The three inside the shack listened, and so did Durkin and Maget. The booming sounds swelled louder and the earth of the valley shook. "I t'ink we better go," said Espinosa gruffly. "I agree with your son, Professor." "No, no. We can conquer this, what ever it is." "You see, father, while you were away, we broke through into a natural cavern, an underground river. It was then that the trouble started. You know the effect of the stuff on the insects and birds. It enlarged a cricket one hundred times. You saw that yourself. Six of the peons have disappeared--they didn't run away, either. They went down the shaft and never came back." "Oh, they probably fell into the water and drowned," said old Gurlone impatiently. "Even if they did not, we can kill anything with these large bore rifles." "We'd better pull out and let it alone for a while," said young Gurlone gravely. "The peons have been trying to bolt for several days. They'd be gone now if I hadn't penned them in and electrified the fence." * * * * * Maget put his hand on his friend's shoulder. "I'm starving," he whispered. Durkin nodded, and they turned away, toward what they had marked as a supply shack. They heard a low murmur from the peons' pen, as they began to break off the hasps of the lock which held the door of the storehouse. They got inside with little trouble, and began to feel about in the dark for food. They located biscuits and canned goods which they split open, and these they wolfed hungrily, listening carefully for sounds from outside. "Here they come," said Maget, gripping Durkin's arm. They looked out the window of the supply shack, and saw old Gurlone issue from the building outside which the two tramps had been listening. In one hand, the old Professor, brave as a lion, carried an old fashioned double-barreled elephant gun, and the rays from a powerful electric torch shone across the barrel. At least, they thought the bizarre figure was old Gurlone, from the size. For the man was clad in a black, shiny suit, and over his head was a flapping hood of the same material in which were large eyeholes of green glass. Behind this strange form came a larger one, armed also with a big bore rifle and with another powerful flashlight. The blind Portuguese was armed, too, but he was not clad in the black suit. He took his stand beside the mouth of the cavern, and waited while the two Gurlones entered the mine. "My foot hurts," said Durkin suddenly, breaking the silence. "I'm going out and see what happens," said Maget. * * * * * Durkin limped after Maget, who now took the lead. They crept close as possible to the mine opening, and saw the big Portuguese standing there in silence, listening carefully. Any sounds the two might have made were drowned in the great bellowing from within the cavern. These noises, so like the croak of bullfrogs but magnified a thousand times, were terrifying to the heart.