Presently the mystery was explained.

Here in the cliff face there was a cleft, though one invisible even from a few paces away, since its outer edge projected over the inner wall of rock.

Moreover, this opening was not above four feet in width, a mere split in the huge mountain mass caused by some titanic convulsion in past ages.

For it was a definite split since, once entered, far, far above could be traced a faint line of light coming from the sky, although the gloom of the passage was such that torches, which were stored at hand, must be used by those who threaded it.

One man could have held the place against a hundred--until he was killed.