Presently a yawning hole, fitfully lighted by bulbs suspended from its earthen roof, confronted her. So this was the tunnel of which the Keeper of the Door had spoken. She entered it resolutely. The quality of the air had altered subtly: it had taken on a puzzling, faint, acid tang, which she could not account for until she came to the exit. She must have travelled three hundred yards before that, and then she noticed that there was only darkness in front, and the bulbs no longer hung from above. She switched on her flash again, and a moment later she left the tunnel. A blank, tile-faced wall was before her, and she looked from right to left in surprise. She was in a second tunnel which ran almost at right angles to the one she had left, and at distances lights hung on the walls. She bent the rays of her torch downwards, and they were reflected by three parallel ribbons of gleaming metal. And then she understood. She was in the Tube! CHAPTER XIII INTEREST IN "H" Making a closer examination with the aid of her flash, she realised her good fortune, for a heavy circular door hung back against the tiling of the tube tunnel. Swinging it cautiously, she found that when closed it fitted snugly into the round gap where the Sud-Scandinavia burrow debouched into the City and South London Railway, so that without a minute study of the surface it was impossible to know that a door existed at all. Remembering that between the hours of two and five ack emma, when the Tube is closed, there are repair gangs moving about its subterranean ways, she realised the necessity for some such camouflage, and guessed that the door had only been left open by an oversight. Scrutinising the inside of it, she found that it was fitted with a strong iron bolt. There was nothing to be gained by remaining, and she retraced her steps with all speed. On the return journey she noticed that two irregular ruts ran on either side of the earthen floor of the boarded lane. Some heavy-wheeled vehicle had made that trip frequently, but what it might be she had no notion.