It went down at an acute angle for some fifty feet, the floor being covered with broken stone. Thence there extended a long, straight passage cut in the solid rock. I am no geologist, but the lining of this corridor was certainly of some harder material than limestone, for there were points where I could actually see the tool-marks which the old miners had left in their excavation, as fresh as if they had been done yesterday. Down this strange, old-world corridor I stumbled, my feeble flame throwing a dim circle of light around me, which made the shadows beyond the more threatening and obscure. Finally, I came to a spot where the Roman tunnel opened into a water-worn cavern--a huge hall, hung with long white icicles of lime deposit. From this central chamber I could dimly perceive that a number of passages worn by the subterranean streams wound away into the depths of the earth. I was standing there wondering whether I had better return, or whether I dare venture farther into this dangerous labyrinth, when my eyes fell upon something at my feet which strongly arrested my attention. The greater part of the floor of the cavern was covered with boulders of rock or with hard incrustations of lime, but at this particular point there had been a drip from the distant roof, which had left a patch of soft mud. In the very centre of this there was a huge mark--an ill-defined blotch, deep, broad and irregular, as if a great boulder had fallen upon it. No loose stone lay near, however, nor was there anything to account for the impression. It was far too large to be caused by any possible animal, and besides, there was only the one, and the patch of mud was of such a size that no reasonable stride could have covered it. As I rose from the examination of that singular mark and then looked round into the black shadows which hemmed me in, I must confess that I felt for a moment a most unpleasant sinking of my heart, and that, do what I could, the candle trembled in my outstretched hand. I soon recovered my nerve, however, when I reflected how absurd it was to associate so huge and shapeless a mark with the track of any known animal. Even an elephant could not have produced it. I determined, therefore, that I would not be scared by vague and senseless fears from carrying out my exploration. Before proceeding, I took good note of a curious rock formation in the wall by which I could recognize the entrance of the Roman tunnel. The precaution was very necessary, for the great cave, so far as I could see it, was intersected by passages. Having made sure of my position, and reassured myself by examining my spare candles and my matches, I advanced slowly over the rocky and uneven surface of the cavern.