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 gla.s.s. 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.

The sweep of wings sounded on the night air, and Espinosa drew back and squatted close to the ground, as immense green creatures, flying on dusty wings, issued from the mine.

"G.o.d, those are moths," breathed Maget.

Yes, unmistakably, they were moths, as large as condors. The green ones, but for their size, were lunar moths, familiar enough to the two tramps. More bats came, disturbed by the entrance of the two Gurlones.

Durkin broke, then. "I'm--I'm--I guess you're right, Maget," he whispered, in a terrified voice. "We should have never come. If my foot wasn't hurt, I'd start for the river now. Curse it, what a place!"

The booming, vast croaks filled the whole valley, reverberating through the hills. Wails sounded from the peon camp.

The big Portuguese was shouting to the Gurlones. "Come out, come out!"

Maget gripped his own rifle, and stood up, bravely. His fear, though it was great, seemed to have brought out the better side of the man, while Durkin, so brave at first, had cracked under the strain.

"Look out, they'll see you," whimpered Durkin.

Maget strode forward. A blast of fetid, stinking air struck his face, and he choked. The noises were now ear-splitting, but above the bellows came the sounds of the big rifles, the echoes booming through the recesses of the cavern.

Then the two Gurlones, running madly, burst from the mine entrance.

"Run," they screamed. "Run for your life, Espinosa!"

"I'll help you," cried Maget, and Durkin could detain him no longer.

The Gurlones hardly noticed the newcomer, as they ran madly towards the shelter of their houses. Espinosa joined them, going swiftly in spite of his blind eyes.

The croaking made Maget's brain scream with the immensity of the sound. Luminous, white disks, three feet in diameter, glared at him, and the creature, which progressed with jerky leaps toward him, almost filled the mouth of the mine.

It was hot in pursuit of the fleeing Gurlones. It squatted and then jumped, and presently it was out in the night air.

Its form was that of a gigantic frog, but it stood some twenty feet in height, and from its throat sounded the terrific bellowing which rivalled the thunder.

Maget bravely stepped forward, and began to fire into the huge, soft body. The great mouth opened, and as the dum-dum bullets tore gashes in the blackish green batrachian, the thunderous croaks took on a note of pain.

The odor of the creature was horrible. Maget could scarcely draw his breath as he fired the contents of the magazine into the big animal.

Two more jumps brought the frog almost to Maget's feet, and the tropical tramp felt a whiskerlike tentacle touch his face, and bad smelling slime covered him.

The frog was blind, without doubt, from its underground life, but the tentacles seemed to be the way it finally located its prey, for it turned on Maget and made a final snap at him. The great jaws closed like the flap of h.e.l.l, and Maget leaped back with a cry of triumphant terror.

The bullets had finally stopped the big frog, but at its heels came a strange, jellylike creature, not quite as bulky as the frog, but pushing along on its legs and with a tail some eight feet thick and fifteen feet in length. This, too, evidently a polywog, was blind, with whitened discs for eyes, but it slid along at a rapid rate because of its size. Maget's gun was empty; he turned so flee, but the polywog stopped and sniffed at the thick blood of its fellow. Then, to Maget's relief, it began to hungrily devour its companion.

Utterly filthy, and ferocious, the polywog in silence snapped great chunks from the dead giant frog.

"h.e.l.lo. Who are you?"

Maget turned, having forgotten the amenities of life in the excitement. Professor Gurlone and his son, still clad in their black suits, but with their helmets off, were standing beside him, clutching their guns and lights.

The big Portuguese, Espinosa, appeared, and Durkin was beside him.

"Why," said Maget, between gasps, "we just happened to be out exploring, and we saw your camp. We were on our way in when we heard the noises and came to investigate."

"I see," said old Gurlone. "What made you head in this direction, and where's your outfit?"

"Oh, we cached most of it back there," said Maget. "My partner's hurt his foot, so he can't walk well. Isn't that so, Durkin?"

"Yeh," growled Durkin. "I got a sore foot, all right."

Old Gurlone was suspicious of the vague story which Maget and Durkin concocted as the explanation of their presence in the valley. But evidently the Professor was too worried about the situation in which he and his friends were, to question the two tramps very closely. In fact, he seemed rather glad that he had two more pairs of hands to aid him and he thanked Maget for his bravery.

They dispatched the great polywog as it tore its parent to bits, and then the five men, the two Gurlones, Espinoza, Maget, and the limping, cursing Durkin, retired to one of the shacks.

The living quarters of the Gurlones was quite elaborate. There were many books on rough shelves, and there was a small bench filled with gla.s.s phials and chemicals, though the main laboratory was in one of the long buildings.

Professor Gurlone poured drinks for the five, and welcomed Durkin and Maget as allies.

"We'll need every man we can get, if we are to cope with these great creatures," said old Gurlone. "The peons are too frightened to be of use. Luckily, it was a frog we came upon on the banks of the subterranean river. There is no telling how many more creatures of the same or greater size may be down there. We will have to destroy them, every one."

Maget and Durkin shuddered. "Say," blurted Durkin, his face working nervously, "how the h.e.l.l did that frog get so big? I thought I was seein' things, Professor."

"No, no," said Professor Gurlone. "You see, the ore in the mine contains radium, that is, salts of radium. It is a pitchblende deposit, and it happens to be so rich in radium content that throughout the ages it has affected all the life in the cavern. The arid land surrounding the ore--this has been, generally, one of the characteristics of radium deposits--has kept most of the jungle creatures away, but underground beings such as reptiles, worms and frogs, have gradually become immune to the effects of the ore and have grown prodigiously and abnormally under the stimulation of the rays given off by the radium.

"Now, this is nothing strange in itself, but never before has such a rich deposit been discovered, so that the amounts of radium available have been too small to really check its effect on growth in animals.

That is our chief scientific object in coming here: we realized, from Senor Espinosa's description of the played-out silver mine he had, and from his loss of sight, that he had stumbled upon a valuable deposit of radium. It usually occurs with silver, that is, the uranium mother ore does, through the disintegration of which radium is formed. The content of radium per ton in this ore proved unbelievably rich: we were delighted. I have always suspected that the animal cell might be stimulated into abnormal growth by exposure to radium salts, for such a thing already has been hinted at in the scientific world. Not till our chance came here, however, has enough radium been available for the experiments."

Maget and Durkin listened with open mouths. Radium meant but vague things to them. They had heard of radium paint which shone in the dark on the dials of watches and clothes, but of the properties of the metal and its salts they were utterly ignorant.

"That radium stuff is what makes the funny light in that mine, then?"