Lying there, Bob heard what seemed to be the first signal. It was as though some giant hand had tapped the solid rock with his club. Then faster came the blows, and more and more did the din increase, until it was fairly deafening. Only for his intense eagerness to hear every sound Bob might have been tempted to thrust his fingers into his ears in order to shut out the awful clamor.
To him it seemed as though a thousand anvils were being beaten in chorus, with a few other minor chords thrown in for good measure.
And what interested Bob most of all, as he crouched there listening, was the fact that all this dreadful noise seemed to be coming directly from the spot where his comrade had pointed out as the opening of a cave.
There was not the faintest trace of lightning accompanying the manifestation; and this proved, beyond all question of dispute, that the mystery connected with Thunder Mountain had nothing to do with an electrical storm. Possibly the observing Indians had many years ago discovered this same thing; and it had strengthened their belief that the great Manitou spoke to his red children through the voice of the wonderful mountain.
It took longer, this time, for the noise to die away; just as though, whatever its cause, there was increasing reluctance to subside again.
"That was a screamer, sure enough!" said Frank, when he could make himself heard above the declining roar.
"And Bob, you noticed, didn't you, that it seemed to come right out of that hole? All right, it begins to look now as if we were Johnny on the spot, if we've got the nerve to push things. Somewhere in there, Bob, lies the explanation of the mystery. Do we take the dare; or stay out here and wait till the fuss is over before entering?"
Bob possibly swallowed hard before replying. It was no easy thing for him to say the words that would thrust them up against so terrible a thing as this unknown peril awaiting them in the gloom of that crack of the great mountain. But his hesitation was brief. In fact, he only wanted to catch his breath, shut his teeth hard together, and summon his Kentucky blood.
"It's a go, Frank!" he said, with determination in his voice; "the chance may never come to us again. Let's go in, and discover for ourselves the secrets of the Indian god they say is guarding Thunder Mountain. I'm ready, so lead on!"