The floor, apparently, had been smoothed by human effort, but for the rest, the corridor was, to judge from the evidence, entirely natural for the walls of shiny black rock bore no marks of tools.
At intervals, other passages branched off from the main one we were following, at greater and less angles, but these were much narrower, and had very apparently been hewn in the solid rock. Like the central passage, they were utterly deserted.
"We'll be coming out on the other side, pretty soon," commented Correy after a steady descent of perhaps twenty minutes. "This tunnel must go all the way through. I—what's that?"
We paused and listened. From behind us came a soft, whispering sound, the nature of which we could not determine.
"Sounds like the shuffle of many feet, far behind," suggested Kincaide gravely.
"Or, more likely, the air rushing around the corners of those smaller passages," I suggested. "This is a drafty hole. Or it may be just the combined flarings of all these jets of flame."
"Maybe you're right, sir," nodded Correy. "Anyway, we won't worry about it until we have to. I guess we just keep on going?"
"That seems to be about all there is to do; we should enter one of the big subterranean chambers Fetters mentioned, before long."
As a matter of fact, it was but a minute or two later, that we turned a curve in the corridor and found ourselves looking into a vast open space, the roof supported by huge pillars of black stone, and the floor littered with rocky debris and mining tools thrown down by workmen.