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.