level which showed several inches of water.

"We can't wade through that!" Tommy declared.

"Well," Sandy suggested, "if we go back a little ways, we can follow a cross heading and get into the mine by another way."

The boys followed this plan, and, after winding about several half-loaded cars which had been left on the tramway, found themselves in a large chamber from which numerous benches were cut.

"Where does all this gas come from?" asked Tommy stopping short and putting a hand to his nose.

"There must be a blower somewhere," Sandy explained.

"What's a blower?" demanded Tommy. "What does it look like, and does it always smell like this?"

"It doesn't look like anything!" replied Sandy. "It's composed of natural gas, and they call it a blower because it blows up out of crevices in the coal and in the rocks."

"If I should light a match, would it set it on fire?" asked Tommy.

"I wouldn't like to have you try it!"

The boys continued on their way for some moments, and then Tommy stopped and extinguished his light, whispering to Sandy to do the same.

"What's that for?" demanded the latter.

"Didn't you hear that noise behind the cribbing?" asked Tommy.

"Rats, probably!"

"Rats nothing!" replied Tommy. "Rats don't make sounds like people whispering, do they? Keep still a minute, and we'll find out what it is!"

"You'll be seeing a light next!" Sandy suggested.

"I see it now!" answered Tommy.

Sandy saw it, too, in a moment. It seemed at first to be floating in the air at the very top of the gangway. It moved from side to side, and finally dropped down nearer to the floor. There seemed to be no one near it or under it. Its small circle of illumination showed only the empty air.

"What do you make of it?" asked Tommy.

"Is this Tunnel Six?" asked his chum.

"I don't know! If it is, we've seen the light the caretaker referred to. We'll have a great story to tell in the morning!"

The boys stood in the darkness of the gangway watching the light for what seemed to them to be a long time. Now the light advanced toward them, now it receded. Now it lifted to the roof of the gangway, now it dropped almost to the floor.

At intervals, the noises behind the cribbing to which Tommy had referred were repeated, and the boys at last moved over so as to stand with their ears almost against the wooden walls.

"There is some one behind the cribbing, all right!" Tommy declared. "I hear some one breathing."

"Aw, keep still!" whispered Sandy. "If there is anyone there, you'll frighten them away! I thought I heard some one myself!"

"I'll tell you what I think," Tommy suggested in a moment, "and that is that either Will and George, or both of them, beat us to this gangway. They are hiding behind there on purpose to give us a scare."

"That's a dream!" replied Sandy. "We left them both asleep."

"Dream, is it?" repeated Tommy scornfully. "You just listen to the sound that comes from behind this cribbing, and tell me what you make of it!"

Both boys listened intently for a moment, and then Sandy switched on his light and moved swiftly along the cribbing as if in search of an opening. Tommy gazed at him in astonishment.

"You've gone and done it now!" he said.

"There's some one in here all right!" Sandy explained. "Did you hear the call of the pack a minute ago? There are Boy Scouts in there, and what we hear are the signals of the Wolf Patrol."

"That's right!" cried Tommy excitedly. "That's right!"

Chapter III

WHO CUT THE STRING

"Do you suppose he would understand the call of the Beaver Patrol?" asked Sandy. "I'm going to try him, anyway!"

The boy brought his hands together in imitation of the slap of a beaver's tail on the water, and listened for some reply.

"He'll understand that if he's up on Boy Scout literature," suggested Sandy. "He ought to be wise to the signs of the different patrols if he's a good Boy Scout."

There was a short silence, broken only by the constant drip of the water in an adjoining chamber, and then the call of the pack came again, clearly, sharply and apparently only a short distance away.

"What did Mr. Canfield call those two boys we are looking after?" asked Sandy, after waiting a short time for the repetition of the sound.

"Jimmie Maynard and Dick Thompson," replied Tommy.

Sandy threw out his chest and cried out at the top of his lungs:

"Hello, Jimmie! Hello, Dick!"

The lad's voice echoed dismally throughout the labyrinth of passages, but there was no other reply. Tommy and Sandy gave the call of the Beaver Patrol repeatedly, but the call of the Wolf pack was heard no more.

"I'll bet it's some trick!" exclaimed Sandy after waiting in the chamber for a long time in the hope of hearing another call from the boys who were hidden somewhere behind the cribbing.

"What do you mean by trick?" demanded Tommy.

"Why, I mean that some of the breaker boys, out of work because of the stoppage of operations, may have sneaked into the mine on purpose to produce the impression that there are ghosts here."

"But ghosts wouldn't be giving signals of the Wolf Pack, would they?" asked Tommy.

"Not unless they were Scouts," replied the other.

"Oh well, of course the kids would want to test us, wouldn't they, seeing that we were only boys?"

"Well, we've discovered one thing by coming down," said Tommy, "and that is that there really are people in the mine who have no business here."

"Then we may as well go back to bed," advised Sandy.

"Do you know how many corners we've turned since we came in here?" asked Tommy.

"About a thousand, I guess," replied Sandy.

"Yes, and we'd have a fine old time getting out if you hadn't brought that ball of twine!"

"Tell you what we'll do," Sandy said, as the boys turned their faces down the gangway, "we'll pass around the next shoulder of rock and then shut off our lights. Perhaps the kids who gave the cry of the pack in there will then show their light again."

"That's a good idea, too!"

The boys came at length to a brattice, which is a screen, of either wood or heavy cloth, set up in a passage to divert the current of air to a bench where workmen are engaged, and dodged down behind it, first shutting off their lights, of course.

"Now, come on with your old light," whispered Tommy.

As if in answer to the boy's challenge, the light showed again, apparently but a few yards away from their hiding place.

A moment later the call of the pack, sounding louder than before, rang through the passage. The boys sprang to their feet and switched on their lights.

"Why don't you come out and show yourselves?" shouted Tommy.

"I don't believe you're Scouts at all!" declared Sandy.

There was no answer. The boys could hear the drip of water and the purring of the current as it crept into a lower gangway, but that was all.

"That settles it for tonight!" exclaimed Tommy. "I'm not going to hang around here waiting for Boy Scouts who don't respond to signals!"

"That's me!" agreed Sandy. "We'll go to bed and think the matter over. There may be some way of trapping those fellows."

"Suppose it should be Jimmie Maynard and Dick Thompson?" asked Tommy.

"Then we'd have the case closed up in a jiffy!" was the reply.

Before leaving that particular chamber, Tommy selected a large round piece of "gob," placed it in the center of the open space, and laid another small piece of shale on top of it.

"What are you doing that for?" demanded Sandy.

"Don't you know your Indian signs?" demanded the boy. "That means 'This is the trail.' Now I'll put a stone to the right, and that will tell these imitation Boy Scouts to turn to the right if they want to get out."

"I guess they can get out if they want