through the black opening that confronted the boys came no sound. it was dark and mysterious. even ruddy, brave as he was, seemed to feel some mystic spell as he left, for a time, the hole beneath the gnarled tree and came to stand beside the lads. they saw him slink back and his tail droop between his legs.

“shall we go in?” asked chot again, rather puzzled by the silence of his chum. “ruddy doesn’t seem to like it, but maybe he’ll follow when we go in.”

rick shook his head.

“not yet,” he answered. “let’s wait until morning, and then we’ll take lanterns, ropes and things.”

“and something to eat,” added chot. “we may be gone all day. and are you going to tell uncle tod?”

“not until we find something that’s worth while telling,” was rick’s answer. “he and sam rockford would only laugh at us if they came here and found out we’d chucked aside these stones just to uncover a hole in the side of the hill.”

“i think it’s more than just a hole,” declared chot. “don’t you think it’s part of the tunnel?”

“i’m sure it is!” asserted rick. “you wouldn’t get that much air coming from just a hole or cave. there wouldn’t be any current. but you can feel how hard this wind pours out.”

“it sure does,” agreed chot, and, indeed, there was a very decided current of air coming from the opening they had uncovered by moving the stones.

“that shows there’s a shaft, or tunnel, with air coming in the other end,” declared rick. “now the thing for us to do is to go in and—”

“find lost river,” interrupted chot with a laugh.

“that’s it,” agreed his chum. “but we’ll go back to camp and start out again in the morning.”

“and aren’t you going to tell uncle tod?” chot asked.

“nope!” decided rick. “let’s have something worth while to tell him.”

“all right!” agreed chot.

and so it was decided. perhaps the boys were foolish in this, but they did not stop to consider the risks they took. few boys do. it is not the quality of youth to think. rush into danger, and, if possible, rush out again. that is why youth does so much—it seldom stops to count the cost.

“come on, ruddy!” called rick, for the dog, after a brief inspection of the “tunnel,” as the boys called it, an inspection which did not seem to indicate that he liked it—had gone back to the hole beneath the tree.

through the gathering darkness, but along a trail they now well knew, the boys and their dog tramped back to uncle tod’s camp. they went by the “outside route,” as they called it, as distinguished from the way leading through the tunnel in which lost river once flowed to wash out the pay dirt at the mine.

“where in the world have you lads been?” demanded uncle tod, as rick, ruddy and chot appeared some time after supper had been served.

“oh, prospecting,” answered rick, vaguely enough.

uncle tod laughed.

“guess he’s a chip from the old block—meaning myself,” he said to sam. “did you find any nuggets?” he asked.

“not yet,” answered rick with a look at chot to make sure his chum would say nothing of their discovery, which, after all, might amount to nothing.

“well, sit up and have some grub,” invited sam. “i kept the beans warm for you.”

“thanks,” murmured rick.

fortunately uncle tod and sam were too much occupied, in talking about a promising prospect they had discovered that day, to pay great attention to the boys, and so the men did not closely question rick and chot.

the two boys did not sleep as soundly nor as easily that night as they had on other nights since coming to lost river camp. the reason was they were thinking too much about what might lie in that dark and mysterious hole they had uncovered.

however, youth does not need very much sleep to refresh it, and what rick and chot obtained was enough to make them as fresh as daisies next morning. they were up, if not exactly with the lark, very shortly following that bird famed for early rising, and after breakfast uncle tod said:

“boys, sam and i are going off prospecting. it’s in a hard place, or we’d ask you to come along. i don’t like to leave you here at the camp, but—”

“oh, we don’t mind,” rick was quick to say. “we’ll go off by ourselves and have some fun.”

“all right,” agreed uncle tod, “but be careful, and take ruddy with you. that dog knows a lot.”

“he sure does,” assented rick.

matters were turning out just as he and chot hoped they would. the boys and dog could take what supplies and food they needed and spend all day exploring the mysterious tunnel.

“it couldn’t be better,” said rick exultantly as uncle tod and his partner shuffled off down the trail.

“that’s right,” agreed chot. “and if we come back and tell ’em we’ve found lost river—”

“oh, boy!” chanted rick.

they took with them everything they thought they would need in making the exploration, including food for themselves and ruddy. they also carried water bottles, for though they were on the trail of a disappeared river they might not find it.

behold them then, a little later, penetrating into the blackness of the tunnel, flashing on the sides and roof gleams from lanterns they carried—oil lanterns, with electric flashlights in their pockets for use in emergencies.

“do you think we might get walled up in here?” asked chot, as he and his chum, with ruddy, passed beneath the overhanging arch of fantastic boulders, below which they had dug the hole for themselves.

“walled up; what do you mean?” asked rick.

“i mean if these rocks took a notion to tumble down they’d fill the opening we made and maybe we couldn’t get out.”

“oh, i don’t believe that will happen,” said rick with the careless and joyous abandon of youth.

and so they went in. ruddy hung back for a moment, as if a bit suspicious of the undertaking, but when rick called to his dog the faithful companion of more than one exciting adventure came on with a wag of his tail as if saying:

“well, if anything happens it’s your fault.”

the boys had not penetrated many hundred feet into what was, undoubtedly a tunnel under the hill, or mountain, before they saw unmistakable signs that water had, at no distant time, flowed there. marks on the floor and walls showed them this, and there were, on rocky ledges several feet up from the floor, masses of dried sticks, leaves and other debris that indicated how the tunnel stream, at times, rose to higher levels. in receding, this debris was left caught in cracks and on ledges.

“but where is the river now?” asked chot, for there was no sign of moisture. the sides and bottom of the tunnel were very dry.

“i think some one took it,” was rick’s answer.

“you do? took it?”

“sure! i mean some one has changed the course of this stream. lost river used to run through this tunnel. now it doesn’t, and some one blasted out a lot of rocks from the end where we just came in and piled them up to hide the tunnel. i believe some one wanted the water of this river for their own mines, or maybe for farm irrigation, and they just changed the course of it.”

“how could they?”

“that’s what we’ve got to find out,” said rick. “come on, it may be a long way to the other end.”

the tunnel they were now in was as black, as dismal and as mysterious as the one they had walked through, starting at uncle tod’s camp and ending at the heap of stones. they went carefully, to avoid falling into holes or deep cracks, and swung their lanterns to and fro. ruddy, contrary to his usual habit, did not run on ahead, to explore on his own account. he kept close to the boys as if afraid.

the tunnel wound to right and left, like some gigantic snake. it was about twenty-five feet wide on the average, sometimes more and sometimes less. in places the roof was not more than ten feet above the heads of the boys and, again, they would be unable to see it in the gleam of their most powerful flash lights.

“must be a hundred feet up or more,” said rick after one of these tests.

“i believe you,” chot answered.

on and on they went, stopping now and then to listen for any sound that would indicate water. but no trickling, murmur or a louder thunder, that might mean a hidden waterfall, came to their ears.

“where do you reckon that river is?” asked chot, after a while.

“you’ve got me,” admitted rick. “but it has been here, that’s sure, and we haven’t come to the end yet.”

this was true, for the tunnel still stretched its black, winding and mysterious length ahead of them. the way was not without its dangers, for, more than once, rick found himself stepping on the very edge of a black hole.

and once chot would have fallen into a dismal chasm but that he caught hold of a projecting spur of rock and so saved himself. however these dangers seemed to the boys no more than others they had encountered when on previous excursions afield and in the forest. they were young and active, and to them a miss was as good as a mile, or “even a mile and a half,” as rick said.

it was nearly noon, which fact chot ascertained by a look at his cheap but reliable watch, and he was about to propose that they stop and eat when suddenly the hitherto silence of the tunnel was broken by a strange, mysterious noise. it was like some dismal giant groaning in agony.

“what’s that?” asked chot in a tense whisper.

“i don’t know,” answered rick. “listen!”

ruddy set up a frightened howling.