They hunted around until they found a small slab of stone which they wedged under the door, after they had dropped down into the space below. Then, with Frank in the van, with his flashlight sending its rays ahead of them, they ventured slowly into the unknown, feeling their way with the utmost caution. The stone floor was uneven and damp, and at times they stepped into pools of noisome water that was covered with green scum. The sides of the narrow passages were covered with mold, and the air was heavy and offensive. Suddenly Frank stepped back with a sharp exclamation, and at the same instant there was a squeal, and a gray form scurried away into the darkness. "A rat!" he murmured to his friends behind him. "I stepped fairly on him. A mighty big fellow he was, too." They went on a little further, keeping close together, for there were several passages that branched off from what seemed to be the main one, and if they became separated it might be difficult for them to get together again, especially as Frank was the only one of the trio who had a flashlight. And now their ears were assailed by soft patterings and shufflings that seemed to increase in number as they progressed. Their eyes caught certain red points that flared like sparks and then vanished, only to reappear. It was as though a host of eerie things were keeping tab on their movements, and after a while this silent mustering of unseen watchers got on their nerves. Billy, who came last, was passing one of the passages that branched off to the left when he thought he caught a glimpse of light. He went into this side passage for a few steps to make sure, and verified his first impression. There, sure enough, was an electric bulb, on the opposite side of which he could see the outline of a door. He was hurrying back to tell his comrades what he had seen when he heard an exclamation from Bart that quickened his steps still more. Bart's right hand was holding on to his left, and in the light that Frank had directed on it he saw that the hand was bleeding. "It was a rat," Bart exclaimed wrathfully, as he nursed his wounded hand. "The beggar jumped straight at it. It feels as though he'd made his teeth meet through it" Billy whipped out his handkerchief and was binding it around his comrade's hand, when a gray form sprang from the darkness and fastened its teeth in his trousers leg just grazing the skin. Frank made a kick at it, but as he did so, his foot slipped on the damp stone and the flashlight flew out of his hand, leaving them in utter darkness. He stooped to try to find it, but his hand touched a furry coat and he drew back just in time to escape a savage snap.