At length he bethought him of another test--that of throwing lumps of rock against the walls. There were fragments in plenty scattered loosely at his feet. The first one he threw went straight out ahead--and presently thumped on something solid. He reckoned the distance some sixty feet away, but admitted it might have been eighty. Every missile he cast right, left, or at an angle promptly reported a wall; and some plumped back into water. The cave was not gigantic, but all its floor was apparently flooded. His hand, which he thrust in the water where he stood, groped blindly and found no bottom. He rolled up his sleeve and tried again, without more definite results. The water, however, was warm. "Good place for a swim, in any case," he told himself, aloud; and, planting his torch with a sudden determination that he would not retreat with information so utterly meager, he stripped off his clothing at once. He let himself into the ebon depths, with his torch held well above the water. He had rather expected to be able to wade, but he sank to his neck without sounding to the bottom. Swimming almost perpendicularly, employing one hand only, he presently lost all sight of the walls and was out in an unknown pool of blackness. Save for a slight sensation of its weirdness, the experience was decidedly pleasant. He tasted the water as he swam and found that it was fresh. He turned to look out at the opening, but could barely see light through the weeds. Some twenty or thirty feet from the ledge, his feet encountered a ridge. It was stone, and across it he waded to a greater depth beyond. Yet once again he was soon enabled to stand erect and walk along the bottom. The broken, uneven surface that he felt with his feet made his progress slow and careful. He had presently crossed the underground pond, up the sloping bank of which he was soon making rapid progress. He emerged on a dry ledge beyond. Even then the walls were not to be seen till he walked a rod straight onward. The briefest examination sufficed to establish the fact he had come to a sort of natural antechamber to the larger cavern he had crossed. Also, apparently, the entire place was as empty as a last year's bird's-nest. Vaguely disappointed, though he hardly knew why, the man surveyed the place anew, by the light that entered at the opening as well as by that of his torch. He saw at once that, could it be drained, the place would afford a retreat of amazing security for anyone needful of shelter. He was also certain he could drain it in a day by blasting through the ledge of rock that blocked the entrance from the sea and so retained the pool. With one more brief and cursory examination of the rocky structure about him, he was turning away when something foreign about a slab of stone, that seemed a fragment of the solid wall, attracted his attention.