Indeed, I have little doubt that this peculiar passageway had been constructed by artificial means.

Every now and then, when a faint light from without straggled through the interlaced boughs overhead, I caught a glimpse of the evidences of human labor.
This odd passage, crooked and intricate, at times so steep as to require the chiselling of steps in the solid rock, wound in and out along the side of the cliff, then ran back into the very face of the precipice, for more than a hundred and fifty yards.

Suddenly we emerged, fifty yards back from the crest, in the heart of a great circular hole resembling the crater of a burned-out volcano, having great ragged points of rock, blackened as if incased with lava, jutting up upon every side, and forming as desolate and barren a picture as ever eyes looked upon.
I was completely fagged by this time, the climb being a heavy one, and I noticed De Noyan was ghastly of face, his body trembling like that of a palsied man.

But our relentless drivers permitted no halting to recruit strength.

The Chevalier was evidently in greater distress than I, so from pity I bade him lean on my shoulder; but as he sought to draw near, the merciless brute on guard struck him savagely, and there was such shaking of spears and fierce uproar on the part of our escort, we could do naught else than set our teeth to it, and go staggering on.
The slight path, if it might be named a path, led in and out among the black lava cones in such labyrinthine fashion that no man could hope to retain memory of its course, while the floor being of irregular stone, the passing feet left no trail for future guidance.