The day was calm, so we ventured in. At first I thought it merely a gorge in the rock, but even while peering for the end wall we slipped under the archway and found ourselves in a vast room. Our eyes were dazzled so we could make out little at first. But through the still, clear water the light filtered freely from below, showing the bottom as through a sea glass. We saw the fish near the entrance, and coral and sea growths of marvellous vividness. They waved slowly as in a draught of air. The medium in which they floated was absolutely invisible, for, of course, there were no reflections from its surface. We seemed to be suspended in mid-air, and only when the dipping oars made rings could we realise that anything sustained us. Suddenly the place let loose in pandemonium. The most fiendish cries, groans, shrieks, broke out, confusing themselves so thoroughly with their own echoes that the volume of sound was continuous. Heavy splashes shook the water. The boat rocked. The invisible surface was broken into facets. We shrank, terrified. From all about us glowed hundreds of eyes like coals of fire--on a level with us, above us, almost over our heads. Two by two the coals were extinguished. Below us the bottom was clouded with black figures, darting rapidly like a school of minnows beneath a boat. They darkened the coral and the sands and the glistening sea growths just as a cloud temporarily darkens the landscape--only the occultations and brightenings succeeded each other much more swiftly. We stared stupefied, our thinking power blurred by the incessent whirl of motion and noise. Suddenly Thrackles laughed aloud. "Seals!" he shouted through his trumpeted hands. Our eyes were expanding to the twilight. We could make out the arch of the room, its shelves, and hollows, and niches. Lying on them we could discern the seals, hundreds and hundreds of them, all staring at us, all barking and bellowing. As we approached, they scrambled from their elevations, and, diving to the bottom, scurried to the entrance of the cave. We lay on our oars for ten minutes. Then silence fell. There persisted a tiny _drip, drip, drip_ from some point in the darkness. It merely accentuated the hush. Suddenly from far in the interior of the hill there came a long, hollow _boo-o-o-m_! It reverberated, roaring. The surge that had lifted our boat some minutes before thus reached its journey's end. The chamber was very lofty. As we rowed cautiously in, it lost nothing of its height, but something in width. It was marvellously coloured, like all the volcanic rocks of this island. In addition some chemical drip had thrown across its vividness long gauzy streamers of white. We rowed in as far as the faintest daylight lasted us. The occasional reverberating _boom_ of the surges seemed as distant as ever.