the five friends did not have it altogether easy. They fought hard, and they were wounded more than once, but not seriously enough to keep them long away from the firing line.

And now, after having fought hard and been given a short leave of absence, part of which they had spent in the old German dugout, the Khaki Boys are again in trouble, caused by the collapse of the underground place when the big shell or the bomb exploded.

Putting aside, for the moment, all thought of trying to dig their way out of their underground prison, the Khaki Boys proceeded to do what they could for the unfortunate Iggy. Their flashlights gave considerable illumination to the otherwise dark hole, and by looking around Jimmy and the others saw what had happened.

The shell, bomb, or whatever it was, must have landed squarely on top of the dugout. And though this top proper, being made of concrete, was some distance underground, it had been made to collapse as one might push in the end of an egg. The roof of the dugout was depressed while the walls still stood upright, though much cracked. The place was filled with débris.

It was owing to the fact that the strong, thick walls held that the boys' lives were not at once crushed out. They had been along the edges, and not standing in the middle, when the crash came.

"Lift him over to one side, and we'll see if we can tell what's the matter with him," suggested Roger.

Accordingly they scraped as much dirt and débris off the Polish lad as possible, and then half lifted and half dragged him over to a pile of blankets.

"Feel any better?" asked Jimmy, as he gave his friend another drink from a canteen.

"Yes, of a little betterness am I," was the answer. "But do not of mind me. I can get alongs. Look for a way out of these place if you can find him. Do not mind me."

"Well, maybe I'd better be looking to see if I can find the old tunnel," said Jimmy. "We'll need to find that, anyhow, to get ourselves out, as well as Iggy. Roger, you come with me. Bob and Franz can stay with Iggy. We'll be back as soon as we can," he added.

The two boys climbed over the débris, flashing their pocket torches at intervals, so as not to use up the batteries too quickly. Bob and Franz made Iggy as comfortable as possible.

"Seems to me the place was over here," observed Jimmy, as he pulled aside some loose and splintered boards.

Quickly Jimmy flashed the gleams of his small lamp over the cracked and broken walls of the dugout. At first he had no success, but finally he found what he was looking for.

"Here it is!" he cried. "An old door! It's been closed for some time, but here it is."

"Can it be opened?" asked Roger.

"That remains to be seen," replied Jimmy, as he limped nearer to it, for he had been struck on the leg by a piece of timber and he was painfully lame.

An examination showed that the door, though broken and jammed, was capable of being forced open. This was done by Roger and Jimmy, using pieces of planks for levers. At last the broken door swung to one side on its burst hinges, and beyond it a dark tunnel was seen.

"Here's the place!" cried Jimmy. "Now if we can go along this until we reach the other bomb-proof we shall get out all right."

"Yes—if we can," said Roger significantly. "Well, we've got to try, that's sure. Can't stay here very long. It may come down about our ears."

"I don't know about Iggy, though," went on Jimmy. "Of course we could carry him, but it would be hard work."

However, they were not compelled to do this. For when Roger and Jimmy went back to where they had left Bob and Franz looking after the wounded Polish lad, they found that Iggy could walk by leaning on the shoulder of someone.

"That's good!" exclaimed Jimmy. "Now then for the tunnel! But take it easy, everybody. We may be going into a blind lead, or a place worse than this. So go slow!"

And slowly and cautiously they entered the tunnel, Roger and Jimmy in the lead. One of them flashed a pocket torch at intervals, for the way was rough and treacherous.

Suddenly Roger, who had got a little in advance of Jimmy, caught the latter by the arm, and, pointing ahead, in a whisper, asked:

"What's that? Who are they?"

Jimmy, looking to where his friend pointed, uttered a guarded exclamation. He was gazing at a strange sight.

CHAPTER III
ANOTHER PUZZLE

"What's the matter up ahead there?" came the demand from Schnitzel, who, with Bob, was helping along the disabled Iggy. "Why don't you go on, Jimmy?"

"Is the way blocked?" asked Bob. "Are we stuck again?"

"Keep still back there!" fiercely whispered Sergeant Jimmy. "Don't make such a row, or they'll hear you!"

"Who?" asked Franz, and this time he lowered his voice to the desired pitch. "Are there some Germans up ahead? Did the blowing up of our dugout mean that our lines are smashed?" His voice was anxious. He and Bob could not see beyond the place where Roger and Jimmy had come to a halt.

"Just wait a minute," advised Jimmy, still keeping to a whisper. "Rodge and I just saw something that may be all right, and may be all wrong. We're going to see what it is. We'll tell you when we come back. Stay where you are with Iggy. It may not be safe to go on any farther."

Bob and Schnitzel let Iggy lean up against the tunnel wall. The Polish lad closed his eyes and made himself as comfortable as possible. His two companions looked ahead along the dark shaft which connected the two former German dugouts. They could dimly see Jimmy and Roger moving ahead, now and then cautiously flashing their pocket torches.

And the strange sight that had so startled the two leading Khaki Boys was this. In the second dugout, which did not seem to have been much damaged by the blast that, for a time, had buried the Khaki Boys, Roger and Jimmy saw four men. They stood in the middle of the old dugout, which had not been used in some time, and on a table, about which they were congregated, burned a candle stuck in the neck of a bottle.

But the curious fact about it all was that while two of the men wore the regulation American army uniform, the other two were in civilian attire. And it needed but an instant's thought on the part of Roger and Jimmy to make them understand that there was something vitally wrong here.

Civilians were not only not supposed to be so far within the front lines, but they were actually forbidden. It was against all military rules and regulations. No one without a uniform was allowed so near the front—even the newspaper correspondents being rigidly required to conform to certain rules in this respect.

The reason for this was obvious. So stern were the necessities of war that it was imperative that each man bear some distinctive mark. He was either a friend or a foe, and the only way this could be told, where there were so many thousands, was by a uniform.

Of course, the wearing of a uniform did not guarantee that the man inside it was a friend. He might be a spy. But the appearance of men in civilian garb within the army lines caused suspicion at once. And this suspicion was at once engendered in the minds of Roger and Jimmy.

"What do you think of that?" whispered Roger.

"I don't think very much," was Jimmy's answer, as they paused at an angle in the tunnel and gazed forward into the candle-lighted dugout. "It looks bad to me."

"That's what I say. Those are two doughboys, or some of our Sammies, anyhow. As for the other two—say, I haven't seen anyone in civies for so long it looks strange. What do you think those two civilians can be doing there talking to two of our men?"

"I give it up—at least for the present," said Jimmy. "It's another puzzle—like the time when we saw Captain Frank Dickerson at the red mill, maybe."

"They could be French refugees," went on Roger. "Maybe they have been held prisoners by the Germans, and just got