We had proceeded in this manner some hundred yards or more, advancing cautiously, when we came to a break in the wall.  A few feet farther the wall began again.

“It’s a tunnel,” said Harry.

I nodded, forgetting he could not see me.  “Shall we take it?”

“Anything on a chance,” he answered, and we entered the passage.

It was quite narrow—­so narrow that we were forced to advance very slowly, feeling our way to avoid colliding with the walls.  The ground was strewn with fragments of rock, and a hasty step meant an almost certain fall and a bruised shin.  It was tedious work and incredibly fatiguing.

We had not rested a sufficient length of time to allow our bodies to recuperate from the struggle with the torrent; also, we began to feel the want of food.  Harry was the first to falter, but I spurred him on.  Then he stumbled and fell and lay still.

“Are you hurt?” I asked anxiously, bending over him.

“No,” was the answer.  “But I’m tired—­tired to death—­and I want to sleep.”

I was tempted myself, but I brought him to his feet, from some impulse I know not what.  For what was the use?  One spot was as good as another.  However, we struggled on.

Another hour and the passage broadened into a clearing.  At least so it seemed; the walls abruptly parted to the right and left.  And still the impenetrable, maddening darkness and awful silence!

We gave it up; we could go no farther.  A few useless minutes we wasted, searching for a soft spot to lie on—­moss, reeds, anything.  We found none, of course; but even the hard, unyielding rock was grateful to our exhausted bodies.  We lay side by side, using our ponchos for pillows; our clothing at least was dry.

I do not know how long I slept, but it seemed to me that I had barely dozed off when I was awakened by something—­what?

There was no sound to my strained ears.  I sat up, gazing intently into the darkness, shuddering without apparent reason.  Then I reflected that nothing is dangerous to a man who faces death, and I laughed aloud—­then trembled at the sound of my own voice.  Harry was in sound sleep beside me; his regular breathing told of its depth.

Again I lay down, but I could not sleep.  Some instinct, long forgotten, quivered within me, telling me that we were no longer alone.  And soon my ear justified it.

At first it was not a sound, but the mere shadow of one.  It was rhythmic, low, beating like a pulse.  What could it be?  Again I sat up, listening and peering into the darkness.  And this time I was not mistaken—­there was a sound, rustling, sibilant.

Little by little it increased, or rather approached, until it sounded but a few feet from me on every side, sinister and menacing.  It was the silent, suppressed breathing of something living—­whether animal or man—­creeping ever nearer.

Then was the darkness doubly horrible.  I sat paralyzed with my utter helplessness, though fear, thank Heaven, did not strike me!  I could hear no footstep; no sound of any kind but that low, rushing breathing; but it now was certain that whatever the thing was, it was not alone.