Such immense caves exist in various parts of the world, natural crypts dating from the geological epoch of the globe. Some are filled by the sea; others contain entire lakes in their sides. Such is Fingal's Cave, in the island of Staffa, one of the Hebrides; such are the caves of Morgat, in the bay of Douarucuez, in Brittany, the caves of Bonifacier, in Corsica, those of Lyse-Fjord, in Norway; such are the immense Mammoth caverns in Kentucky, 500 feet in height, and more than twenty miles in length! In many parts of the globe, nature has excavated these caverns, and preserved them for the admiration of man. Did the cavern which the settlers were now exploring extend to the centre of the island? For a quarter of an hour the boat had been advancing, making _detours_, indicated to Pencroft by the engineer in short sentences, when all at once-- "More to the right!" he commanded. The boat, altering its course, came up alongside the right wall. The engineer wished to see if the wire still ran along the side. The wire was there fastened to the rock. "Forward!" said Harding. And the two oars, plunging into the dark waters, urged the boat onwards. On they went for another quarter of an hour, and a distance of half-a-mile must have been cleared from the mouth of the cave, when Harding's voice was again heard. "Stop!" said he. The boat stopped, and the colonists perceived a bright light illuminating the vast cavern, so deeply excavated in the bowels of the island, of which nothing had ever led them to suspect the existence. At a height of a hundred feet rose the vaulted roof, supported on basalt shafts. Irregular arches, strange mouldings, appeared on the columns erected by nature in thousands from the first epochs of the formation of the globe. The basalt pillars, fitted one into the other, measured from forty to fifty feet in height, and the water, calm in spite of the tumult outside, washing their base. The brilliant focus of light, pointed out by the engineer, touched every point of rock, and flooded the walls with light. By reflection the water reproduced the brilliant sparkles, so that the boat appeared to be floating between two glittering zones. They could not be mistaken in the nature of the irradiation thrown from the centre light, whose clear rays broke all the angles, all the projections of the cavern. This light proceeded from an electric source, and its white colour betrayed its origin. It was the sun of this cave, and it filled it entirely. At a sign from Cyrus Harding the oars again plunged into the water, causing a regular shower of gems, and the boat was urged forward towards the light, which was now not more than half a cable's length distant.