rick and chot, to say nothing of ruddy, were in their element—just where they most delighted to be—engaged in something venturesome and penetrating into the unknown. for the tunnel or cavern, through which lost river had formerly flowed, was certainly unknown to the boys.

“and i don’t know much more about it than you do,” confessed uncle tod. “i wouldn’t risk taking you lads in, under those circumstances, except that we have ruddy with us. i depend a good deal on your dog, rick.”

“you mean to drive away any mountain lions if any come at us?” asked chot as they slowly made their way farther into the dark cavern.

“shucks! i don’t believe there are any mountains lions around here!” scoffed the miner. “nothing worse than skunks, and they’ll give us plenty of warning. no, it isn’t animals i’m afraid of.”

“what then?” asked rick, curiously.

“well, i don’t exactly know. there’s some sort of danger in here, but what it is nobody seems able to tell. sam says it’s ghosts, but shucks! i never saw a ghost yet that was worth a mess of beans! but, for all that, other miners around here say they wouldn’t venture into this tunnel.”

“maybe they’re afraid of the water suddenly coming back,” suggested chot.

“well, there may be something in that,” agreed uncle tod. “but if lost river starts to come back we can hear it and get out of the way. besides, the river never covered more than a small part of the bottom of the tunnel—that is when it was running at its best. there was room to walk on either side of it, and it wasn’t deep in the middle. so even if the water should come back it wouldn’t harm us.”

“unless,” said rick, “we happened to be in a narrow part of the tunnel where the river filled it completely.”

“well, yes,” admitted uncle tod, “in that case it might be dangerous. but we won’t enter any narrow part unless we see it’s safe. no, it isn’t the water i’m worried about. it’s some unknown sort of danger that sam fears, and that other miners around here fear.”

“have other miners spoken of it?” asked chot.

“yes, several of ’em since the water stopped. when my mine went dry, and there wasn’t any more chance of working it, i said i was going in this tunnel and see what the trouble was. i was advised against it by several. they said there was a story that, years ago, the water stopped running. some indians went in to see why and—well, they never came out again.” uncle tod shook his head dubiously.

“did the water start flowing once more?” rick wanted to know.

“yes, it must have, for it’s been running for years. no one around here has ever seen it dry—it’s just a rumor that it was.”

“i don’t see what there is to be afraid of,” remarked chot. “if it isn’t animals, and the water itself doesn’t nearly fill the tunnel, what can it be?”

“i wish i knew,” sighed uncle tod. “it’s like looking for something you don’t know about and in the dark at that—for these lanterns don’t give much light. but, as i said, i wouldn’t have brought you boys out here except i believed you might happen to think of some things sam and i couldn’t. you boys are smart, and so is ruddy. i trust a dog where i wouldn’t a man, in sensing danger.”

“that’s right!” cried chot. “’member the broken bridge, rick?”

“i should say i did,” and when they repeated this story in detail to uncle tod—for they had barely mentioned it before—the old miner exclaimed:

“there! what’d i tell you? ruddy is what we got to depend on. he’ll give us warning of danger, and i might as well say that what i fear worst is getting lost in here or tumbling down some deep hole. so mind your steps, boys! we have ropes to help us in case we take a tumble, but watch out just the same.”

thus warned the boys stepped cautiously enough, and ruddy, too, seemed filled with a wholesome respect for the place, as he did not rush about blindly, nosing here and there as he did out in the open. he kept close to his friends, going only a little way ahead, and not out of range of the glimmer of the lanterns. and then, looking back, he would wait for the party to come up to him.

“ruddy knows his business,” said uncle tod. “i thought of him first shot when i saw what had occurred here, and that’s why i wanted you to bring him, rick. two boys and a dog are equal to any mystery that ever happened.”

they were now fairly within the long, winding cavern or tunnel that led under the mountain and served as a course for lost river when that stream condescended to be found. just now no one knew where the river had hidden itself, though it was reasonable to suppose that it followed the general law of water and ran down hill. in that case it must either be flowing under the feet of the explorers, perhaps a mile or more below them, or it was off to their left or right, more or less underground.

there are underground rivers in many parts of the world, and they are always more or less of a mystery. i, myself, have explored some of them, and have been puzzled, as nearly everyone has, over the strange behavior of the streams. they appear on top of the earth, and then suddenly dip down into a gorge that they have worn away, often through solid rock. then they disappear beneath the surface of the earth, to appear miles farther on, having gone through passages never seen by mortal eyes.

often it cannot be said whether the reappearing river is the same one you start to trace, or another that has taken its place underground. it is all guess work, and as such is very fascinating.

so you can easily see that rick and chot were delighted with this opportunity of being with uncle tod, and the danger of it did not at all worry or impress them.

“we’ll find lost river!” declared rick.

“sure we will!” agreed chot.

ruddy, of course, said nothing, but from the manner in which he nosed about it might be assumed that he would do his share of the exploration work and warn of any danger he sensed.

flashing their lights to and fro—for each carried a lantern—the travelers in the tunnel looked about them. under the suggestion of uncle tod the boys were searching for some side passage, or downward dip into which lost river might have slipped, thus keeping away from the mine where its waters were much needed.

suddenly, as chot stepped a little ahead of his two friends, and off to the right, the boy gave a cry of astonishment, mingled with fear and then came the thud of a fall.

“chot’s gone!” exclaimed uncle tod, hurrying to the place where the lad was last seen.

ruddy uttered a bark of warning it seemed as he dashed up along side of the miner, and only just in time, for uncle tod stopped short on the edge of a deep and black hole. rick, at his uncle’s side, gave a gasp of fear and swung his lantern over the chasm.

“are you there, chot?” he cried desperately.

to the great relief of the two, back came the boy’s voice in answer:

“sure i’m here! it wasn’t much of a tumble, but my lantern went out. didn’t break, though, i guess.”

“are you hurt?” asked uncle tod.

“nope!” cheerfully answered chot. “there’s a lot of old leaves and stuff down here and i fell on that. if you lower the rope i can get up all right.”

“it’s good we brought the rope,” said rick, as his uncle uncoiled it.

“yes, i figured on something like this,” said the miner.

it was not difficult to pull chot up, for the hole into which he had fallen was not deep. the lad was bruised and shaken up, but not otherwise harmed.

“we have got to be more careful,” declared uncle tod.

but, with all their care the same accident happened again, a little farther on, only it was rick who fell in. and he fell harder and deeper than chot, with the result that he received a badly bruised left arm which gave him great pain.

“want to go back?” asked uncle tod.

“no, sir!” exclaimed rick, gritting his teeth to keep back a groan of anguish.

so they went on. but when uncle tod himself slipped over a small ledge, turning on his ankle with force enough to make him limp, and when chot just saved himself from plunging into another hole, uncle tod said:

“i’m through! i’m going to give up!”

“what?” cried rick. “why, we haven’t explored half the tunnel yet!”

“yes we have,” was the answer. “there’s the end now! it is shorter than i thought, and there isn’t a sign of water. i’m through i tell you. might as well give up the mine.”

“what do you mean—the end of the tunnel?” asked rick.

in answer his uncle pointed to the right and the boys could see daylight glimmering where, before, only invisible blackness had been ahead of them.

what did it mean?