So far, Frank had been too busy and too much engrossed by the novelty of his situation to devote much time to thinking; but now, when the excitement and hurry was over and he had leisure to turn his attention to other matters, second only in importance to securing all there was of value in the schooner, he concluded to make a thorough exploration of the island and the grim, conical-shaped ledge of rocks that formed its upper, or southern part. So, the fifth day of his landing on the island, he got ready the small boat, placed in it a bottle of water and a good supply of food, and set out to row around the reefs. He made a complete circuit of the island, and found it to be one of the many results of volcanic eruption common throughout the Pacific Ocean and the Caribbean Sea. At low tide, a long, black reef showed its frowning edge above the restless surf, connecting with the higher point of rocks overlooking the narrow strip of fertile land lying between it and the sandy beach, where the Sea Eagle had stranded, and still maintained the strange and lonely anchorage she had made for herself. Frank, curious and venturesome as he might be, was yet keenly alive to hidden dangers, and, as he rowed around among the rocks, kept a sharp lookout for treacherous currents and submerged ledges. The meridian sun was pouring down its fiercest rays, and he was thinking of returning to his tent and the grateful shade of the palm-trees, when, just as he had rounded the jagged spur of a particularly ugly-looking coral reef, he suddenly saw before him a deep, dark line of perfectly smooth water, over-arched by a natural bridge of grayish-white limestone, and flowing, as it seemed to him, directly under the island. The entrance to this odd underground water-way was not more than four feet in height by six wide, but he unhesitatingly entered the narrow channel, bent upon seeing what there was of it and where it led to. Drawing a long breath of surprise and satisfaction, he ceased rowing, and, as the boat came to a stand-still on the glassy surface of this subterranean sea, he uttered an exclamation of wonder, and looked around him in a maze of doubt and admiration. The cool, grotto-like atmosphere and dim, half-twilight contrasted pleasantly with the heat and glare outside, though the silence was something oppressive, and different from any he had ever before known. No sound of wave or sigh of wind or howl of tempest seemed ever to have been heard here. The water along the edges of the rocks was absolutely without motion, and the light from either extremity of the cave--as one might call it--nearly lost itself before it reached the vaulted centre. Frank shouted loudly, and in answer the rocks sent back only the faintest and most weirdly far-away echoes.