g 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