Nature had no doubt begun, but man had certainly finished it.

For the floor level was comparatively free from harshness, and the outjutting projections of the sides and roof had been tempered, and progress was not difficult.
It was very narrow, however, and very low, and quite dark.

He could only drag himself along on his stomach like a worm.

But he pushed on with all the ardour of a discoverer.
Was it silver?
Was it smugglers?
Or what?
Wholly accidental formation he was sure it was not, though he thought it likely that man's handiwork had only turned Nature's to account.
The fissure had probably been there from the beginning of time, or it might be the result of numberless years of the slow wearing away of a softer vein of rock, but some man at some time had lighted on it, and followed it up, and with much labour had smoothed its natural asperities and used it for his own purposes.

And he was keen to learn what those purposes were.
To any ordinary man, accustomed to the ordinary amplitudes of life, and freedom to stretch his arms and legs and raise his head and fill his lungs with fresh air, a passage such as this would have been impossible.
Here and there, indeed, the walls widened somewhat through some fault in the rook, bur for the most part his elbows grazed the sides each time he moved them.
Even he, used as he was to such conditions, began at last to feel them oppressive.