So closely did the youths look about that they did not notice the wall of rock that appeared suddenly before them. Only Dr. Rander’s voice served to rouse them.
“Here we come to the first secret,” he asserted, pointing to a small opening in the wall of rock.
“So soon?” asked Bob wonderingly. “Why, I thought——”
“It is a tunnel,” explained the old explorer. “One that was made by the Incas. As far as I know, I am the only person who is aware of its existence.”
At once the boys were aflame with interest.
“And—and we must pass through it?” Bob peered at the narrow opening, which seemed no different from many other crevices they had seen.
“Yes. There is no other way to reach the cave of gold. At least not from this direction.”
The adventurers got through the opening easily, but the mules experienced more difficulty. And they did not at all like the idea of plunging into a dark tunnel.
But finally they were forced through by Joe, who had gone back outside. Then, with the aid of flashlights, the party proceeded to thread their way in the narrow passage.
“How much of this is there?” asked Joe, when fully five minutes had passed.