Even as Blake and his chums looked about for some place of refuge, the firing of the German anti-aircraft guns began. These weapons, designed especially for shooting straight up and sending shrapnel shells to a considerable height, were rapidly manned and fired by crews that seemed to be in readiness for just such danger.

The raid of the French and American airships, quickly as the defensive preparations were made, seemed to take the Germans by surprise. That is the only way the boys could account for the fact that their guarding escort deserted them. For deserted they had been, some of the Germans running back in the direction whence Blake and the others had come, while a few, under orders from one of the German officers, helped to man the guns of which several score were now shooting at the aircraft high above the Hun position.

Joe, Blake and Charlie paused a moment, before seeking some shelter, to watch the thrilling sight. On came the aeroplanes, like a flock of great birds, and they did not resemble anything else quite so much, high up as they were. They came on in regular formation, for the day of the lone attack by an aeroplane was passed, except under special circumstances.

Straight for the German camp—if camp it could be called—came the flying squadron. As yet the airships were too high to be hit by the German guns, however great their range.

But the airships came on. Their speed was not apparent at so great a height, but it must have been wonderful, for but a few minutes seemed to have elapsed from the time they were first sighted, far down on the horizon, until they were almost overhead.

"And now's the time for us to get under cover!" said Blake. "When they begin to drop bombs, there'll be something doing around here."

"Where'll we go?" asked Charlie.

"Oh, there ought to be plenty of bomb-proofs and dug-outs in this camp. The Germans must have been air-raided before, or they wouldn't have the anti guns ready. The most likely place to find the best cyclone cellars will be near the officers' headquarters, I think. Trust those fellows to have a safe place ready."

"Do you think they are making the raid to help us?" asked Joe.

"Hardly," replied Blake. "They probably don't even know that we have been captured. No, I guess this has been in preparation on our side for some time, judging by the number of craft in it. I hope they wipe out this dump!"

"But not until we get under cover!" said Joe. "Look! There goes one of our ships!"

As he spoke a white cloud seemed to burst in the vicinity of one of the aircraft. The machine, which with the others had come lower down, was seen to dip and plunge. Then, after what seemed a dizzy fall, it straightened out again and kept up with the others.

"Hit but not disabled," murmured Blake, as he and his chums paused in their race for shelter. "The Germans are getting the range, I guess."

"Why don't we drop some bombs?" cried Joe, speaking as though he and his friends were personally engaged.

"I guess they're waiting until they get in a favorable position," returned Blake. "Look out! Here comes one!"

Something black dropped from one of the airships. It fell in a long curve, landing in a spot which the boys could not see, and an instant later there was a terrific explosion.

"That hit an ammunition dump, all right!" cried Charlie. "Duck, fellows!"

"In here!" yelled Blake, for at that moment they came opposite what looked like the entrance to a tunnel. It was lighted by small electric lamps and appeared to extend some distance into the earth. No one could be seen in it or entering it as the boys made a dive for it.

And it was well that Blake, Joe and their assistant found shelter when they did, for an instant later the whole area was under bombardment by the airships. The boys, racing through the tunnel, dug underground and timbered and braced as is a mine shaft, could not see what went on, but they could hear and imagine.

By this time the American and French aeroplanes were directly over the German camp and were dropping tons of explosives. The bombs struck and burst, some of them setting off stores of ammunition and powerful powder designed for the big guns. And these explosions, combined with the firing of the weapons aimed to bring down the flying enemy, made a pandemonium which penetrated even to the tunnel along which the boys were fleeing.

"That's some fight out there!" cried Joe.

"If we could only film it!" added Charlie, his voice and that of his chum ringing hollow in the tunnel.

"We'd stand about as much chance as we did when the volcano let loose in Earthquake Land," answered Blake. "Come on, fellows! This isn't over yet."

"I only hope we don't run into a party of Huns who'll drive us out," murmured Joe.

But, so far, they had met no one, though ahead of them they could hear a sound as though others were running through the underground shaft seeking a place of safety.

"Where are we going, anyhow?" asked Charlie at length.

"Going until we stop," answered Joe.

"And that'll be soon," added Blake, "for I see the last of the lights."

The boys looked down the long passage, which was well made and was high enough to permit them to run upright. It was wide enough, also, for three to go abreast. As Blake had said, the string of incandescent lights, suspended overhead, came to an end a little farther on. They stopped under the third light from the last and looked about them.

"Isn't this as good a place as any?" asked Joe. "If we go on any farther we may get into a hole we can't get out of. I say, let's stay here. We'll be safe from the airship bombs."

"I don't know about that," said Blake. "If you'll notice, we have come along pretty much on the level. This tunnel wasn't dug in the side of a hill. It went into the ground slanting, and at such a gradual slope that the top can't be very far under the surface."

"What does that mean?" asked Charlie.

"It means that we haven't much dirt over our heads, and if a bomb were to drop directly above us we'd be in a bad way. I think we'd better keep on until we get to a deeper part of the cave, or whatever it is."

"But we'll have to go on in the dark," objected Joe. "There are only three more lights, and——"

Suddenly came a muffled explosion, and the lights went out, leaving the place in black gloom.

"Now there aren't any lights," said Charlie, when the echo of the dull roar had passed away. The tunnel had been shaken, and there was a pattering sound all about the boys, as if little particles of earth had been dislodged, but no other damage appeared to have been done.

"It is dark!" said Blake. "But come on. Use your pocket lights. No, hold on. We'll use only one at a time. No telling how long we may need them."

Bringing out his own light, he flashed it on and led the way. Above them a continuous roar could now be heard, and they guessed that the airships were attacking in force, directly over the German camp, and were being fired at from all sides.

"One bomb must have splattered Fritz's electric plant," observed Joe, as he and his chums hurried on as best they could in the somewhat dim light of the little pocket lamp Blake carried.

Hardly had he spoken when there came a tremendous explosion—one that staggered the boys and seemed to crumple up the tunnel as though it were made of paper.

They had no time to cry out. They were thrown down and felt rocks and stones falling about them, while their ears were deafened by the roaring sound.

Then came silence and darkness—a darkness that weighed heavily on them all, while Blake, who had been in the lead, tried to move his hand to flash on the electric light that had gone out or been broken. He could barely move, and as he felt dirt and rocks all about him there was borne to his senses the horrible message:

"Buried alive!"

After that thought mercifully came unconsciousness.