The search of the ruins was prosecuted with vigor for several days more before they stumbled upon anything in the way of “te-ter-treasure,” as Ding-dong Bell called it. But during that time the boys’ eyes had been so satiated with wonders of ancient architecture and carvings, that they had almost forgotten about the more material part of their quest.

One afternoon Nat and Joe had set forth to explore a temple which, hitherto, had not been entered. The professor would have accompanied them, but he was busy working up his field notes into his journal, and compiling in systematic form descriptions of the wonders of the island. Mr. Tubbs and Ding-dong had gone off making photographs, of which a goodly number had been taken, not forgetting several motion pictures, showing the explorers at work.

“Suppose we take a look over that queer, oblong building,” said Joe, as they set out, indicating a smaller building than the others, not so very far removed from the grand circle of structures fronting on the circular Sun Temple, which formed the “hub” of the island.

“Very well,” said Nat; “but I don’t suppose it contains anything but a replica of what we’ve seen already.”

“Well, inasmuch as the professor has made up his mind not to leave the island till everything has been explored and recorded, we might as well see what we can see in there,” went on Joe.

So the two lads set forth on their tour of exploration. The door of the temple they had elected to investigate was in fairly good preservation, the lintel post not having cracked, as was the case with most of the other buildings. The usual condition was an evidence of the severity of the earthquakes that must, from time to time, have shaken the island.

Passing through the entrance they found themselves in pitchy darkness. But, as they had long since found electric flashlights needful articles in searching the ruins, they soon had drawn out a couple of these and illuminated the gloom.

“This is a queer sort of place,” remarked Nat, looking about him as they flashed the lights hither and thither, “I wonder if the same peculiar feature about it has struck you as it has me.”

“What is that?” asked Joe.

“Why, in every other one of these old temples and ruins we have seen, there was every provision for the admittance of light; in fact, the old Incas were sun worshippers.”

“I see what you mean now,” cried Joe eagerly. “This place hasn’t a window in it.”

“No; that’s odd, isn’t it? I wonder if, by any chance, this can be the Temple of the Moon that the professor was anxious to discover.”

“By George! I shouldn’t wonder if you’ve hit on the explanation, Nat.”

“Do you think so?”

“I do.”

“Well, let’s carry on our investigations.”

“By all means. We may be on the verge of a great discovery of some sort.”

“I hope we don’t discover any more snakes.”

“Same here. Those beasts get on my nerves.”

“We’ve seen enough of them in the last few days to make you get accustomed to them.”

“That is true; but just the same, the more I see of them the less I like them. These ruins all seem to be alive with them.”

“I guess they are common in every part of this country.”

“Ugh! I can never think of that one that almost got poor Ding-dong without a shudder.”

“Well, let’s push on. This place seems to have a sort of dome for a roof.”

As he spoke, Nat flashed his light up till its beam of radiance showed a finely modeled but low dome above them. As the light fell on the concave structure, the lad gave a cry.

“Look, Joe! Look!”

“What? Where?”

“Up there, right above us!”

“Why, it’s a huge silver moon embossed on the dome!”

“That’s what it is. There is almost as much silver there as there is gold on the sacred dome. Those old fellows were not sparing with precious metals.”

“I should say not. But what’s that over there, Nat? Surely it’s a door.”

“Looks like one, anyhow. Let’s try it and see.”

The two lads crossed the stone floor, upon which the dust of the ages lay thick and rose in choking clouds, and reached the portal which Joe had pointed out. The great ring affixed to one side of it was of some peculiar sort of metal, not unlike bronze, and was untarnished.

Not without a faster beating of his heart, Nat turned the ring. It moved easily, and as it did so the door swung outward. It was of stone, and massive as the living rock itself.

Within they made out a flight of stairs that led steeply upward into the darkness.

“Are you game to try them?” asked Nat.

“Am I? I wouldn’t go out of here without seeing where they lead.”

“Well, go easy. They might give way. Heaven only knows how old they are.”

But the stairs proved solid. They wound upward steeply, worming their way around a central pillar covered with carvings. At last the boys emerged on a kind of platform at the top, which was roofed in by an irregularly shaped covering. Right in front of them were two round holes placed at some distance apart, and at their elbows were some curious-looking bits of apparatus. One of these looked like a gigantic bellows, and another was not unlike a megaphone in form.

“Well, where on earth are we now?” gasped Joe.

“I don’t know, but light is coming in through these holes. Let’s look out and see.”

The boys each took one of the circular windows and peered out. To their astonishment they looked into a vast cavernous chamber, lighted from the summit which admitted sunshine, the roof of which was supported by pillars. It was so vast that it took the breath away almost, to gaze into its great distances and heights.

The floor of this place was marked with a circle, about which were inscribed signs at regular intervals.

“Must have been their equivalent for the signs of the zodiac,” breathed Nat, awestruck at the enormous spaces before him.

“Then this was a temple,” said Joe looking down from his window at the great floor, which was fully twenty feet below where the boys stood peering.

“It must have been,” gasped out Nat, “and—and—Joe, we are in the very holy of holies of this island.”

“What do you mean?”

“Can’t you see? Look below you. We are peering out of the eyes of a huge idol made out of the rock. That stuff at the head of the stairs must have been the apparatus the priests used to make the idol speak and utter terrifying noises.”

There was no question but that Nat was right. Both boys could now make out beneath them, the rounded outlines of a huge squatting figure. In the head of this monstrous figure—its eyes, in fact—were the two circular holes through which they were looking.

“Gracious, what a sight it must have been when that temple was full of people of the vanished race, adoring this great idol,” murmured Nat, in awestruck tones.

“And what a job the priests must have had fooling them through that megaphone and that big bellows,” said Joe, the practical.

“That wouldn’t have detracted from the grandeur of the scene. It must have all been very real to them. Why, this place must be as vast as the hugest cathedral.”

“It gives me the shivers,” said Joe. “Hark, how your voice goes echoing off there among the pillars.”

“I wish there was some way of climbing down through these eyes. I’d like to explore that temple. I wonder where the entrance is.”

“Must be on the other side of the island. In the meantime, let’s look at the head of the stairs there, and see if we can discover anything else.”

The boys flashed their lights about among the pile of mouldering relics and machinery of the ancient priests. Suddenly Nat gave a shout of triumph.

“What do you make of this?”

“This” was a huge chest, the lid of which, bound and embossed with dully glittering metal, was open. It was full of various articles, some of which gleamed and flashed with gems. Nat plunged in his hand and drew out a golden breastplate. Joe followed this discovery by drawing forth a cup of what seemed to be pure turquoise. Various head-dresses of precious metal, more cups and vessels of gold, all jewel studded, followed.

“Well, we’ve found it,” breathed Nat; “we’ve found it, Joe, old boy.”

“Yes, and now we have, let’s take what we can of this stuff and get out of here,” said Joe. “We’ll come back with more lights and company. It’s getting kind of creepy and lonesome in the dark here.”

The boys loaded themselves with all they could carry, including the turquoise cup, and stumbled down the stairway. It did not take them long to retrace their steps and dump down their prizes in front of the astonished professor. He declared that the value of the turquoise cup alone was inestimable, while the jewels in some of the breastplates and vessels were worth more than he dared to name.

“I should say that what you have here would fetch two hundred and fifty thousand dollars in the value of the jewels alone,” he said. “As to what they are worth as relics of a vanished race, I am not prepared to say.”

Half an hour later, while they still sat awed and silent about the pile of wonderful relics, Ding-dong Bell appeared lugging an armful of photographic plates.

“We got some dandy pictures,” he began, “we—— Wer-wer-well, I’ll be jer-jer-jer-jig-gered!”

For the first time in his life Ding-dong Bell was fairly taken aback and bereft of all speech. He could only stand and blink in owl-like fashion at the marvelous display laid out before him.

“Nat! Nat! wake up!”

The voice sounded in the ear of the leader of the Motor Rangers, and was accompanied by a violent shaking of his shoulder.

“What is it, Joe? Here, quit shaking my bed, I——”

“I’m not shaking your bed, Nat. It’s the whole island that’s shaking! Quick, help me arouse the others!”

Nat was awake in a flash. As he hastily drew on some clothes a strange moaning noise filled the air. It was followed by a rushing sound overhead.

“It’s an earthquake!” exclaimed the professor, as soon as he was awakened.

As he spoke the whole structure of the Discoverer was shaken as if by a giant hand beneath her.

At the same instant the voice of old Matco was heard calling out as if in prayer.

“Get her loose, for heaven’s sake!” cried Mr. Tubbs, “or we’ll be destroyed!”

“It is the vengeance! The vengeance!” cried old Matco in Spanish, bursting into the cabin.

“Switch on the lights,” ordered the professor.

Joe sprang into the pilot house and threw the switch. A blaze of light illumined the aircraft. It showed a strange scene in her cabin. Half-dressed, and wholly bewildered, the adventurers were being thrown about like so many ninepins. The substructure of the Discoverer shook like an ague-stricken human being, as the earth beneath her rocked and rumbled.

Nat and Joe, the most self-possessed of any on board, sprang out upon the decks. The ropes had been tied, it not having been anticipated that they would want to leave in a hurry.

“Cut them!” shouted Nat above the hubbub about them.

The sky was being ripped and seared by livid lightning, while the flashes of light showed the lake to be a mass of white foam. The air was filled with a strange, roaring sound.

It was the voice of the earthquake. Nat had heard it once before in California.

As the boys’ knives fell on the ropes, the Discoverer shot upward. Up and up into the lightning-riven sky she arose, while beneath them the earth shook and rocked and rumbled.

“Great Scott!” cried a voice,—it was Nat’s,—“if ever we get struck by a flash of that lightning,—good-bye!”

The words sounded flippant, but the danger was real. The boy recalled reading of the fatal disaster to the great Zeppelin dirigible in a thunder-storm. But still they could not seek a refuge on the earth, at any rate not on the island. The air was the only place for them to seek safety.

The noise all about was nothing less than terrific. Voices could not be heard unless raised to a shout. The rigging of the dirigible creaked and groaned as the great bag swayed, and added to the distracting turmoil.

Paralyzed by the very suddenness and utter unexpectedness of it all, the adventurers for a time merely clung to the rails of their swaying, madly careening craft. How that night passed, none on board was exactly able to tell in after days.

They got the engine going, and held the big cloud cruiser as close to the earth as they dared, using the descending planes to steady her under the wild swaying of the great gas bag. A furious wind accompanied the earthquake, and when the lightning died away it seemed as if there was to be fresh and even more deadly peril, from the possibility of the great gas container being ripped bodily from the substructure.

But the rigging held tightly, and dawn found the disturbance almost at an end. It was a shaken, white-faced crew that regarded one another in the gray light. The night had been one to try the nerves of a man of iron, and the Motor Rangers were only youths.

However, the storm died out almost as swiftly as it had come, and breakfast and hot coffee heartened them wonderfully. Even old Matco plucked up his spirits, although, during the night, he was certain that they were bound to perish in the anger of the old gods of his country.

After the morning meal they began to look about them. They found that, during the night, they had been blown far to the southward of the site of the lost city, but they could still make out the ragged peaks that marked its locality.

The professor called a meeting, and it was unanimously decided to wing back and find out how the island of the dead had fared. They reached the spot by noon, and sailed over the peaks and gazed down into the place where the island should have been.

But no island was there!

It had vanished as completely as if it had been a dream. Only the waters of the lake rippled as placidly as of yore, hiding forever under their azure surface the city that had been and now was not.

Silent and stunned the adventurers turned the Discoverer’s prow toward the westward once more.

“If it wasn’t for those relics in the cabin,” said Nat pensively, “I should think that we’d dreamed it all.”

As he spoke he looked back toward the far horizon. Already the ragged peaks were fading on the sky and soon would be out of sight.

“After all,” said the professor at length, “perhaps it is better so than if that noble city of a vanished race had become the resort of gossiping tourists.”

And in after days they agreed with him; but with Nat and Joe it was long a bitter thought that they had left in the Temple of the Moon some of the most marvelous remains of an ancient civilization ever discovered.

The untimely ending of the existence of the wonderful island put an end also to the Motor Rangers’ aerial adventures, for the professor decided to abandon all attempts at relocating it and employing divers, as had been his first intention.

The voyage north was made on the staunch old Nomad, and Mr. Tubbs and the professor accompanied the boys. Old Matco received a substantial reward, and decided to spend his last days in the shelter of Bolivian cities rather than to take once more to the life of the forest.

As for Captain Lawless and his rascally mate, they were last heard of roaming about Bolivia, still seeking for the lost city, of whose destruction they were not aware. They had engineered an expedition with their remaining money for this purpose, but not, of course, till after their release from prison for firing at the airship. But as this was only a brief incarceration, it did not delay their plans much. The present chronicler is not in a position to state their ultimate fate.

It may be of interest to state here, that the crew they had so basely deserted, managed to regain their schooner from the rascally old island chief and sail her home, where they collected salvage from the owners.

The Motor Rangers enjoyed a long rest at home and then visited New York to aid in classifying and arranging the pictures and relics of the lost city. The cloud cruiser was sold to a syndicate, which long used her as a passenger craft at fairs and exhibitions, and it is safe to say that not one of her passengers ever dreamed of what the airship that carried him had passed through.

Their exciting adventures above the earth will ever remain to the trio of boys among their most thrilling recollections, says Nat; but in a recent letter to a friend he hints that tiring of inactivity he and his two chums have already started out in search of fresh incident and adventure.

From what Nat says the tale of their experiences should form a suitable sequel to the other volumes of this series, and it will be called: The Motor Rangers’ Wireless Station.