“Ice or—­”

“There it goes again!” exclaimed the Doctor.

This time the blow was heavier.  It sent them against the side of the compartment.

“Ice beneath the ocean?  Impossible!” exclaimed the Doctor.  “Must be rocks!”

Another blow hurled them in the opposite direction.  Both realized the gravity of this new peril.  If one of these blows caught the craft squarely it would crush the sub like an egg-shell.

But the boat was slowing up.  There was hope in that.  Dave, attempting to look out of one of the portholes, was thrown to the floor by another shock.  And this time the craft seemed to have stuck, for she did not move.

“Where can we be?” asked Dave, rubbing a bruised head.

It was a strange sight which met their eyes as they looked from the conning tower.  On every side appeared to be giant pillars of ice.  Between these were narrow water passages, while above they could make out a mass of ice far more opaque than any they had yet passed beneath.

“One of two things,” said the Doctor.  “We are beneath an iceberg or the end of a glacier.  Probably a glacier, and the pillars which support it reach to the bottom, which must not be far below us.”

“We have driven between two pillars and stuck there like a mouse in a trap,” said Dave, “and if we cannot set ourselves free, we are—­”

“It must be done!” exclaimed the Doctor.  “Start the power slowly and see what the propeller will do.”

Dave gave the signal.  There followed a harsh, grating sound, but the boat did not move.

“Stuck!” muttered Dave.

“Not so fast.” said the Doctor.  “There’s hope yet.  Shut off the power and order all hands aft.”

“Now,” said the Doctor, when they were assembled.  “We will go to the starboard side, then all together dash to port, and throw our weight against the side.  Then turn and rush back—­we want to make her roll.  Are you ready?  Go!”

The craft stirred a trifle at the shock.  The second attempt seemed to promise still better.  After they had repeated the operation half a dozen times, they were getting considerable side-wise movement out of the trapped submarine.

“Now,” said the Doctor, “start the power slowly, engine reversed.  The instant she is free, shut off the power.  On the precision of this operation depends all our lives, for should the propeller strike one of those pillars it will be torn away and our hope of escape gone.”

Dave’s hand trembled as he moved the lever.  For one second the propeller spun around.  Then, with a shudder, the craft started backward.  That instant Dave shut off the power.  The submarine drifted free.  So far, they were safe.

The Doctor consulted his watch.

“Time of low tide,” he observed “Guess we should be able to rise and get some air.  Try it.”

Slowly they rose to the surface, and there the craft rested.