“CAREFUL, now.”

Dr. Rander was slowly and dangerously hanging from the heavy wire with his hands, working his way steadily across the roaring rapids below. If he were to fall, it would probably seal his doom, even though the youths held one end of a stout rope that was tied securely around his waist.

“Think he’ll make it?” Bob looked on anxiously, half expecting to see the old man let go and plunge into the seething water below.

Slowly but surely he swung across, holding on with a grip of steel. It seemed remarkable that a person of his age could withstand the arduous tasks connected with exploration. But no doubt he was hardened to an eventful outdoor life of adventure.

“He’s over,” cried Joe happily. “Made it all right. Didn’t even threaten to let go. Now I wonder if we can do as well?”

The youths had yet to see.

Joe was next to swing across. He walked over to the cable and gazed doubtfully first at the boiling stream below, then at Dr. Rander on the opposite cliff.

“Go ahead,” said Bob. “Might as well get it over.”

“Yeah. But I wish it weren’t necessary to do this. I suppose, though, that I can do it if Dr. Rander can.”

Joe sat down on the edge of the cliff, took hold of the heavy wire, and slowly let himself down.

For a brief moment it seemed as if his arms would be torn from their sockets, for, although he was not heavy, the strain was very great.

“I’ve got to make it!” he told himself, gritting his teeth. He refused to think of what might happen if he were to plunge into the roaring rapids below.

“Don’t let go, whatever you do!” Bob, on the rock, was holding the rope tightly, ready to pull his friend to safety if the latter should be forced to release his grip.

Joe swung across with a certain determination that was luckily with him in every emergency. He did not look below for fear of becoming frightened.

At last, when he felt that he could stand no more of this torture, the youth felt his foot touch rock, and he knew that he was safe.

“Hurrah!” cried Bob Holton, waving his arms in the air. “Now here I come.”

Bob would find the feat more difficult, since he was heavier than either of the two who had crossed. But usually when he set his mind on doing a thing he was able to do it.

But first he walked over to the mules, which had been tethered securely by Dr. Rander, and got out a stronger rope, which he tied around his waist and shoulders. A small weight he fixed at the other end, and then moved back to the edge of the rock.

“Here. Catch this,” he called, and threw the rope over to Joe on the other side of the river.

When everything was in readiness he slowly lowered himself until he could grasp the wire that stretched across the stream. Then, setting his nerves for the trying task, he let his feet drop.

For one awful minute Bob’s heart stood still. Then he got a grip on himself and swung easily across to the other side.

“That was fine!” praised Dr. Rander. “You never flinched. Now let’s hurry up the mountain to the treasure.”

“Do you suppose the mules will be safe over there?” queried Joe anxiously. “We couldn’t take any food with us across the river. If anything should happen to them——”

“There is no danger,” the old man assured him. “Probably no one has been in this section for years and years.”

Notwithstanding this, the young men were still worried. They realized that they would be in a grave predicament if anything should befall the provisions.

This anxiety gradually wore off, however, as they neared the treasure cave.

“Hard climb, but nothing will stop us now,” smiled Joe happily.

“You will be amazed when you look upon the wonders in the cave,” Dr. Rander told them. “It will exceed your wildest dream.”

“How did they all get there?” inquired Bob.

“I do not know. They were placed there by some wealthy Inca—perhaps a king—for safe keeping. It might have been that this was at the time of the Spanish invasion, and that the owner was later killed by the conquering troops. At any rate, the secret remained as such until I accidentally found it.”

“You sure were lucky,” remarked Joe. “Perhaps it wouldn’t have been discovered at all if you hadn’t located it.”

At a point near the ledge, the mountain grew so steep as to make climbing a very arduous task. A single misstep would have meant a horrible plunge into the roaring rapids below.

At last they came to the top of the ledge, before the notch cut in the mountainside.

“See that dark entrance?” asked Dr. Rander. “That is a tunnel that leads to the cavern. Come.”

The youths followed the old man into the opening, which was made light by the latter’s flashlight. The passage seemed to lead steadily upward, probably into the peak of the mountain.

“Do you suppose this tunnel was constructed?” asked Joe, after five minutes had passed.

“I believe so,” Dr. Rander returned. “When we get farther on, you will think so too.”

Fifteen minutes of walking brought them to the end of the tunnel. From all appearances there was nothing but natural rock before them.

“Watch,” said Dr. Rander, moving over to the end of the cave.

He reached up and pressed a mysterious button. Then he stepped back and waited.

Suddenly the youths gasped in awe, as they saw the huge stone wall slowly swing inward. As it made an opening, a beam of natural light flashed into the cave, making the use of a flashlight no longer necessary.

“Huh! That’s funny!” muttered Joe. “Must be an entrance to the outside somewhere.”

“If you can find it, it is more than I can do,” laughed Dr. Rander.

The opening grew larger, and the light brighter. When the ponderous stone wall had swung away to the utmost, Bob and Joe followed the old man through the entrance.

As they did so their eyes opened wide and their jaws dropped at the sight that lay before them. Ahead was a great stone room, fully fifty feet square, in which were scores of objects carved from stone. Statues of men, birds, llamas, pumas, and many other animals were all about. Ears of corn, plows, chairs, pottery—all these and many more objects were carved out of stone.

It was a wonderful collection, one that would delight the eye of any archÆologist. Bob and Joe inspected the various objects with interest, knowing that Dr. Rust and his fellow scientists would give a great deal to know of the existence of this place of wonder.

“All this is very interesting,” remarked the old man. “But what we really came after was treasure—if there is any.”

“Yeah. Where is the place where you said it might be?” Bob was anxious to look for something still more valuable than the stone objects.

“I will show you.”

Dr. Rander stepped over to one corner of the large room, where was a huge statue of a man, perhaps an Inca king.

“Now look closely,” he said, when the youths had followed him.

The old man walked up to the statue and stood directly in front of it. Then he waited.

At that moment something happened that caused Bob and Joe to gasp in wonder. Slowly, surely, the statue sank into the floor until it disappeared from sight. Directly below the resulting opening was a ladder, which led down into another gigantic room.

“Well, of all things!” cried Joe in utter amazement. “That sure is a secret if there ever was one.”

“Let us go down.” Dr. Rander was making his way down the ladder, motioning for the youths to follow.

They did and soon found themselves in a huge cavern cut out of solid rock. Here were more stone objects similar to the ones in the room above them.

“This way,” pointed out the old man, walking over to one side of the cave.

“Wonder where the light comes from?” pondered Bob. “It’s from the outside, all right, but how does it get in?”

Dr. Rander bent over and pressed something in a crack in the floor, straightening up a moment later.

Again the youths were filled with amazement. A huge stone block swung away on unseen hinges, leaving an opening perhaps four feet in width.

“Now we must use flashlights,” Dr. Rander said, stepping inside. “There is no opening to the outside, apparently.”

The beams of the electric torches revealed the fact that they were in another cave.

“Over here,” explained the old man, “is a sort of bin cut out of the rock. It is covered with a stone that I cannot lift. But I feel sure that all three of us can.”

“Let’s have a look at it,” said Bob.

Together they heaved on the stone block with all their strength. Before long they saw that their efforts were not in vain. The stone was slowly slipping from the top of the bin. At length it fell to the floor with a dull thud.

The explorers turned the beams of the flashlights into the opening, looking about eagerly.

“Here’s something,” announced Joe, bringing out a small iron box.

“Open it,” directed Dr. Rander.

Joe did—and then cried out in wonder as he saw its contents.