through the air, over the heads of rick, chot and ruddy, now seemingly on one side and now on the other—surrounding them, as it were—the mysterious noise came and went. now it almost died away—an expiring groan it might be from some unseen inhabitant of the tunnel. then again, it would fairly howl around them. and at the conclusion of one of these weird howls ruddy again joined his voice to that of the unseen one, making so nerve-racking a combination of notes as to cause cold shivers to run down the spines of the lads.

“whew!” whistled chot, as the sound seemed to vanish into the mysterious black recesses of the place, “this is too much for me!”

“you’re not going to quit; are you?” cried rick, for he saw the light of chot’s lantern drawing away.

“why not?” demanded chot. “this is fierce! you aren’t going to stay; are you?”

“i’m going to stay and i’m going on!” declared rick firmly.

“well,” went on chot, “i’m not going to desert, but when ruddy howls like he did—that’s enough. there’s something unhuman here, rick.”

“it doesn’t sound very pleasant,” admitted the boy. “there it comes again!” he cried, as, once more, the mysterious noise filled the black tunnel, which the lanterns of the boys seemed to make only the darker.

around them, above them, on all sides of the lads circulated that weird sighing, howling, groaning and yelling noise, as though hundreds of imps of blackness were calling to each other in the gloom, laughing in fiendish glee at the plight of the boys.

ruddy once more howled dismally, ending with such a queer note of protest in his voice that, in spite of his fears, rick laughed.

“what’s the matter, old fellow?” he asked, as he patted the dog’s head. “can’t you stand a little groaning?”

“if we only knew what it was,” spoke chot in rather a chattering voice. “do you reckon that’s just the wind making echoes in here, rick?”

“first i thought it was the wind, maybe blowing through holes in the rocks,” said rick. “i remember reading in the book ‘tom brown’s schooldays’ how there was a ‘blowing stone’ as it was called. a man in an inn blew through a hole in the stone back of the fireplace and the sound came out of a hill half a mile off. i thought maybe it was like that here, but there’s no wind.”

“no,” agreed chot, “or, anyhow, there isn’t enough wind to make all those howls. it blows a little, but not enough for that.”

the boys, as i have told you, noticed a wind blowing toward them through the tunnel as soon as they opened the closed end by removing the barrier stones. and after entering the black horizontal shaft they had been aware of a constant current of air in their faces, showing that there was an opening at the farther end which they had not yet reached. but, as chot remarked, there was not enough of the wind, or air current, to account for the noises.

“if the wind made it,” said chot, “we’d feel a sudden breeze as soon as the sound came.”

“that’s right,” agreed rick.

again echoed the howls and wails, like those of the fabled banshee of ireland, but the boys only felt the same gentle air currents in their faces.

“it might be there is a current of air higher up, away over our heads, that we don’t feel,” suggested chot. “that might cause it.”

“we’ll see,” said rick. he carried a long pole, and on the end of this he put the handle of an oil lantern, raising the light as high as he could toward the roof. “if there’s a current there it will flicker the light,” rick told his chum.

the two boys watched the lantern. it’s flame burned as steadily as when rick had held it, showing that there was no increase in the air current higher toward the roof. and yet the strange sounds kept up.

“well they can’t hurt us; that’s sure,” said rick, as he brought his lantern down. “i say let’s go on.”

“all right,” answered chot, but there was not much enthusiasm in his voice.

the mysterious sounds kept up as the boys and the dog advanced, but ruddy no longer howled in concert with them. perhaps he felt that it would do no good, and then, too, the confidence the boys exhibited, though perhaps they did not feel, made an impression on the setter. at any rate he seemed more contented.

and then, almost as suddenly as they had started, the noises died away. gradually they became less in volume until finally the boys noticed it. rick was the first to speak about it.

“say,” he called to chot, “we haven’t heard that howling for some time; have we?”

“no, and i wish it would stop forever,” said chot fervently.

“it might be that you can only hear it in that part of the tunnel where we were,” went on rick.

“how do you mean?” asked his chum.

“well, i mean it’s an echo and you can only hear it in certain places. you know back in frog hollow at home, there’s one place where there’s a big echo, but ten feet on either side of it you can’t make it echo at all.”

“yes, i remember that,” admitted chot. “it might be like that. anyhow the howls have stopped.”

and so they had—at least the boys did not hear them any more. this was a relief to them, and they began to feel hungry. they found some flat rocks, raised from the floor of the old river tunnel bed, and sat on these to open their lunch packets and water bottles, feeding ruddy on the scraps and pouring out some water for him in the hollow of a rock.

“he hasn’t lost his appetite, anyhow,” remarked rick with a laugh, as he noticed how eager ruddy was for crusts and bits of meat.

“he hardly ever does,” agreed chot.

then the boys kept on again, moving cautiously through the black tunnel. at one point they came to a ledge of rock over which, it was evident, some underground waterfall had tumbled when the river ran through the concealed cavern. but now the cascade was dry.

“guess we’re stuck,” remarked chot, as he looked at the abrupt face of the rock over which water had formerly toppled.

“maybe we can climb it on one side or the other,” suggested rick.

this they found they could do, ruddy managing to scramble up after they had helped him over the worst places.

again they found a fairly level road before them—a road that sloped slightly upward, this slope giving the downward current to lost river where it had emerged at uncle tod’s mine.

suddenly, as chot walked along a little in advance, he gave an exclamation.

“what is it?” asked rick, who was flashing his light upward, trying to ascertain how high the roof was.

“i see daylight!” cried chot.

rick hurried to his chum’s side. gleaming ahead of them was unmistakably daylight, coming through an irregularly shaped opening like another mouth to the tunnel. and, as the boys advanced nearer they saw, moving about, in the open beyond the tunnel’s mouth, several men.

“go easy!” whispered rick, catching his chum by the arm.

“all right,” assented chot. ruddy was held back. the boys cautiously advanced until they could look out upon a level place, seemingly in some valley and there, hidden from view as they were in the tunnel, they saw a strange camp.

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