been drawn by some gigantic artist who used a bolt from heaven for his brush.

But the eyes of Tom and Mr. Damon took all this in at a glance, and then their gaze went forward to where the avalanche had torn away a great part of the mountain.

"Whew! I should say it was a landslide!" cried Tom.

"Bless my wishbone, yes!" agreed Mr. Damon.

Below them, in the valley, lay piled immense masses of earth and stones. Boulders were heaped up on boulders, and rocks upon rocks, being tossed about in heaps, strung about in long ridges, and swirled about in curves, as though some cyclone had toyed with them after the lightning flash had tossed them there.

"But the mountain isn't half gone," said Tom, as his eyes took in what was left of the phantom berg. "I guess it will take a few more bolts like that one, to put this hill out of business."

Though the landslide had been a great one, the larger part of the mountain still stood. An immense slice had been taken from one side, but the summit was untouched.

"And there's where the diamond cave is!" cried Tom, pointing to it.

"I think so myself," agreed Mr. Jenks, who came from the tent at that moment, and joined the lad and Mr. Damon. "I think we shall find the cave somewhere up there. We must start for it, as soon as we have eaten, and we may reach it by night."

The three stood gazing up toward the summit of the great mountain. Suddenly, as the sun rose higher in the heavens, it sent a shaft of rosy light on the face of the berg that had been scarred by the landslide. Tom Swift uttered an exclamation, and pointed at something.

"See!" he cried. "Look where the trail is--the trail down which the phantom must have come. It is on the edge of a cliff now!"

They looked, and saw that this was so. The increasing light had just revealed it to them. When the lightning bolt had torn away a great portion of the mountain it had cut sheer down for a great depth and when the earth and stones fell away they left a narrow pathway, winding around the mountain, but so near the edge of a great chasm, that there was room but for one person at a time to walk on that footway. The uncertain trail up Phantom Mountain had all but been destroyed.

"The way up to the peak is by that path, now," spoke Tom, in a low voice.

"Bless my soul!" cried Mr. Damon. "It's as much as a man's life is worth to attempt it. If he got dizzy, he'd topple over, and fall a thousand feet. Dare we risk it?"

"It's the only way to get up," went on Tom. "It's either that way, or not at all. We've tried the other side without success. We must go up this way--or turn back."

"Then we'll go up!" cried Mr. Jenks. "It may not be as dangerous as it looks from here."

But it was even more dangerous than it appeared, when they went part way up it after a hasty breakfast. The trail was a mere ledge of rock now, and in some places, to get around a projecting edge of the mountain, they had to stand with their backs to the dizzy depths at their feet, and with both arms outstretched work their way around to where the trail was wider.

"Shall we risk it?" asked Tom, when they had tried the way, and found it so dangerous. "We can't take anything with us--even our guns, for we couldn't carry them, and if we reach the month of the cave, and find those men there--"

He paused significantly. The adventurers looked at one another. The search for the diamond makers was becoming more and more dangerous.

"I say let's go on!" decided Mr. Damon, suddenly. "We want to locate that cave, first of all. Perhaps, when we do find it, we may see some easier way of getting to it than this. And if those diamond makers do attack us--well, I don't believe they'll shoot defenseless men, and they may listen to reason, and give Mr. Jenks his rights--tell him how to make diamonds in return for the money he gave them."

"I don't believe those scoundrels will listen to reason," replied the diamond man, "but I agree with Mr. Damon that we ought to go on. We may find some other means of reaching the cave--if we can discover it, and we'll take a chance with the men."

"Forward it is, then!" cried Tom. "I have a revolver, and I can supply one of you gentlemen with another. They may come in useful in an emergency. Let's go back to camp, take a little lunch in our pockets, and try to scale the mountain."

They were soon on their way up the dizzy path once more, and, as they advanced, they found it growing more and more dangerous. In some places they found it almost impossible to get around certain corners, where there was barely room for their feet. As Tom remarked grimly, a fat man never could have done it. Fortunately they were all comparatively thin, for their hard work, and not too abundant food, since they had left the airship, had reduced their weight.

Up and up they went, higher and higher, sometimes finding the path wide enough for two to walk abreast, and again seeing it narrow almost to a ribbon. They hardly dared look down into the chasm at their left--a chasm filled, in part, with the rocks and boulders tossed into it by the lightning bolt.

Tom was in the lead, and had just made a dangerous turn around a shoulder of rock--one of those places where he had to extend both arms, and fairly hug the cliff before he could get around.

But, when he had made it, and found himself on a broad pathway, cut in the living rock, he gave a great shout--a shout that caused his companions to hasten to his side. They found the young inventor pointing to a clump of bushes and small trees.

But it was not the shrubbery that Tom desired to call to their attention. They saw that in an instant, for, dimly seen through the leaves, was something black, and, as they looked more closely, they saw that it was a great hole in the side of the mountain--a vast cavern, opening like a tunnel.

"The cave! The cave!" cried Tom. "The diamond makers' cave

Hardly had he spoken than two men, each one carrying a gun, showed themselves in the mouth of the cavern, and, instant later they both ran toward the little party of adventurers.