The purpose of the great lake of heated water behind me appeared to be evident. It must be the source from which the great expanse of ordered growth was irrigated, and perhaps fed. The stream that came through the tunnel might be a mere overflow, which was drained off into the sea, or it might be used for the filling of subterranean pipes lower down the slope. In either case, it did not greatly concern me,--or so I thought, not foreseeing how greatly I should need its help in the coming peril. The yellow sand on either side supplied a sufficient space on which to walk upright beneath the shelving walls. It was dimly lit from above, and obscured by the steam which rose from the water, but I could see that it ran straight on for a long distance. Actually, it was a length of about twelve miles, as I learnt afterwards. It appeared that, being entered, it would offer no exit until I reached the further end, however far it might be. But there was no appearance of any possible danger, and I knew that it was the way which I had been directed to take. The only warning I had received was to traverse it as rapidly as possible, and it certainly did not appear to be an inviting avenue in which to linger. Perhaps it was the fact that I must emerge from it on the threshold of a new experience, the nature of which I could only guess very dimly, that made me rest so long, even when I waked from the sleep I needed, before I entered the passage, but I remember that I did it with a great reluctance, and started at a pace which, though it might not be equal to the light swift running of my instructress, was sufficient to take me a long way forward in safety.